The mill receives 200 tons per clay of crude mine ore. After being crushed to 2-inch size, this ore is passed over a picking-belt, where one ton of high-grade ore and four tons of waste are removed each day. The remaining 195 tons of second-class ore is crushed in stamp-batteries to pass a 4-mesh screen. Lime-water, in the proportion 7 of water to 1 of ore, is added in the battery. The pulp from these is classified roughly, the coarse sand being ground in a Hardinge mill to pass a 20-mesh screen. The pulp is again classified roughly into four sizes of sand and one size of slime. The sand is concentrated on Wilfley tables and the slime (after being settled to 7:1 in cone-bottom tanks) is concentrated on Deister tables.
The slime-tailing, from the Deister tables, is re-concentrated on vanners. The tailing from the vanners settles to 3½ tons of water per ton of slime; the water being further reduced to ¾ : 1 in a vacuum- filter. The filter-cake is washed with weak barren solution before being sent to the cyanide plant.
The sand-tailing from the Wilfley tables of the stamp-mill is classified carefully in mechanical classifiers; the slime under-size joins the slime-tailing from the re-concentrating vanners, and the sand (after the addition of cyanide solution) enters the tube-mill circuit, where it is joined by 50 tons per day of dump-tailing. All tube- milling is done in cyanide solution. After passing through the tube- mills, the combined current and dump sands are re-concentrated on Wilfley tables. The tailing from these tables is classified, the coarse sand re-entering the tube-mill circuit and the slimed sand being thickened in three 24-ft. Dorr vats before entering the cyanide plant.
The recovery therefore is: gold, 65; silver, 48; copper, 35; and lead, 45%.
NECESSITY FOR BETTER CONCENTRATION
The above data show that a little more than half the gold is being recovered in concentration, and that the recovery on silver, lead, and copper is less than half the contents in the original ore.
Tests indicate that more than 90% of the metal in the original ore occurs in the form of sulphides. Hence half of the metallic sulphides of the original ore enters the cyanide plant. This is undesirable for the following reasons:
1. The extraction of silver from sulphide metals is poor in the cyanide plant; the concentrate produced from panning current residue assays between 50 and 100 oz. silver per ton, and the tailing from panning invariably assays below 2 oz., even when the residue assays as high as 5 oz.; showing that the poor extraction is due to undissolved silver in the sulphides.
2. The presence of metallic sulphides increases the cyanide consumption. The chemical consumption of cyanide is reduced from 4 to 1 lb. per ton when the metallic sulphides are removed from the heading to the cyanide plant. The present excessive cyanide consumption is due almost entirely to the solution of copper from the ore.
3. The presence of copper and zinc sulphides in the cyanide pulp fouls the solution, thus decreasing the extraction from the rest of the ore.
The possibility of improving results by better concentration of the ore has long been recognized. For this reason, arrangements were made for re-concentrating. Both arrangements have effected a reduction of assay-value in the final residue and a corresponding increase of profit.
METHODS FOR IMPROVING PRESENT CONCENTRATION.
Lately, extensive tests have been made to determine the possibility of still closer concentration. Careful panning reduces the average feed to the cyanide plant from 18 oz. silver to 10 oz. per ton.
Canvas tables give slightly poorer results. A full-sized canvas table, treating tailing from the Deister tables (assaying 20 oz. per ton) produced 15 oz. tailing—an extraction of 25%. Further tests along this line were discontinued on account of securing much better results from laboratory flotation tests.
LABORATORY FLOTATION TESTS
All flotation tests, made in the laboratory, were run in separatory funnels. The general procedure in the tests was to mix 100 grams of minus 200-mesh ore with water in proportion of four of water to one of ore. Suitable amounts of oil were then added and the mixture shaken violently. After allowing the pulp to settle for a few moments, the bottom cock of the funnel was opened and the tailing run into a second separatory funnel for another flotation treatment; the cock being closed before the froth began to run out. This process was repeated, on the tailing, from five to seven times. Several hundred tests have been run, all possible variations of conditions being tried. The results of the tests led to the following conclusions:
- All ores from the mine may be treated by flotation. Semi-oxidised ore from one level yields a tailing assaying 10 oz. silver per ton, while the oxidized ore from another level gives a tailing containing only 2 oz. per ton. The tailing from average ore, when conditions for flotation are right, is 4.5 oz. silver per ton.
- The grade of tailing appears to be independent of whether the original ore is treated by flotation, or whether wet concentration precedes flotation.
