Bald Mountain Mill Technical History Bald Mountain Gold Mill, Lead South Dakota
Bald Mountain mill was built into a natural hillside and utilized gravity to aid the flow of materials through the building. Its siting allowed construction of a leveled area above and behind the mill to accommodate mine car tracks and attendant buildings. Cut into the hillside below part of these tracks were the crude ore bins and primary coarse crusher facilities. A belt conveyor covered the short distance to the main mill building which was set on a lower level than the base of the ore bins.
The mill itself was constructed with four terraced levels that essentially remained the core of the plant throughout its operative life. The uppermost level supported the main crushed ore bin, and later storage tanks. This level was the only one to undergo appreciable linear extension (on a north-south orientation) by being built further back into the hillside to create later sub-levels. These sub-levels initially accommodated the ore sampling facilities and over the mill's history were enlarged to include dust collecting, secondary coarse crushing, and part of an unoxidized (blue) ore treatment circuit.
The second distinct level has retained a strong degree of integrity as the milling and classification level. It also included primary thickeners when extended laterally (to the west). The third level originated as the sand tank floor, but later accommodated thickeners and was also extended laterally (to the west) where it partly encroached on the level above. The lowest level featured a raised floor above the mill sumps and air compressors, though the floor itself accommodated pregnant solution tanks, agitators and the precipitation stage.
The basic four levels were a good deal less inviolate than the above summary depicts. However, it is valuable to use them as a starting point for viewing the mill's growth. Additions at the mill were often quite piecemeal. A one-story, sloping roofed shed built onto a previously external wall was a common method of extension. Foundations and retaining walls were either of rough- hewn stone or poured concrete, and the structure was wooden, with some exceptions. Roof and wall coverings were fashioned from wooden boards, roofing felt, rubber and metal sheet.
Periods of financial investment and mine acquisition, particularly changes in ownership, had profound effects on the mill. The history of the mill can be divided into three phases representing three different owners: American Eagle Company (1906-10), Trojan Mining Company (1910-28) and Bald Mountain Mining Company (1928-59). Each phase is described in a fashion that follows the flow of ore through the mill for easier comparison of operation and plant. Ancillary buildings have been given a separate heading within each phase.
The American Eagle Mining Company 1906-1910
Information about machinery and practice at the American Eagle mill is somewhat scarce. It has been necessary to rely on early documentation from the Trojan Mining Company's ownership period, as well as photographs and field work. In its original phase this Bald Mountain mill was a small cyanide leaching plant. The cyanide leaching process was a relatively simple one and the subdivisions of that process within the plant were clearly defined by the four levels.
The main crude ore bin was built into the hillside, with the wooden bin structure set against concrete retaining walls. A proposal drawing shows vertical walls dividing two new bins and a front wall, with four windows, enclosing the front (the pitched roof house had been removed), but it is likely that no alterations were carried out until the Trojan Mining Company took over in 1910 and the capacity of the mill was increased. In an early photograph of the bin, the concrete wall is complete and the framework and sloping sides of only one bin can be seen. It is the central of the three proposed, set furthest back into the hillside and directly behind the crusher house, a small pitched roofed structure. It is possible that the additional bins were constructed at some point in the Eagle phase to form a trio set around a central crusher house.
The bins were emptied by manually operated gates where ore fell through or over grizzlies (iron grills) into or past the crusher beneath. If the ore was small enough to pass through the grizzly bars it fell directly down sloping wood walls to the bottom of the bin. Material that could not pass through the bars was diverted by them into the primary coarse crusher, the largest pieces possibly being broken by workers with sledgehammers. The crusher was a heavy-duty machine designed for breaking up large pieces of rock, although its exact type is unknown. It was powered by an electric motor running on 440 volts, as were all machines in the mill.
Beneath the primary coarse crusher was the base of a 16 inch-wide inclined belt conveyor that received both the crushed and suitably small ore. Concrete retaining walls that stood parallel to the belt at its base still survive. The conveyor led to the main mill building over a distance of 117 feet and an elevation of approximately 30 feet. Over this distance it was enclosed in a weather boarded housing with a shallow pitched roof and pairs of small windows. The conveyor housing was supported on four wood trestles of increasing height.
At the top of the conveyor, ore was deposited into an ore bin with a 365-ton capacity. The ore bin was built of wood with heavy exterior timbers providing additional support on the lower section and an upper floor housing the head of the conveyor and providing room for maintenance and observation. The outer wall was extended on the east side to enclose the three flights of stairs to the upper floor, and windows were placed on east, west and north sides.
Ore was discharged from the north side of the 365-ton ore bin by two "Challenge" feeders, known to have still been present in 1912. These feeders consisted of a large hopper from which ore could flow onto a revolving disc, driven by a bevel gear from beneath. As the disc moved a specified amount of ore could pass between a pair of adjustable metal wings. In the Eagle mill the timber cube framework of each feeder would have been positioned above and behind the Chilian grinding mills they fed. The Chilian mills were manufactured by the Monadnock Company and had seven-foot diameter bowls and nine-foot diameter flywheels. The ore was ground to a fine consistency by the action of rollers, driven from a central shaft, crushing against a ring in the base of the bowl. The bowls had outside diameters of 84 inches and the rings had an internal diameter of 46½ inches. The bottom of the mill pans sloped downwards to the discharge side of the mill in order to use gravity flow. The two mills worked as independent primary grinding mills, receiving and discharging ore separately and without passing it on to secondary grinding.
The retaining wall at the lower extent of the ore bin level was constructed of stone with a concrete sill (possibly a later addition). Angled slots, a feature not found in later phases of operation, are present which could have corresponded to the line of descent between the bin and the former site of the primary (Chilian) mills. Power was transmitted from an overhead line shaft arrangement which was used "as we inherited it" by Trojan in 1913.
A single, unspecific reference to "rolls" and a "classifier" requires mention here. "Rolls" would seem to imply that a secondary fine grinding system, composed of roller mills, was in place when the Trojan Company took over the mill. Their exact place in the production flow is unknown, and it is even possible that they were superseded by the Chilian primary mills and merely left in the mill. Although the Eagle Company was not dividing its pulp into sand and slime fractions, a classifier could have been used to return coarse material to the mills for further grinding. There was no other method of grading sands present at this stage so a classifier could have been placed after the Chilian mills. No other reference to this machine has been found and its type is unknown.
