Grinding the ore to minus 100-mesh gave satisfactory liberation of the minerals. Standard flotation procedure with potassium ethyl xanthate, cresylic acid, and lime produced good-grade copper concentrates with over 85 percent recovery of copper and about 60 percent recoveries of gold and silver. This stop was employed in two methods of treatment developed during laboratory testing.

Following flotation of copper in the first method of treatment, the combined flotation cleaner and rougher tailings, amounting to 96 percent of the ore, were cyanided for 4 hours, with resulting further recovery of about 30 percent of the gold and silver, giving total recoveries of about 90 percent of the gold and silver. Cyanide consumption during this treatment was 5 pounds per ton of ore.

In the second method of treatment, copper was recovered by flotation in the same manner. This step was followed by flotation of “scavenger” concentrates amounting to about 35 percent of the original ore. More xanthate was used for this purpose. The “scavenger” concentrates were reground to pass a 400-mesh screen, washed, and then cyanided for 4 hours. This, treatment yielded substantially the same over-all recoveries of gold and silver as the first method. Cyanide consumption was 2.5 pounds per ton of ore.

The advantages of this method of treatment are the reduction in amount of material to to cyanided from about 96 percent of the ore to about 35 percent together with reduction in cyanide consumption. Offsetting these advantages arc the cost of additional flotation treatment and cost of regrinding and was washing the “scavenger” concentrates.

Economic considerations together with plant capacity and equipment must determine which of these treatment methods is preferable.

The test work had demonstrated that the present ore contains gold and sliver associated only in part with the copper sulfides, and that the remaining precious metal values occur in pyrite. Also, part of the pyrite to is nearly barren of precious-metal values. “Scavenger” flotation of only the readily floated pyrite recover satisfactory amounts of gold and silver.

Reduction of agitation time during cyanidation to 4 hours showed satisfactory extraction of gold and silver, indicating that much of the excessive cyanide consumption was due to oxidation of the sulfides while being treated.

Cyanide consumption was reduced to about 5 pounds per ton of ore during treatment of the entire copper flotation tailings and to 2.5 pounds per ton of ore during treatment of the reground and washed “scavenger” concentrates.

Many western ores containing gold, silver, and copper can be mined and concentrated at a profit only when the recovery of these metals is high and treatment costs are low. Usually, ores containing easily dissolved copper minerals, or copper minerals that become soluble to some extent through oxidation during treatment, present a real problem in cyanidation. Copper cyanide compounds are formed that tend to precipitate and that cannot yield regenerated cyanide and metallic copper on contact with zinc. This results not only in some loss of copper through solution in cyanide, but in excessive consumption of cyanide above the amount required for the extraction of gold and silver. Any means therefore of minimizing the loss of cyanide while at the same time effecting high recoveries of the several values contained in the ore is of wide interest.

A sample of typical low-grade ore of gold, silver, and copper, containing copper-sulfide minerals, van obtained from the MacLaren Gold Mines Co., Cooke, Park County, Mont., for laboratory investigation by flotation and cyanidation.

The sample contained about 50 percent pyrite, fine-trained and disseminated in a siliceous gangue. Copper was present as fine-grained chalcopyrite, and some covellite coatings were noted on both copper and iron pyrite grains. Analyses showed:

Gold, ounce per ton…………………………………………………0.23
Silver, ounce per ton………………………………………………..0.68
Copper, percent………………………………………………………..1.10
Sulfur percent……………………………………………………………22.0
Arsenic, percent…………………………………………………less than 0.02

Satisfactory liberation of the minerals was obtained by grinding to minus 100-mesh.

Two methods of treatment were developed, either of which gave good recovery of copper, gold, and silver with comparatively low consumption of cyanide. It was found that cyanide consumption could be minimized by shortening the period of citation to 4 hours. Apparently, longer agitation resulted in increased consumption of cyanide by the products of oxidation of the sulfide minerals present in the pulp. The short agitation period was long enough to permit satisfactory extraction of gold.

By the first method of treatment, copper was concentrated by flotation according to standard procedure. In a typical test, 86.4 percent of the copper was recovered in concentrated containing 21.51 percent copper, 3.29 ounces gold, and 9.40 ounces silver per ton; 62.7 percent of the gold and 61.2 percent of the silver also were recovered in these concentrates. The copper flotation tailings were cyanided for 4 hours; 28.9 percent of the total gold and 35.8 percent, of the total silver were extracted, giving overall recoveries of 91.2 percent of the Gold and 97.0 percent of the silver. Cyanide consumption was 4.7 pounds per ton of original ore. Lime consumption was 1.4 pounds per ton for cyanidation, in addition to 2.5 pounds required in the copper flotation circuit.