- The alkalinity during flotation must be between 0.01 and 0.05 lb. per ton of solution. The best results are secured when the alkalinity is 0.025 lb. When the alkalinity is too low, the extraction is poor although the concentrate is clean. When the alkalinity is too high, both the extraction and grade of concentrate are poor. When the alkalinity is right (0.025 lb.) both the extraction and grade of concentrate are best. The maintenance of proper alkalinity will require the most care of anything in the plant; although it will not be more difficult than the maintenance of proper cyanide strength in the cyanide plant.
- The dilution may range between 3½:1 and 7:1; with the best results, on average ore, between 4:1 and 6:1. When the pulp is sandy a low dilution is best: pure sand, ground to 200-mesh, requires a dilution of 1:1. Average slime, like the Deister feed, on the other hand, requires a dilution of 8:1. Good extraction may be secured on either sand or slime, provided approximately the proper dilutions are secured in each case. Proper dilutions will be easy to maintain in the plant, for the range for the best work is comparatively large; and when either an excess of sand or an excess of slime occurs in the ore, the proper dilution will automatically adjust itself; for the sand of itself will settle to a thick pulp, while the slime will not settle well, but will remain thin.
- The temperature is not a matter of vital interest. The extraction is slightly better and the grade of concentrate considerably higher when the temperature is over 100°F., but good results have been secured with the temperature as low as 40°F. It will not be necessary to arrange for heating the pulp, especially at the start.
- Fine grinding is necessary for good results in flotation. When the mill-heading was crushed to 60-mesh, the tailing from flotation assayed 0.08 oz. gold, and 11 oz. silver; when the same ore was crushed to 100-mesh, the tailing assayed 0.04 oz. gold and 5 oz. silver; and when the crushing was carried to 200-mesh, the tailing assayed 0.02 oz. gold and 3.75 oz. silver.The question of the kind of crushing mechanism best adapted to preparing ore for flotation is vital; at present the tube-mill, ball-mill, and disc-crusher hold the field.
- The best flotation agents, so far tested, are pine-oils. Low- grade pine-oil gave as good results as the higher-grade varieties.
S. S. pine-oil, of the General Naval Stores Co. (cost 26c. per gal., f.o.b. factory) has given exceptionally good results. For the best work in flotation it is necessary to have this oil present to the extent of 0.6 lb. per ton of ore. In actual plant-practice, where the water is returned again and again to the top of the mill, the consumption of oil will probably be about ¼lb. per ton of ore. This oil will cost, delivered, 8c. per pound.
Pine-tar oil is much cheaper. It gives good extraction, but the grade of concentrate is low. Cresylic acid, when used with pine-oils, increases the extraction about ¼ oz. silver per ton. This hardly pays for its use.
- In the laboratory tests the grade of concentrate was low, averaging 200 oz. silver per ton. This concentrate could be raised to 1100 oz. by re-treating the concentrate by flotation.
- Cyanide tests, run on tailing from the flotation tests, produced residues assaying less than 1 oz. silver per ton, with a cyanide consumption of less than 1 lb. per ton.
- The dump-tailing cannot be easily treated by flotation. When the methods of flotation that are applicable to mine ore are applied to the pump-tailing, the results are nil. Furthermore, when the water that has been in contact with the pump-tailing is used for diluting mine-ore, the mine-ore cannot be treated advantageously by flotation. Experiments show that both these effects are due to the presence of soluble sulphates (chiefly those of magnesium and calcium) in the pump-tailing. The injurious effect of magnesium sulphate can be overcome largely by an excess of oil. No method of overcoming the injurious effects of calcium sulphate has yet been discovered in the tests.
When the dump-tailing is washed in fresh water half a dozen times, before being treated by flotation, the results of flotation are as satisfactory as is the case with mine-ore. However, a plant for washing the dump-tailing would be more expensive than the small tonnage of this material warrants, and the operation of such a plant would necessitate the waste of more water than is available. Some other method of rendering the dump-tailing susceptible to flotation may be devised; but the small tonnage does not warrant any extended investigation. The best thing to do, especially at the start, is to send the dump-tailing direct to the cyanide plant (after concentrating the ground sand on Wilfleys) as at present.
- As a result of the laboratory experiment, it was decided that full-sized tests should be conducted in the plant on run-of- mine ore.
For this purpose, there were set aside for the flotation circuit: one battery of five stamps, two Wilfley tables, one classifier, one tube-mill, one 24-ft. settler, and one pump for returning the water from the settling-tank to the head of the mill.