At the eastern side of the primary mills stood a mill solution storage tank. From here cyanide was introduced to the ore in the mills, making it into pulp. The crushed ore was transferred from the primary mills by launders to six sand leaching tanks, twenty-eight feet in diameter and eight feet deep, situated on a lower level of the mill. (This proposed arrangement assumes the absence of secondary fine grinding and classification). Cyanide solution was also channeled down to this level in pipes from the mill solution tank. The bottoms of the sand tanks were fitted with metal grates overlaid with a filtering screen, usually of canvas, through which the gold and silver bearing solution could pass. Solids were periodically cleared through a discharge gate in the bottom of the tank. At Bald Mountain this was probably done by hand, though expensive mechanical excavators were used in large mills and sluicing was practiced in areas with a more plentiful water supply.
Ideally ore would be crushed to a consistency that would allow percolation of each cyanide wash at a rate of one to three inches per hour. A common number of washes was four: one of strong cyanide, two of weak cyanide and one of water. Each leach would have taken about fifteen hours to complete and the water wash about twenty hours. Combined with charging and discharging the whole process took from five to ten days. Two methods of adding the solution were commonly used. Liquid could either be poured in from the top of the tanks or could be pumped in from the bottom until the tank was full, and then allowed to drain out. The latter method agitated the ore pulp and improved dissolution of the metals. The exact nature of the operation at Bald Mountain is unknown, although the arrangement of the sand tanks below the mills implies the use of a gravity aided top pouring method.
At the sand tank level, the building extended latterly from the main structure to accommodate the tanks. The main slope of the roof continued in the central four ranges of the level. The pairs of four bays placed at either side (accommodating two tanks each) were built lower with pitched roofs and small windows punctuating them at two levels. The level below the sand tanks was constructed with a stone retaining wall. Stone foundations were added for the support of gold and silver bearing solution tanks ("gold tanks"). Two wood gold tanks (12 feet deep and 14 feet in diameter) remain on-site, and stone foundations are found underneath the Trojan period agitators that have been capped with concrete. It is possible that these are the sites of other Eagle gold tanks. How the mill's sand leaching operation continued if these are gold tanks that were removed is unclear, since the number of sand tanks was not correspondingly reduced. Possibly less washes were used (and therefore less solution drained off) after the application of finer grinding when sliming was introduced and the gold tanks were no longer required.
Beyond the gold tanks the precipitation level continued. A raised wood floor was constructed to compensate for the ground level falling away below the tanks. Details of the precipitation stage at the Eagle mill are sketchy. "Burt" filter presses were used to remove solids from the bullion bearing solution. An unknown number of these devices probably operated by mechanically pressing solution between the leaves of a series of filter bags. The fluid would pass through and drain away while the solids remained on the leaves. As far as is known, no other filtration method was used and there was no vacuum system for removing oxygen from the solution. Although one is not known to have existed in this period, a tank of hydrochloric acid was the preferred method for cleaning the filter leaves, "every week or two."
After filtration, the solution moved to the zinc precipitation stage. Information from 1912 indicates that the Trojan company inherited a Johnson zinc lathe. Such a machine would have been used to manufacture zinc shavings, suggesting that zinc boxes were used by the Eagle company. The zinc box was an elongated trough divided into a series of compartments, each with a mesh bottom supporting a layer of zinc shavings. Gold and silver bearing solution was fed into the first compartment, and as each compartment filled, it would fill the next by rising through the mesh. In this way the solution was brought into contact with the zinc. A drain beneath the box took away the solution and the water from a subsequent zinc washing stage. Zinc compounds from the first box were then taken for refining while the ones below (having precipitated less gold and silver) were moved up one compartment each.
Barren solution leaving the zinc boxes flowed to sump tanks constructed under the raised floor, at natural ground level. The earliest Trojan drawing of the mill shows a pair of large circular sump tanks. If these both date from Eagle Company operations, they would have served to receive barren solution which probably had cyanide added (to make mill solution) before being pumped back up to the mill solution tank. A second storage tank, sited near the first on the western side of the primary mills, is listed as a "barren solution tank" in 1916, but may have served Eagle as a mill solution storage facility. A single story, sloping roofed "clean up room" was built onto the north side of the lowest floor of the mill. This facility was used to clean the filter leaves which would periodically become clogged with lime.
No details are known of the equipment used in the refinery during this time. Most likely an oven for drying the precipitate would have been present as well as a furnace for melting it, after fluxes had been added to separate any slags (impurities). Refinery furnaces were generally of the reverberatory type, so that the graphite crucible full of precipitate would be heated without direct contact with flame. In the early Trojan period inquiries were made about converting the furnace to oil firing. This implies that the Eagle Company had been using either solid fuel or gas firing.
Several buildings were constructed outside the main mill complex during the Eagle phase in order to supply essential services to both mine and mill. A single story warehouse was built, to the east of the mill, beside the road to the railroad and to Portland (later Trojan). The mill proper was served by three ancillary buildings. A small electrical substation was located close to the mill on its east side, about half way down the hillside. This substation converted main current from the public supply used by the mill. A small, wood single story assay office, for the testing of ore samples, was constructed on the sand tank level, to the east of the mill. Also on the east side of the precipitation level was the refinery, a small, wood pitched roofed structure with a short steel furnace chimney.
The Trojan Mining Company 1910-1928
The major change at the mill during the ownership of the Trojan Mining Company was the gradual introduction of sliming technology. This new technology brought alterations to the fine grinding stage and additions to the sand tank and precipitation levels. In terms of the overall development of gold milling technology, the mill at this phase stands between the early all sand leaching and later all-sliming systems.
By 1912 there were three crude ore bins at the top of the mill in order to increase capacity. The two additions were in front of the original and divided by a pair of sloping walls, in the form of a pitched roof. Although it is possible that the two additional bins had been built before the Trojan Company took over, an increase of capacity to 1000 tons in 1914 would correspond to changes during the early Trojan period. The primary coarse crusher was situated between and beneath the bins, which had small windows added for illumination. The primary coarse crusher was a Gates Number 5 "K style" gyratory machine belt driven by a 35 HP motor running at a speed of 850 RPM. The motor was situated in a chamber in the concrete base of the ore bins and placed above the crusher. A gyratory crusher operated by the rotation of an internal grinding cone revolving about an eccentric axis. Rock was caught between the cone and the inner wall of the crusher. This gap could be set to grind to a specified grade, in this case 1 3/8 inches. A belt drive from the crusher flywheel transmitted motion to a bevel gear system powering the main conveyor to the mill and a pan feeder, an 18- inch-wide belt conveyor acting to carry material between the crusher discharge and the main conveyor.
To take selected ore samples for assaying from the 365-ton crushed ore bin, a sampling system was constructed in 1912. This system was housed within a sloping roofed structure built against the uphill (south) side of the ore bin. A bucket-like device at the top of the main conveyor from the crude ore bins was used to divert a given amount of ore to a sampling bin, created by a diagonal division of a fraction of the 365-ton bin. The sample bin was in three parts which could be released independently to the sample room below where a vibrating screen retained the larger pieces for inspection. To return the unwanted sample material to the ore bin, a bucket elevator was situated beneath the sample room floor which rose to deposit ore, through a chute, into the 365-ton bin.