The second method of treatment consisted of production of cooper concentrates in similar manner, followed by flotation of part of the pyrite from the copper tailings amounting to only one-third of the original weight of ore. These secondary concentrates were reground, washed to facilitate cyanidation, and agitated with cyanide for 4 hours. Total recoveries of gold and silver were 90.6 percent and 93.6 percent, respectively. Cyanide consumption was 2.9 pounds per ton of ore. Lime consumption during cyanidation was 0.6 pound. This method of treatment appears to be the more economical as gold recovery was nearly equal to that of the first method described, a much smaller portion of the original ore was treated by cyanidation, and cyanide consumption was considerably lower. These advantages were offset to some extent by the additional flotation treatment required along with the necessary re grinding, and washing of the secondary concentrates.

In the correspondence received from the MacLaren Gold Mines Co. concerning this work, previous testing done at the Reno Station under Project 270 was mentioned. A search of the station files revealed a report dated October 27, 1937, describing tests made on a similar but higher-grade ore from the same property. The results of this previous testing are of interest, and are included for comparison with the results obtained in the present work. These previous results were summarized in the report of Project 270, as follows

1. Over 90 percent of the gold and over 80 percent of the silver were recovered in a low-grade concentrate by bulk flotation of the minus 100-mesh product.
2. About 80 percent of the gold and 60 percent of the silver were recovered with over 85 percent of the copper in a relatively high-grade concentrate by selective flotation of minus 100-mesh ore.
3. Cyanidation of the ore or tailing resulting from selective flotation did not appear practical owing to high consumption of cyanide.

The bulk flotation test referred, to under (1) showed production of concentrates carrying 0.80 ounce gold and 1.88 ounces silver per ton in 44 percent of the original weight of ore. Copper content was not given.

In the selective flotation test (2), the concentration contained 3.02 ounces gold and 6.00 ounces silver per ton and 17.00 percent copper in 9.1 percent of the original weight of ore. The reagents need were in pounds per ton of ore: lime, 8.0, and sodium cyanide, 0.6, ground with the ore; and potassium ethyl xanthatc Z-3, 0.05, with cresylic acid, 0.15 for flotation. The pH of the pulp was 9.2.

One cyanidation test referred to in (3) was made on ore ground to minus 100-mesh and agitated for 68 hours, during which time 82.1 per cent of the gold was extracted with consumption of 33 pounds of cyanide and 17.6 pounds of lime per ton of ore. Another tent was made on tailings from selective flotation of copper, in which the pulp was agitated for 72 hours, resulting in extraction of additional gold and silver to make totals in both flotation concentrates and cyanide solution of 95.9 percent of the gold and 80.1 percent of the silver. Cyanide consumption was 5.4 pounds per ton of original ore, and 18.3 pounds of lime were used in addition to the amount used in flotation. The cyanide residue contained 0.02 ounce gold and 0.26 ounce silver per ton. The flotation tailings used in this latter test were not from the selective flotation test described in (2) but from a similar test in which concentrator carrying only 9.89 percent copper were made with higher recovery of copper and about the same recovery of gold. The flotation tailings used in this test contained only 0.11 percent copper, as against 0.27 percent copper in the tailings when flotation concentrates carrying 17.00 percent copper were made.

Preliminary Cyanidation Tests

Several preliminary cyanidation tests were made during the present investigation to determine if the consumption of cyanide could be reduced to a reasonable amount and to discover the cause of the previous excessive consumption. Results of these tents were unsatisfactory, but it was found that 90 percent of the gold could be extracted by cyaniding the ore for only 16 hours after grinding to minus 100-mesh. Cyanide consumption was 12.7 pounds per ton of ore. Longer periods of cyanidation did not give lower-grade residues and consumed more cyanide. The cyanide consumption was caused by continued oxidation of the sulfides in the ore. Replacement of pregnant solution with fresh solution at intervals during treatment did not help to decrease cyanide consumption. Although superior to results of a test mentioned in the report of Project 970, excessive consumption of cyanide still occurred. As analyses of test products showed considerable solution of copper during cyanidation. It seemed evident that any improvement in cyanidation treatment would involve removal of the copper minerals to as great an extent as possible by flotation before cyanidation.