A flotation-cell of the pneumatic type was first tried. When treating 20-oz. heading this machine produced a 290-oz. concentrate and a 15.3-oz. tailing. This was far from satisfactory.
Another machine consisted of a series of mechanical-agitation chambers, alternating with a series of settling-chambers. From the start, this machine has given excellent results. In spite of many mechanical difficulties, and trouble with inexperienced operators, the tailing from the plant has averaged but little above 5 oz. silver per ton, and the concentrate has averaged above 600 oz., without re-concentration.
The chief weaknesses of mechanical agitation, as ascertained in this mill, are as follows:
- The complex system of shafts and counter-shaft, with the corresponding drives, bearings, etc.
- The difficulty of adjustment; any slight change in feed necessitating a change in the valves of each chamber.
- The difficulty of the passages between chambers becoming clogged.
It has been attempted to evolve a flotation machine to overcome these weaknesses, and at the same time give results as good as the mechanical agitation plant. A small machine (capacity 3 tons per day) has been constructed, and this, after many alterations, has yielded a 3.7-oz. tailing and a 680-oz. concentrate, when treating 10-oz. feed. This machine employs a somewhat novel principle of flotation—that of submerged agitation—the mixture of pulp, oil, and air being violently agitated in a partly closed chamber, under the hydrostatic pressure of several feet of pulp in the settling-chambers above.
In construction, this machine is simpler than the machine using mechanical agitation. It consists essentially of a V-shaped box or trough, divided into compartments by a series of vertical partitions. At the bottom of each partition is an agitation-chamber. Agitation is supplied by a paddle-wheel in each chamber. All the paddle-wheels are mounted upon a single horizontal shafting, which passes the entire length of the trough, leaving the end partitions through stuffing-boxes. The pulp enters each agitation-chamber through an opening around the shafting, and leaves the agitation-chamber through an adjustable aperture, at a slight distance from the shafting. The agitator thus acts slightly as a centrifugal pump, overcoming the friction loss in the passage from one compartment to another, and keeping the height of the pulp the same in all the settling-chambers. The adjustment of the aperture is arranged to increase or decrease the centrifugal force. This adjustment occasions much less difficulty than is experienced in mechanical agitation, where the flow from one compartment to another is merely throttled.
Also, in the new plant there are no pipes to become clogged, the passage of pulp from one cell to another being along a rapidly revolving shafting, which keeps all material in suspension. The concentrate overflows from both edges of the trough, thus being removed more promptly than in a plant using mechanical agitation.
Further tests with the small machine are being made, and a larger machine (capacity 40 tons per day) is being constructed, for thoroughly testing the principles involved. The 40-ton machine will be constructed with the idea of using it for re-concentration of the concentrate, should a full-sized flotation plant be installed.
SIMPLE MECHANICAL AGITATION
The machine has now been operating intermittently for a month. During that month it ran six days continuously, treating 25 tons of 29-oz. pulp and producing 6.4-oz. tailing and 600-oz. concentrate. During the first five days of the following month, careful tests were run to compare flotation results with those from current concentrating practice. The following tables give the summary of results from these tests.
Mill-Tests. Flotation v. Present Concentration Practice. Flotation plant takes 25 tons per day of mill heading after being crushed to 20-mesh by stamps, concentrated on Wilfleys, and passed through a tube-mill.
*Throughout this article values are given in U. S. Currency.
During the five days the flotation test was being run, the rest of the mill received 160 tons per day of the same grade of mill-heading. This was treated on Wilfley and Deister tables and the tailing re-concentrated on Wilfleys and vanners.
Labor and repair costs will remain about the same as now. Two high-class operators in the present re-treatment plant will be replaced by three cheap operators in the flotation plant.
Power consumption will be increased about 40 hp. This will cost 5c. per ton.
Oil consumption will be about ¼ lb. per ton of ore. ¼ lb. @ 8c. = 2c. per ton.
The total increase in the cost of concentration will therefore be about 7c. per ton.
The present cyanide consumption per ton of ore is 6 lb., of which 2 lb. is mechanically lost. Small laboratory tests show that the chemical consumption of cyanide when flotation-tailing is treated, is only 1 lb. per ton of ore. This is 3 lb. less than the consumption when current tailing is treated. If this result is sustained in actual plant-practice, the saving in cyanide alone will amount to 3 X 19c. = 57c. per ton of ore.
The present cost of precipitation and melting is 2.56c. per fine ounce.