On the fine grinding floor, the Chilian mills were retained as primary milling devices, though there seems to have been attempts to improve the machinery as the mill moved toward part- sliming. In November 1910, shortly after the mill came under Trojan ownership, manager H.S. Vincent proposed the mill "capable" of using two "Eclipse" roller mills and "2-7 Trent mills". Vincent seems to have been thinking in terms of a two stage fine grinding system to bring ore to the required slime grade with the Trent mills grinding ore wet (with the addition of cyanide) and the rollers grinding dry.
There is no evidence that this proposal was ever carried out, though a reference to "mill no. 3" in September 1912 seems to have been a record of a brief experiment with alternative milling devices. An illustration of approximately the same period shows what appears to be a solitary roller mill positioned on a platform at the north side of the 365-ton crushed ore bin. In addition, proposals for a pair of ball mill/classifier circuits were made as early as 1916, but never realized.
In order to sort the sand from slime a pair of classifiers was installed around 1913. They were positioned on a timber platform raised above the milling floor and projecting northwards over the third (from east to west) sand tank on the level below.
Although this floor has long been removed, a flight of stairs can still be seen rising approximately half the height of the building which may have been a point of access to this upper level. Crushed ore was transported from the Chilian mills up 40 feet to the classifiers' platform by a pair of vertical bucket elevators. Each classifier operated as primary classification units working and discharging separately without being in series to another classifier.
The Dorr rake type classifiers used consisted of a pair of reciprocating moving rakes set in an inclined trough filled with solution and operating at approximately five rakes per minute. The reciprocating motion was supplied by connecting rods and eccentric cams situated above the rakes in order to keep them clear of the liquid. The rakes pulled pulp up the trough but on the return stroke they lifted clear of the bottom of the trough, allowing fine slimes to flow free and leave the lower end of the trough with the overflow fluid. Since there was no facility to return material for further milling once it had entered the classifiers at the Bald Mountain mill, it can be assumed that some form of screen was used on the mills to stop excessively large pieces of ore being discharged to classification. The classifiers were driven by electric motors via belt or chain, though details about these motors are unknown. Individual motors, parallel to the classifiers' drives, are likely to have been used through an overhead line shaft arrangement. Working from a shared motor would also be possible. The classifiers seem to have been installed before any of the other technology required for the treatment of slimes. This change implies that the partial conversion of the mill was undertaken at a slow pace and perhaps that it was undesirable to close the mill down completely while refitting took place.
An overhead crane, spanning the width of the fine grinding level, was installed in 1916 to enable the movement of machinery for repair. It was designed to bear six tons and ran on rails resting on beams upon rows of timber pillars erected inside the existing walls of the building. Additional trusses were added to strengthen the rail support in 1917. On the level below the Chilian mills the sand leaching tank arrangement was maintained intact and continued to operate. Sands were discharged from the classifiers and flowed down to be distributed into the tanks. The slime output from the classifiers passed into pipes or open launders to the primary thickeners. This action would probably not have required the application of pumps due to the roughly four foot drop to the thickeners.
The processing of slimes began in earnest around 1914 with the installation of a thickener 40 feet in diameter and 14 feet deep, constructed from timber staves bound in steel tie hoops. 5 6 The Dorr thickener allowed solids to settle out from the pulp and be removed while the solution containing gold and silver remained at the top of the tank and flowed away. A central shaft driven from above the tank turned radial arms at the bottom, clearing settled solids away through a central drain. Pulp was fed into the thickener tank by a launder passing over the surface to the center, while the level was kept constant by a discharge launder. Having separated from much of the solid pulp, the "clear" solution left via the discharge launder.
Primary thickener number one was constructed on timber cross beams set on concrete piers. These were sited on the milling level, at the western side of the Chilian mills, and raised the tank considerably above the floor level. This level of the mill was extended both to the west and south, uphill below the west side of the 365-ton crushed ore bin level. To create space on the milling floor, the barren solution sump that stood to the western side of the primary (Chilian) mills was moved up to an extension of the 365-ton crushed ore bin level above it. The mill solution tank at the opposite end of the milling floor was also moved to the east of the 365-ton bin. In this way the top level of the mill building was expanded laterally, continuing the pitched roof line from the 365-ton crushed ore bin to house the mill and barren solution tanks.
The clear solution overflow from primary thickener number one was either returned to the primary mills to aid the cyanidation and become further enriched or flowed downhill to the main mill solution sump. At this early stage, the mill's precipitation department did not have the capacity to handle the extra solution directly. The pulp leaving the bottom of the thickener (the "underflow") ran by gravity through a pipe to three 16 foot deep agitators which were installed between 1912-16. These 17 foot diameter timber tanks were sited along the tier below the sand tanks where the gold solution tanks stood.
The agitators were round, flat bottomed tanks constructed of redwood staves bound by steel bands. Slime was fed in from the top of the tank and any solids would settle on the bottom. Here they were scraped towards a centrally positioned shaft by two rotating arms, each with a series of ploughs, that were attached to the same shaft. Each shaft was driven by a belt from overhead line shafting via bevel gearing. Compressed air was introduced, forcing solids up the inside of the shaft to a pair of revolving launders at the top of the tank, where they were distributed back into the tank. A launder at the top of each agitator took the solution, including suspended solids, to the next so that the agitation process continued through all three tanks, running west to east. Two existing gold tanks were retained for receiving the pregnant solution from the sand tanks. It is uncertain what happened to the solution from the agitator series after it had left the final tank. It is unlikely that this extra volume was being put through precipitation at this stage as only three zinc boxes were in operation in 1912.
In place of the earlier filter press a Butters' vacuum filter was in operation in 1914. A series of filter bags stretched around perforated metal tubes were placed in an elongated trough filled with pulp. A vacuum in the tubes would suck any solution present in the pulp through the bags and away through pipes connecting the tubes. After this operation a wash of water was used to clear the bags of barren pulp which was then flushed away as tailings. Filtered solution was passed to the gold tanks by vacuum pump of 14 inch bore and 12 inch stroke.
The position of the Butters' filter is unknown, but it is likely that it survived to be recorded on a plan of 1916 which shows two bays having been added to the north side the mill building to accommodate a 11 feet wide, 29 feet long filter. By 1925 a vacuum pump of 14 inch bore and 12 inch stroke was added.
These two bays also housed an air compressor engine on the ground floor below. This compressor would have been installed around 1913 in order to provide air to the agitators. It was built by Ingersoll-Rand and run by a 40 HP electric motor. An air receiver tank, 3 feet in diameter and 18 feet long, was placed against the north wall of the mill next to the compressor/filter housing to maintain pressure in the agitator supply system.