Flotation of Copper

Tests to study the flotation of copper from the ore were made. Standard flotation procedure with lime to depress the pyrite, potassium ethyl xanthate as collector reagent, and cresylic acid for frother gave good results. No cyanide was used in flotation, although such use had been reported under Project 270 and was presumably an additional depressant for pyrite. However, the use of 0.25 pound of sodium silicate per ton of ore as a conditioning reagent in the rougher flotation circuit appeared to give cleaner froths. Very little xanthate was required, and by adding this reagent in increments of 0.01 to 0.03 pound per ton of ore, good selection of copper mineral was obtained. A limit of about 0.15 percent copper in the flotation tailings was noted regardless of rougher concentrate grade, this amount of copper being lost when rougher concentrates contained 4.6 to as much as 16.2 percent copper. Apparently the use of more than 0.04 pound of xanthate per ton of ore resulted only in flotation of more pyrite. Copper recovery in rougher concentrates was approximately 90 percent. Simple cleaning in one stage without additional reagents produced concentrates carrying 19.8 to 25.3 percent copper, representing from 82.7 to 81.6 percent recovery. When rougher concentrates from several batch tests were combined for cleaning in one stage, better results were obtained for example, concentrates contained 21.51 percent copper, with 86.4 percent recovery. Two-stage cleaning of the copper rougher concentrates without additional reagents produced final concentrates containing 27.93 percent copper, representing 80.6 percent recovery. By close regulation of the addition of xanthate in plant operation, together with normal countercurrent cleaning in the copper flotation circuit it should not be difficult to produce high-grade copper concentrates with at least 85 percent-recovery from this ore. The copper concentrates; would amount to about 4 percent of the original weight of ore.

Flotation of Gold with Copper

The copper concentrates mentioned in the preceding paragraph that contained 21.51 percent copper and represented 86.4 percent recovery were typical of those obtained in several similar tests in which the copper rougher concentrates were cleaned once. These concentrates carried 3.29 ounces of gold per ton and 9.40 ounces of silver, representing 62.7 percent recovery of gold and 61.2 percent recovery of silver. It was reported wider Project 270 that when 86.3 percent of the copper was recovered in concentrates carrying 17.00 percent copper, 81.1 percent of the gold and 58.2 percent of the silver also were recovered, the concentrates assaying 1.02 ounce a gold and 6.00 ounces silver per ton. It was concluded at that time that the highest net return per ton of ore treated probably can to realized by treatment of this ore by selective flotation for production of a copper-gold concentrate for marketing at a smelter.

Although in the tests made on the present ore sample a similar recovery of copper was made, it was evident that a considerable portion of the gold did not occur with the copper minerals. It was observed in the present work that whenever a low-grade copper rougher concentrate was produced, gold recovery increased but the material mostly pyrite, that was dropped in cleaning the rougher concentrate-carried considerable gold and little copper. This was believed to indicate that although more than half of the gold probably was contained in the copper mineral, the remainder was contained in the portion of the pyrite that floated quite readily. Consequently, tests were made “to produce two concentrates , the first carrying as much copper as possible while maintaining a good grade of copper, and the second to recover as much of the remaining gold as possible without floating all the pyrite.

“Scavenger” Flotation of Cold

In normal plant operation of the copper flotation circuit, the material produced as cleaner tailings in laboratory tests would be retained in the circuit during countercurrent cleaning and would continually be distributed between the final cleaner concentrates and the copper circuit tailings.

These cleaner tailings consisted for the most part of the more readily floatable pyrite and carried considerable gold but little copper, as previously mentioned.

By adding 0.04 to 0.08 pound of xanthate and no additional frother reagent to the copper circuit tailings, a “scavenger” concentrate was produced as a dull brassy froth. When more xanthate was added, the froth took on a brighter silvery appearance caused by the flotation of the remaining pyrite. This latter pyrite contained very little gold, and after several tests had been made to study optimum conditions only enough xanthate was used to float the dull pyritic mineral. The “scavenger” concentrate thus made amounted to about one-third of the weight of the ore. As about half of the original ore was pyrite, it would appear that at least one-third of the total pyrite was nearly barren of gold. About 40 percent of the total gold was contained in the scavenger concentrates when high-grade copper concentrates were made by two-stage cleaning, and about 30 percent when copper concentrator carrying about 21 percent copper were made. The gold content of the “scavenger” concentrates varied from 0.25 to 0.12 ounce per ton of concentrates and the copper content from 0.4 to 0.1 percent. In one test, an attempt was made to clean the scavenger concentrates, and a small amount of cleaner concentrates containing 2.25 ounce gold and 4.50 ounces silver per ton with percent copper was produced. However, the cleaner tailings from this operation still contained about half the total gold in the original “scavenger” rougher concentrates. It was also attempted to produce a smaller amount of “scavenger” concentrates by using less xanthate. This effort resulted in production of primary and secondary concentrates, the latter containing 0.08 ounce gold per ten which, was too much to discard. It is possible that by very close control of reagents in plant operation, a smaller amount of higher-grade “scavenger” concentrates could be produced without materially sacrificing gold recovery; say, 25 percent of the original weight of ore instead of 30 to 35 percent, as done in the laboratory tests.