In present practice 16.6 fine ounces are produced per ton of ore. When flotation tailing is treated, only 3.7 oz. are produced per ton of ore. This means an excess of 12.9 oz. produced in present practice: 129 X 2.56c. = 33c. per ton of ore.
Increased profit per ton from flotation, 200 tons per day at $1.64 = $328 increased profit per day or $9840 increased profit per month.
The above is calculated on the basis of results from a single mill- test of 5 days’ duration. During this interval the heading to the mill and the residues were excessively high; indicating a greater advantage in favor of the flotation plant than is actually warranted.
The estimate of probable profit may be revised roughly by using the metallurgical results of the past two months for the basis of calculations. During two months the heading to the cyanide plant has averaged 17.8 oz. silver, and the residue has averaged 2.75 oz. per ton. The residue from cyanide-flotation tailing would assay 1 oz. per ton. This indicates an increased extraction of 1.75 oz. silver per ton of ore. 1.75 oz. at 41c. = 72c. increased profit per ton.
The indicated decrease in cyanide consumption (as determined solely in the laboratory) is 3 lb. per ton of ore. Reducing this to 2½ X 19c. = 47c. per ton of ore.
The decreased cost of precipitation and melting may be figured as follows::Cost of precipitation and melting, per fine ounce, has been 2.4c. The decreased production of bullion, due to flotation, would be 11 oz. per ton of ore. 11 oz. at 2.4c. = 26.4c. When fixed charges are considered, this should be reduced to 20c. per ton.
When the profit from marketing an increased amount of lead and copper is balanced against the increased loss occasioned by marketing the silver and gold as concentrate instead of bullion, there is a deficit of 17c. per ton of ore.
The average tonnage of mine-ore for the past two months has been 5761 tons. Hence the indicated increase in monthly profit would be 5761 X $1-22 = $7028.42.
INSTALLATION OF FLOTATION PLANT
Should a flotation plant be installed, operations in the stamp-mill will continue as at present; though it may be deemed advisable, after the flotation plant is running smoothly, to eliminate concentration in the stamp-mill, and depend upon the flotation plant for all concentration.
Re-concentration of current tailing on vanners and Wilfleys will be discontinued from the start.
The dump-tailing will be treated as at present, with the exception that this material will enter the plant only in the day-time. One tube-mill, one classifier, one elevator, and the re-concentrating Wilfley tables will be kept separate from this circuit, which will be in cyanide solution. All lime for the cyanide plant will enter this circuit. One of the 24-ft. tanks and one of the pumps must be reserved for the dump-tailing circuit.
All the tube-milling of current sand tailing will be done in mill-water, instead of cyanide solution.
The ground sand, together with the current slime, will be settled in two of the 24-ft. thickening-tanks, and will then enter the flotation plant. From the flotation plant, the tailing will flow to the two 33-ft. thickening-tanks. The thickened pulp from these tanks will be de-watered and washed in the vacuum-filter before entering the cyanide plant.
All lime for the mill-circuit will be added as an emulsion to the flotation-tailing launder, where it will be under direct control of the flotation-operator. The water in the 23-ft. thickening-tanks will contain about 0.4 lb. dissolved lime per ton. This is ample for good settling. The overflow from these tanks will be reduced, by consumption, to about 0.1 lb. per ton. This is sufficient for fair settling in the cone-bottom tanks of the stamp-mill. By the time the pulp reaches the 24-ft. thickening-tanks the lime will be reduced to 0.03 lb. per ton. This low lime will be extremely detrimental to good settling in these tanks.
The matter of supplying proper settling and de-watering facilities will be the most serious and most expensive part of the installation.
The two 24-ft. thickening-tanks, to be used in the flotation-circuit, must be triple-decked. It will also probably be found necessary to double-deck the 32-ft. steel thickening-tank. The work on settling- tanks will cost about $6000.
By increasing the settling capacity, the pulp will probably be settled to a sufficient thickness so that the vacuum-filter will be able to handle the combined sand-slime feed.
It may be found necessary, however, to add another unit to this plant. This will cost $2000.
The flotation plant will consist of two units (each of which will be able to treat the total tonnage of current tailing) and one smaller clean-up machine. The whole plant will cost about $3000. A filter- press for handling the concentrate will cost $2000. Tanks, air-lifts, launders, and buildings will cost $2000. Thus the whole installation will cost $15,000, or $20,000 at the most. The addition of the flotation plant, for treating current tailing, will increase the profit about $7000 per month. Practically all ore from the mine may be treated by flotation.