The Butters' filter does not seem to have sent agitator solution to the zinc boxes, but instead deposited it in the mill solution sump. The mill solution and barren solution sumps located beneath the floor of the filtration and precipitation level were 26 feet in diameter, 7 feet deep and illustrated as being circular, possibly of wood construction, as were many liquid storage tanks in the mill.
In an attempt to stop the leaching of pollutants into False Bottom Creek, from the tailings dumped on flat ground on the north side of the mill, a concrete dam was constructed 100 yards north of the mill. When completed in September 1913, the dam featured barrels filled with charcoal placed in the far wall of the dam to trap the traces of bullion still contained in the tailings. These yielded $10 per day.
It is possible at this time that the mill capacity was too small and as a temporary measure until further changes took place, pregnant solution from the slimes was only used to enrich the sand leaching. In many ways the changes of the first part of Trojan's ownership were tentative preparations for a larger change in the system, even though the faster decantation sliming method was increasing output over leaching even before these changes took place.
The second phase of Trojan's ownership of the Bald Mountain Gold Mill site was characterized by alterations designed to produce more pulp of a fine slime grade. To this end, ball mills were brought in to supplement the Chilian mills, and better use was made of classification. The addition of a secondary thickener system using counter current decantation and an integrated milling/classification system between 1916 and 1925 established the basis of the Bald Mountain Company mill. It also dramatically changed the flow of materials in parts of the mill.
The ball mill consisted of a steel cylinder rotated by an electric motor, acting upon a large gear ring around its circumference. Ore was fed into the mill to be crushed by numerous metal balls as the cylinder was rotated on its axis. Ore was fed in at the axis by a revolving scoop. In the "high level discharge" type, the content simply builds up until it overflows through a tube set in the axis opposite the point of entry. "Low level discharge" mills featured lifting bars attached to the inside of the cylinder which brought material up to a grate at the axis through which it fell.
The first ball mill was installed at the Bald Mountain mill in 1917. It was built by the Denver Engineering Company ("Dewco") and measured 8 feet in length with a 5 foot diameter. It rotated at a speed of 28 RPM and was powered by a toothed shaft driven by multiple belts from a 75 HP motor. This mill was positioned to the west of the primary (Chilian) mills and was placed in closed circuit with a rake classifier 6 feet wide, and 18 feet, 4 inches long. Sands from the classifier were carried along a trough by a screw and deposited in the ball mill feed to be picked up by the rotating scoop. Discharge from this mill was by the high level method, and a launder conveyed pulp back to the classifier. This ball mill/rake classifier assembly was itself linked to one of the Chilian mills as a secondary milling and classification unit (secondary mill number one). Pulp from the Chilian was fed to the secondary mill circuit via the classifier.
When pulp finally left the ball mill circuit it flowed to the elevators which took it to the rake classifiers raised above the sand tanks. These classifiers were now placed in series, one feeding into the other, and became a tertiary classification facility after the primary mills and the secondary milling/ classification machinery. From here sand went down to the sand tanks while slime flowed to the primary thickeners.
Another ball mill/classifier circuit was installed in 1919, replacing the eastern Chilian mill and taking over the function of primary mill number one in supplying the secondary circuit. The mill and classifier were raised on large concrete foundations to provide gravity flow for the discharge. The new primary number one ball mill was built by the Allis Chalmers Company. It was 6 feet in length, with cylinder 6 feet in diameter and operated with a low level discharge system. The mill was driven by a toothed shaft powered by a 125 HP, 436 RPM motor acting directly on the mill's gear ring. A Dorr rake classifier 16 feet, 4 inches in length and 4 feet, 6 inches wide was connected in closed circuit and used for primary classification before pulp flowed to the secondary mill/classifier circuit. The eastern of the Challenge feeders was replaced by a timber hopper built onto the base of the 365-ton crusher ore bin to feed the new primary mill number one by an 18 inch-wide belt conveyor.
Mill solution was added at the discharge end of the primary ball mill from the mill solution tanks (a second was added beside the first), and at the feed end by an extension of the pipe from the primary thickener overflow. This overflow also supplied the remaining Chilian mill (primary number two) and the other (secondary) ball mill. Pulp from the secondary mill flowed through a launder to the bucket elevators where, with the output from primary mill number one, it was raised to the secondary classifier platform.
In 1917, a Weigand Classifier was installed on a platform adjacent to that of the two rake classifiers and was receiving pulp by bucket elevator from the grinding circuit as well as mill solution from the storage tanks. The Weigand was circular in shape with a 8 feet, 6 inch diameter bowl in which rotary rakes at the bottom discharged settling sands. The flow of solution removed lighter slime particles at the top. Although the power source for the Weigand classifier is unknown, a motor driving a central shaft via bevel gears or line shafting is possible. The rake and Weigand classifiers together acted as a tertiary classification unit, taking pulp from the secondary below sending sands to the sand tanks while the slimes flowed to primary thickener number one. Both these routes used gravity and required no pumps.
Also in 1917, a second primary thickener and a secondary thickener system composed of three tanks were built. To accommodate primary number two, the fine grinding floor was further extended to the west and south. The sand tank level was extended to the west by six bays and then south next to a retaining wall at the edge of primary 2. The new primary thickener was of the same dimensions and similar construction as the first, with high concrete piers supporting heavy timbers to raise the tank above the underflow pipe and provide useable inclines for gravity flow.
Overflow solution from primary thickener number two joined that from number one to be reused in various ways. It could be sent back to the mill solution storage tanks, or into the grinding system, either to primary mill number two (the Chilian) or the secondary mill/classifier. In this way an excess of solution output could be diverted from the limited capacity of the gold tanks and precipitation facilities by being sent into the fine grinding circuits for further enrichment.
Primary thickener overflow that was sent to precipitation was first passed through a "gravity clarifier" set on timber staging just below the thickener discharge launders. This clarifier removed solids from the overflow before joining solution from the sand tanks on its way to the gold tanks. The name implies a simple filter through which solution passed, although the remains of line shafting are still seen at floor level (incorrectly placed for thickener drive), and the presence of a 15 HP gasoline engine suggest it may have had some form of compression action built in. By 1925, the gravity classifier was no longer connected to the thickener outflow, improvements in the precipitation floor having made it redundant. The underflow pulp from the primary thickeners discharged through a connecting pipe to the agitator series where it underwent continued cyanidation.