The “scavenger” concentrates produced in a typical laboratory test contained 0.17 ounce, gold per ton, 0.42 ounce silver, and 0.23 percent copper, representing 29.4 percent of the total gold in the ore, 32,0 percent of the silver, and 8.1 percent of the copper in 35.9 percent of the original weight of ore.

The corresponding copper concentrates in the same test carried 20.57 percent copper, 3.13 ounces gold, and 9.06 ounces silver per ton, representing 85.4 percent of the total copper in the ore, 64.7 percent of the gold, and 64.0 percent of the silver in 4.2 percent of the original weight of ore.

The final flotation tailings contained 0.11 percent copper, 0.03 ounce gold, and 0.04 ounce silver per ton, representing losses of 6.5 percent of the total copper in the ore, 5.9 percent of the gold, and 4.0 percent of the silver.

Cyanidation of Fl0tation Products

Cyanidation of Copper Flotation Tailing

As described under “Flotation of Copper”, concentrates containing percent of the copper, 62.7 percent of the gold, and 61.2 percent of the silver wore made by 1-stage cleaning. The cleaner tailing from this operation were then returned to the rougher tails. This combined material contained 0.15 percent copper, 0.085 ounce gold, and 0.26 ounce silver per ton and comprised 95.8 percent of the original weight of, ore. This pulp had a pH of 7.2 and contained about 20 percent solids. It was settled and decanted, corresponding to the recovery of water from the flotation tailings, which is normal practice in many plants. The thickened pulp was then diluted with fresh water, a step corresponding to the re-use of barren solution in standard cyanidation practice. Lime and cyanide were added, and the pulp was agitated for 4 hours. The pregnant solution was recovered, and the pulp washed. Both pregnant and wash solutions were assayed for gold, silver, and copper, and were also tested for lime and cyanide consumption. The washed residue was assayed for gold, silver, and copper. The assays showed that an additional 28.5 percent of the gold and 36.8 percent of the silver in the original ore had been extracted by cyanidation, bringing total gold recovery to 91.2 percent and silver to 97.0 percent. Of the copper in the cyanide feed, about 40 percent had been dissolved during cyanidation, leaving 0.09 percent copper in the residue. Consumption of cyanide in this test was 4.7 to 5.5 pounds per ton of original ore, and the lime consumption was 1.4 pounds in addition to the pounds used in the flotation circuit. In similar tests, cyanide, consumptions of 4.7 to 5.5 pounds per ton of ore were obtained, so it is probable that in plant practice about 9 pounds of cyanide would be required. This consumption of cyanide is still high but would show a profit under usual plant – operating conditions while treating the type of ore tested, in which a considerable portion of the gold was not recovered, with the copper during flotation of the latter metal.

Tests also were made to reduce the time of agitation with cyanide to less than 4 hours, but results were unsatisfactory.

Cyanidation of ”Scavenger” Concentrates

In the manner as described under “Flotation of Copper” concentrates containing 85.4 percent of the copper, 64.7 percent of the gold, and 64.0 percent of the silver were made by 4-stage cleaning. The rougher tailings from this operation were treated to produce “scavenger” concentration as described under “Scavenger Flotation of Gold.” These concentrates were combined with the cleaner tailing from the copper flotation, the combined product containing 29.4 percent of the gold in the original ore, 32.0 percent of the silver, and 0.1 percent of the copper in 39.9 percent of the weight of the original ore. This product carried 0.17 ounce gold per ton, 0.42 ounce silver, and 0.23 percent copper. The final flotation tailings assayed 0.03 ounce gold per ton, 0.04 ounce silver, and 0.11 percent copper.

The combined “scavenger” concentrates and copper cleaner tailings were dewatered and ground in a laboratory pebble mill to pass a 400-mesh screen. Apparently, considerable oxidation occurred during grinding, as the pulp had a pH of 5.0 when discharged from the mill. The ground pulp was diluted during removal from the pebble mill and washed by decantation. Fresh water was used for cyanidation, corresponding to the use of barren solution in plant practice. Lime and cyanide were added, and the pulp was agitated for 4 hours. 88 percent of the gold and 92 percent of the silver in the pulp were extracted, corresponding to 25.9 percent of the total cold, in the ore and 29.6 percent of the total silver. These extractions, together with the precious-metal values recovered in the copper flotation concentrates, indicated over-all recoveries of 90.6 percent of the total gold and 93.6 percent of the total silver. Cyanide consumption during treatment of the “scavenger” concentrates was 2.5 pounds per ton of original ore and 0.6 pound lime.

The cyanide residue assayed 0.02 ounce gold per ton and 0.04 ounce silver and carried 0.05 percent copper. Apparently, most of the cyanide consumption during this treatment was due to solution of copper from the reground “scavenger” concentrates. Washing the product after grinding removed most of the iron arid copper salts formed by oxidation during grinding which otherwise would also have consumed cyanide.