In 1917, two more agitators were constructed at the western end of the row, in vacant space that was probably intended for that purpose. At this time the flow of the agitator sequence was reversed (east to west) and the height of the tanks and their discharge launders altered, creating gravity flow to the secondary thickeners. Three secondary thickener tanks were constructed in the six new bays at the western end of the sand tank level, the end three of which extended south beside a retaining wall at the edge of primary thickener number two. Each tank was of the same dimensions and construction as the primary thickeners. They also operated in the same way and were driven by 10 HP motors fixed at the top of the central plough/arm shaft.
A counter current decantation system was used in the secondary thickeners. Slime was passed from agitator number five to secondary thickener number one, the solids from which were then drained from the bottom and pumped up to number two by a pair of four inch diaphragm pumps. In secondary thickener number two the underflow slime was transferred by another pair of four inch diaphragm pumps to the third tank, where it was finally discharged to flow back downhill to the filters. While the solids were moving uphill they came into contact with barren cyanide solution that had flowed down from the barren solution storage tank. This solution became pregnant solution (bullion bearing overflow) and was transferred, without the aid of pumps, from secondary thickener number three to number two and number one before flowing to the Butters filter and then to the gold tanks.
To undertake the heavy work of filtering slime from the secondary thickeners, three Portland filters were installed in 1917. Two were located on the western half of the precipitation floor and an extra bay was added to accommodate the third. While the exact operation of these machines is unknown, from a photograph and contemporary description they were clearly a form of rotating drum vacuum filter similar to the Oliver design. Fabric filters were attached to the outside of a rotating cylinder and a vacuum created behind the filter. The lower part of the cylinder was immersed in a pulp bath, and as the drum rotated, pulp was spread against the filters. The vacuum sucked solution through the filters, leaving solid "cake" on the outside to be scraped off by a blade fixed against the cylinder. The drum revolved on two trunnion bearings and was powered via a worm gear drive. Examination of the remaining foundations shows the Portland filter cylinders were orientated east-west, with the feed troughs supplied from the south side and cake probably- deposited on the north. A vacuum receiver 8 feet high and 4 feet in diameter powered by a vacuum pump drew the filtered solution from the filters, and a Triplex pump of 4 inch bore and 5 inch stroke transferred it to the mill sump.
To accommodate the extra output of pregnant solution, four extra zinc boxes were added around 1918. Three were of the same dimensions as the existing three, but one held compartments of slightly over half width. An extra bay was built onto the east side of the precipitation floor to house them and all fed by gravity into the barren solution sump below the floor.
In the clean-up room, in addition to an acid tank for the cleaning of filters, two filter tanks, a settling tank and a sump are shown in 1925. It is possible these were used to collect solids from the filters, perhaps to reclaim any last metallic value they might have. No details of the refinery are known during this period, but it can be assumed that the new oil-fired furnace was installed, (accounting for the construction of an on-site oil store), to handle the increased production of precipitate.
A Trojan Mining Company site plan of 1915 (BHSUL) shows the extent of new facilities the above changes had necessitated in the ancillary buildings. On the hill behind the mill a pair of octagonal roofed water tanks stored water pumped from the Two Johns mine for use in the mill. A stable was constructed on the south side of the tramway to accommodate mine horses. A machine shop was built east of the crude ore bins. Both crude ore bins were extended on the east and west sides, the latter enclosing the track from the Eagle mine and abutting the ore bin covering snow sheds erected around 1912. Behind the east end of the shop a coal depot building and truck garage was built to take coal from the road and place it into mine cars. A lime bin was constructed beneath the ore drops on the west side of the main crude ore bins, making delivery by rail possible. To the east of the mill, a hoist house pulled cars up and down an incline to a coal-fired boiler house that provided heating for the mill. One branch of this incline connected with the refinery and precipitation floor while another led across the base of the ore bin/ mill conveyor to a repair shed, immediately to the northwest of the bins. This shed stored repaired bearings for both mine and mill use. The refinery, assay office and electrical substation remained in their original positions.
The Bald Mountain Mining Company 1928-1959
Two major developments took place at the mill during the ownership of the Bald Mountain Mining Company. First, production was changed from part-sand to all sliming. This technological change involved extra crushing facilities and an enlarged secondary thickening circuit. Pulp could then be subjected to increased cyanidation. Second, a successful method of preparing refractory unoxidized (blue) ores for cyanidation was applied, initially in a small pilot mill and then in a full scale blue ore roaster circuit. This circuit added a lateral branch, or spur, to the main flow of material through the mill, though part of it was still housed within an extension of the main building. Discussion of these changes can be divided into the blue ore system itself and the various other changes to the mill, most of which took place in the early part of the Bald Mountain Company's ownership, from 1934 to 1940.
A magnetic head pulley manufactured by the Ding's Magnetic Separator Co. of Milwaukee was installed at the mill end of the main conveyor in 1938. This device consisted of a drum, measuring 24½ inches wide and 24 inches in diameter, that acted as the end winding drum at the head of the conveyor. It was electro- magnetically charged by a motor-generator set driven by a 3 HP, 1800 RPM motor connected by chain and sprocket drive. Ore passed over the head pulley and ferrous materials adhered to the belt as it came into contact with the magnetic drum. The ferrous materials were released, dropping onto a chute and collection bin, only when the belt passed out of the magnetic field.
A 3 feet wide, 6 feet long Symons horizontal single deck vibrating screen with ½ inch space mesh was placed at the top of the conveyor from the ore, also in 1938. Larger pieces of ore were moved off the screen by its rocking motion and fell down a chute into a newly installed secondary coarse crusher situated on the ground floor below, while smaller particles fell through the screen and were taken, via a chute, to a reversible belt conveyor. The secondary coarse crusher installed that same year was gyratory type 3 feet in diameter, manufactured by Symons. It was driven by a 60 HP motor and set on massive concrete foundations above a 24 inch belt conveyor taking the crushed rock discharge to the foot of a bucket elevator.
Smaller grade ore on the reversible belt conveyor could go in two directions. One deposited ore into the dryer. The other direction took small pieces of ore to the base of the 24 inch belt conveyor below the cone crusher, thus carrying the material underneath the crusher and by-passing it. At the end of this conveyor a 42 feet high bucket elevator was installed in 1938. It was manufactured by Stephens-Adamson of Aurora, Illinois, carried 25 tons per hour at a rate of 144 feet per minute, and was powered by a 5 HP, 1730 RPM motor via chain and sprocket." At the top of the elevator, an 18 inch belt conveyor led to the 365-ton ore bin.
The installation of this secondary coarse crushing facility and the roaster circuit prompted the addition, in December 1939, of a dust collecting system. A 28 foot, 6 inch high dust collecting tower with a maximum diameter of 9 feet, 7 inches and a conical bottom section was placed against the east wall of the 365-ton crushed ore bin. Dust was collected from the Stephens- Adams bucket elevator, secondary coarse crusher and adjacent belt conveyor plus the mill, screen, cooler and inclined bucket elevator used in the blue ore system. Metal hoods secured these machines as dust was drawn into the collector through overhead steel pipes by a 4 foot Norblo fan housed on top of the collector tower. The fan was driven by a 25 HP, 1800 RPM motor connected by rope drive. Approximately 80% of the dust was filtered out before the air was vented through a four foot stack.
In 1946 a new primary coarse crusher was installed. Manufactured by the Traylor company, the new jaw crusher was 15 by 24 inches in diameter and powered by a 50 HP motor. Jaw crushers worked by the action of a hinged crushing surface pressing ore together. The distance between the two determined the grade of crusher rock produced, in this case 1 3/8 inch. The main conveyor from the primary coarse crusher and ore bins was changed from 18 to 24 inches in width and shortened as housing for new crushing and screening equipment brought the top level of the mill building further back into the hillside.
On the milling floor, major changes took place in 1935. Another ball mill/classifier circuit was installed and the remaining Chilian mill removed. The tertiary rake and Weigand classifiers and their platforms were also removed. Primary mill number one (the new ball mill classifier circuit) and primary mill number two (the existing one) both fed their slime output to the secondary mill circuit (the third in line) for further milling and classification while keeping their sands in closed circuit until fine enough for discharge. Primary mill number one was a Stearns Rogers machine, 6 feet in length with a cylinder 6 feet in diameter. It rotated at 24 RPM by belt drive from a 125 HP, 880 RPM motor and discharged by the high level method. The attendant Dorr rake classifier was 6 feet wide, 18 feet long and linked to the ball mill by sloping launders at either end. Crushed brown ore was delivered from an 18 inch wide belt conveyor connecting the western of the 365-ton ore bin hoppers while blue ore came from a launder running beneath the mill solution tanks. Both fed into the launder at the sand discharge end of the classifiers. Mill solution entered the circuit at the ball mill feed from the mill solution storage tanks, which had a surge tank added for extra capacity at about the same time.
The classifier in the second primary milling circuit was replaced in 1935 by another Dorr rake machine 18 feet long and 6 feet wide, directly driven by a 3 HP, 860 RPM motor. Slimes from the two primary mills reached the secondary mill circuit in a wood launder, via a new 4 foot by 4 foot surge tank. Also in 1935, the secondary milling circuit benefitted from the replacement of its rake classifier with a Dorr rake and bowl classifier. The bowl classifier was a steel bowl reinforced with a concentric steel band inside the rim, set above the slime discharge end of a standard rake classifier. Pulp was delivered into a feed well around a central drive shaft, powered by a bevel gear system from above, situated in the center of the bowl. This shaft drove rotating arms in the bottom of the bowl, each with a set of small ploughs. These ploughs moved heavier solids (ie: sands) towards a central drain which discharged into the rake classifier trough beneath. Slimes, light enough to be kept in suspension, flowed over the lip of the bowl and were borne away in a launder.
The bowl classifier used at Bald Mountain was 18 feet long, 6 feet wide, and had a bowl 10 feet in diameter in which the arms rotated at 3/4 RPM. Sand deposited in the rake classifier was moved to the top and returned to the ball mill circuit by a screw conveyor in a wood trough. It was then moved to primary thickening by a pair of 4 inch bore pumps. These pumps were manufactured by the Wilfley company and driven by 25 and 15 HP motors.
The new milling floor arrangement enabled more alternate milling sequences to take place and a finer end product to be produced, due to the more discriminating action of the bowl classifier. In practice maintenance, or an easily milled ore sometimes led to the shut down of one primary, which was easily by-passed. The new milling facilities heralded a change to all- slime production and the removal of the sand tanks. However, a reference to changing to full counter current decantation in 1939 implies that at least some sand leaching continued until this date. After the construction of additional secondary thickeners, and the secondary classifiers Wilfley pumps only two sand tanks could possibly have been in use but there are no documents to confirm or deny this.
The operation of the primary thickeners remained much as it was during the Trojan period. Overflow from the primary thickeners could be returned to the primary ball mills or flow down to the gold tanks, where a 4 foot by 4 foot surge tank was added for excess liquid. Another surge tank, measuring 7 feet deep and 7 feet in diameter, was placed on a timber platform near the primary thickeners, where the gravity classifier had been.
This tank received any excess thickener overflow before releasing it to either of the two destinations. Underflow from the primary thickeners flowed to the agitator sequence where it moved east to west.
Three new secondary thickeners, forming a counter current series independent from the first, were fitted in 1936. They were 24 feet wide, 8 feet deep and of common steel tie-bound timber stave construction raised on concrete piers and powered by individual motors. By siting the new series in the western seven bays of the original sand tank floor, the thickeners took the place of sand tanks four, five and six. Pulp leaving the agitator series was divided between the two secondary thickener series, beginning with number one (the "old" series) or number four (the "new" series). The two series operated as mirror images of each other so that thickened underflow from number one was pumped uphill through number two to three, while underflow from number four was pumped via five to number six. Diaphragm pumps of 4 inch bore were installed in the new series to move pulp uphill. Once it had reached the end of the respective series, the pulp was allowed to flow back to the Portland filters where it was diluted with water ready for filtration. As the pulp made its way up the secondary systems barren solution was brought into contact with it while coming down the series. Barren solution from the barren solution tank was introduced to secondary thickeners numbers three and six and flowed down hill to numbers one and four respectively. From there it overflowed into the mill solution sump.
In the fall of 1939, a seventh secondary thickener, constructed of laminated wood three feet thick, fifty feet in diameter and twelve feet deep, was situated at the western edge of the mill. Adjacent to secondary thickener two, it was located in a semi-free standing 13-sided shed. Drive for the rakes was supplied by a 2 HP, 1750 RPM motor with a speed reducer mounted over the center shaft. Once operation began in 1940, secondary thickener seven received pulp pumped uphill from the top of the two secondary thickener series (thickeners three and six) by a 4 inch bore duplex diaphragm pump. After thickening overflow was returned to the old series at number three while the underflow was diluted with water and pumped to tailings by a 3 inch bore Wilfley.
By 1954, three "fine tuning" adjustments had been made. To limit the amount of agitation the pulp underwent, and therefore the amount of oxygen the precipitation process had to contend with, a bypass was installed so pulp could leave the agitator series at tank number four. At agitator five itself, another by- pass was constructed enabling the whole counter current process to be avoided and pulp to be channeled directly to the seventh secondary thickener. This by-pass was probably used to remove exhausted pulp from the system without the trouble of further thickening. An additional bypass allowed the underflow from secondary thickeners three and six to be returned to agitator five.
The major change in the precipitation section was the removal of both Portland filters and zinc boxes and the installation of new equipment using the Merill-Crowe vacuum process. A pair of vacuum leaf clarifying tanks replaced the filtering action of the Portland and Butters' filters. An oxygen removal stage was added, enabling zinc more effectively to act as a precipitant. The first tank was installed in 1934, measuring 12 feet wide and 8 feet deep, the second, 10 feet wide and 8 feet deep, after 1936. Both tanks were constructed of timber staves bound with steel hoops and contained 17 or 10 filter leaves respectively, 7 feet by 5 feet in dimension. The fabric leaves were reinforced with wood slats, and stretched across a perforated tubular frame. A solution inlet valve kept the level constant, and the leaves entirely submerged to prevent air being drawn in. An Ingersoll Rand vacuum pump created suction in the tubular frames via a receiver tower, pulling solution through the leaves, while the solids were left on the leaves. A second valve controlled solution outflow from the tanks to maintain pressure within the receiver chamber. The solution was broken up into fine streams by wood slats as it fell through the receiver, enabling the air to be removed while the de-oxygenized solution, settling in the bottom, was pumped out.
An emulsion of zinc dust was introduced (at the rate of 0.025 pounds per ton of solution) to a fraction of the liquid diverted from the main flow and then returned. A small belt conveyor supplied zinc dust to a feeder cone that introduced it to the solution while passing through a liquid-sealed pump. Precipitate was retrieved from the now barren solution by pumping it into two tanks containing 60 cylindrical filter bags each, submerged in solution. Precipitate remained in the bags while the barren solution was pumped up to the barren solution tank. The precipitate was removed from the bags and taken for refining.
The mill and barren solution sumps beneath the precipitation floor were removed, probably in 1934 during the alterations to the precipitation machinery, and replaced by a trio of concrete tanks. The easternmost of these, the mill solution sump, received solution directly from the secondary thickener series. Solution was then pumped back to the mill solution tanks by a Triplex pump of 8 ¼ inch bore and 10 inch stroke. The middle sump acted as an emergency overflow from the first and solution from both was removed by another pump. The third sump, beneath the western side of the precipitation equipment, served as a mill drainage sump for material drained from the secondary thickeners and agitators through a system of concrete channels set in the mill floor. A Triplex pump of 7 inch bore and 8 inch stroke removed solution to the mill solution sump. A cyanide feeder tower was constructed above the mill drainage sump. This tower was a simple timber structure with a hinged chute to pour cyanide into the sump below.
In the refinery during this period, the drying oven was a large brick structure with three compartments for trays of precipitate and fire doors at ground level, stoked with wood. The small melting furnace had two gas fired compartments and was of steel, presumably with a firebrick lining. A pouring melting furnace was also used, so bullion could be poured into molds by a gas fired container mounted on trunnions. An additional alteration to Bald Mountain mill was a second dam built in 1934 to retain seepage from the existing one about one mile north of the mill. The water collected there was pumped back to the mill's water tanks for reuse.
Although the higher grades of refractory unoxidized ('blue') ores from the Bald Mountain region had long been transported to specialized custom smelters, much of the lower grade resources were untouched. Many deposits would only yield about 11% of their potential value in gold and silver by the cyanidation process and the blue ores from the Two Johns mine could yield as low as 7% from orthodox grinding and cyanide action. It had long been understood that additional preparatory processes would be required to improve this situation. Experimentation at the South Dakota School of Mines in 1927 encouraged the Bald Mountain Company to institute a pilot roaster project on site in 1935. The pilot mill was constructed to the east of the main mill building, beside the tramway incline for transporting ore to the boiler house. An initial estimate foresaw recovery rates of 85-87% from ore milled to 200 mesh. A trial run from January 23 to March 26, 1935 used both Two Johns blue ore and Portland brown ore to compare the performance of the machinery. First results showed recovery at 73%.
In order to process small amounts of ore under completely controlled conditions, the pilot facility was developed into a scale model of the whole mill process, with the exception of refining. Rather than one downhill linear flow, two parallel flows on three terraced levels were used. The first, housed in the eastern side of the building, contained the crushing and roasting machinery. The roasted ore was then returned to the top of the pilot mill, where it underwent grinding and cyanidation, in imitation of the mill itself. From mine cars, ore was dropped into a bin where it could be released onto an area of sheet metal floor inside the building. From surviving evidence it appears that it was then sorted by hand into one of two chutes. The larger material fell into a jaw crusher and from there to a vibrating screen on the second level. The finer material passed straight down to a roller mill parallel to the screen. All these machines were run by belts from line shafting. A bucket elevator took ore from the screen and rolls. It is unclear what happened to material too big for the screen, but it may have been manually transferred to the rolls. The bucket elevator deposited ore in a timber bin on the third floor where a belt conveyor carried it to the roaster.
Early experiments centered on the inadequacies of the commercially available roasters of the time and for the first test a "blast kiln type furnace" was used. A customized rotary hearth design was constructed for the Bald Mountain Company. This machine consisted of a circular firebrick tunnel within which a steel hearth rotated, driven from beneath. Heat was provided by gas burners set in the walls and ore was agitated to ensure even roasting by sets of rotating blades called "rabbles". One advantage of this design was that the rotary hearth allowed greater control over temperature and length of roasting.
The roasting hearth was 3 feet wide and 32 feet in circumference. The rabbles were driven by 18 meshing cogs arranged around the roaster roof. Two sets of bevel gears, running from a motor, drove the rabbles via these cogs, through the roof, and onto the hearth. Discharged ore left the hearth by a rotating scoop moving it into a screw conveyor which deposited it into a bin or mine cars. At the top of the second part of the mill roasted ore was deposited into a ball mill. From this point most of the machinery no longer survives but it seems that ore pulp flowed past two tanks (possibly cyanide solution) to a pair of steel tanks, linked by launders and served by overhead line shafting. These were probably thickeners. There does not appear to be any sign of agitation machinery but that does not discount its presence. From the tanks ("thickeners") solution probably flowed to the Dorrco rotary vacuum filter that is still extant. Solids fell into a tank built below the roaster and liquid was collected in a small tank. Where and how precipitation took place is unknown.
The construction of a full scale roaster was largely carried out in 1939, though the blue ore preparation facilities included secondary coarse crushing machinery that had been installed prior to that. Quantities of blue ore were put through the mill separate from brown ore in order to isolate them and monitor the processing procedure and its effects. Additional machinery was required to divert ore to the roaster, for preliminary drying, extra crushing, screening and storage. The new roaster circuit was housed within the secondary crushing building behind (south of) the 365-ton crushed ore bin, and partly in an additional building constructed adjacent to it. The roaster itself, and its attendant cooling and storage facilities were set outside the main mill building.
Although a relatively self contained processing arrangement, the roaster circuit forms a branch along which ore was diverted before rejoining the main current of movement within the mill. Processing of blue ore started from the main crude ore bins at the head of the conveyor. As the larger pieces of crushed blue ore passed over the Symons vibrating screen, they were directed via a chute into the same Symons 3 foot gyratory secondary coarse crusher used for brown ore. Smaller particles which passed through the vibrating screen were deposited on a reversible belt conveyor. This belt fed blue ore to a Stearns-Roger cylindrical dryer 4 feet by 20 feet, fitted with feed and discharge hoods. A gas burner at one end heated ore as it was moved along the inside of the cylinder revolving on motor driven rollers. Exhaust heat from the dryer was removed by a steel tubular stack 44 feet high, 24 inches in diameter supported by guy cables, situated outside the mill building and connected by a horizontal flue. The exact nature of the reversible conveyor's connection with the dryer is unclear. It is likely that the conveyor arrangement was altered when the dryer was removed some time after the cessation of blue ore processing in 1942, thus making an assessment based on existing remains difficult.
The method of discharging ore from the dryer is also open to question, although illustrations show a 9 inch cast iron screw conveyor with an 11 foot shaft in position at the discharge end of the dryer. Drive was provided by a chain and sprocket arrangement and the screw operated at 30 RPM. Dry ore, having previously passed the Symons Screen, was passed by a 24 inch belt conveyor directly to the elevator feed.
Large ore pieces, after passing through the secondary coarse crusher, moved on the 24 inch belt conveyor, along with material from the dryer, to the Stephens-Adamson 42 feet bucket elevator. This elevator took the ore to the upper floor where a manually operated gate was used to divert the blue ore straight from the elevator to a newly installed 100-ton steel ore bin. This bin measured 15 by 15 feet with a conical cover, side walls of 3/16 inch and base of 5/16 inch sheet steel. Blue ore was fed from the 100-ton bin by a 18 inch belt conveyor to a 3 feet wide, 8 feet long Symons vibrating screen with an mesh screen, rope driven by a 3 HP, 1800 RPM motor. Material too large to pass through the screen was shaken from the top and conveyed to a new Steams- Roger rod mill. This mill operated like a ball mill but included long steel rods in the milling cylinder. It was 4 foot in diameter, 8 feet in length and powered by a 50 HP, 720 RPM motor situated at the discharge end. The rod mill went into operation in January 1939.
Fine ground material from the rod mill was discharged into a 12 foot long screw conveyor motor 12 inches in diameter, driven by chain and sprocket drive. From this screw conveyor, an inclined bucket elevator was installed to take the ore back up, depositing it in the screen where oversize was retained for further rod milling. Once the ore had reached the required size it left the screen for the roaster building by a 14 inch wide belt conveyor, 48 feet long and driven by a 5 HP, 104 RPM motor. A bucket elevator at the end of this conveyor deposited ore into a 50-ton capacity steel ore bin built alongside the roaster. The 50-ton bin was 12 feet in diameter, 10 feet high and was constructed with 3/16 inch steel sheet sides and 5/16 inch steel sheet base. The top was conical and the discharge door built with a rack and pinion system securing it. A small bucket on chains rotated in front of the bin feed-in point, taking ore samples for assaying.
The octagonal roaster building was constructed on a poured concrete foundation with a floor of compacted soil. Concrete construction was also used in the base of the walls, with steel framing for the upper parts. The roof and upper walls were clad in corrugated sheet metal. The roaster's hearth was basically a large, flat steel ring, 50 feet, 4 inches in diameter outside, 26 feet, 3 inches inside and 12 feet, 1 inch wide with 2 inches of concrete and 4 inches of insulating material on its upper face. The roasting surface totaled 1,400 square feet, although approximately 53 square feet were taken by the charging and unloading mechanisms. The hearth was rotated by an electrically driven gear wheel acting on a loop of heavy chain. This chain connected with lugs built onto rails beneath the hearth. The hearth was also supported by a separate set of rails resting on unpowered rollers beneath it. Power was supplied by a 5 HP, 1730 RPM motor and a complex series of gears.
Ore was deposited evenly at a depth of between 1 inch and 5 inches and heated by natural gas burners set in the walls around the heating part of the circuit. The part closest to the charging and discharging apparatus was unheated, allowing ore to warm and cool during the cycle. Temperature varied with the condition of the ore and its position within the roaster, but a maximum of 1120 degrees Fahrenheit could be achieved. As the hearth rotated the ore was stirred in order to make the roasting more even. The stirring was done by sets of plows on rotating arms positioned just above the hearth. There were 13 plow and drive shaft assemblies, called rabbles, with up to 8 plow blades each (less in the heated part of the circuit), rotating at 10 to 40 RPM. They were placed symmetrically around the hearth circle and driven from overhead by solid shafts and bevel gears from a 15 HP, 1500 RPM motor filling the 14th place in the circle.
The hearth was enclosed in a 9 inch thick firebrick tunnel (with 2 ½ inches of additional insulation on the walls and 3 inches of plastic asbestos on the roof). The tunnel was built over the hearth from foundations on either side, leaving the hearth free to rotate independently within it. Fumes and heat could be drawn off through a 150 feet tubular steel stack outside the roaster building built by Stearns-Roger. The stack had a diameter of 42 inches, weighed approximately 14-tons and was constructed from 1/4 inch steel plate. It was erected on January 23, 1940, replacing an earlier stack 48 inches in diameter and 150 feet high that stood nearby.
A Baker Cooler was installed in September 1938 to cool the ore that was taken from the roaster by an eccentrically rotating scoop feeding a screw conveyor. The cooler consisted of a horizontal steel cylinder 5 feet in diameter and 40 feet long with a screw inside. As the cylinder was rotated in a trough of coolant water, ore was moved along inside. Power to turn the cooler was supplied by a 7 ½ HP motor operating at 1740 RPM, protected from the water and fan cooled. The motor drove the cylinder by acting on a large external gear wheel around the circumference of the cylinder which rotated at 20 RPM. Pulp was fed from the cooler to rejoin the main process of the mill operation at the ball mill circuits. Mill solution was introduced to the pulp as it flowed in a trough beneath the mill solution tanks.
Few additions or changes were made to the mill's ancillary facilities during the Bald Mountain Company period. The machine shop was extended to the north with the inclusion of a small furnace building, while the coal depot and garage adjacent to it were demolished. Further to the east of the mill, a hopper was installed linking the road to the tramway and coal was presumably transferred to mine cars at that point. An additional concrete water tank, capable of holding 100,000 gallons, was built on the hillside south of the mill complex, and the electrical sub-station was moved further to the east of the mill when the roaster building was constructed.