The Munktell Chlorination Process
This method was devised by W. Munktell, who seems to have worked it out without having visited either the vat or the barrel chlorination works already established in other parts of the world. The process enjoys the distinction, according to the published accounts, of having been worked at a profit. It was first used at the Fahlun Copper Works, Sweden, and was afterwards introduced into Hungary, where it was worked at Brade and at Boitzas. The process as worked at Fahlun may be briefly described as follows:—The ore is roasted at a low temperature with the view of obtaining the copper in the form of sulphates. If silver is present, salt is added in the roasting furnace. The calcined ore is then placed in false-bottomed, wooden vats and leached with hot water, followed, if necessary, by hot, dilute sulphuric or hydrochloric acids, by which all the copper and much of the iron are removed from the ore. The solutions thus obtained are run through tanks containing scrap iron, by which the copper and any silver in solution are precipitated. The solution of ferrous sulphate may be saved and used subsequently to precipitate the gold. The residues in the tubs are now in a condition to yield up their gold readily, and are accordingly treated by a solution of 0.6 to 0.7 per cent, of chloride of lime (bleaching powder) in water, mixed with an equal volume of dilute hydrochloric acid of specific gravity 1.002 or 1.003, or of dilute sulphuric acid. These solutions are mixed in troughs just before they flow into the ore-vat. Chlorine is slowly generated by the action of the acid on the hypochlorite of lime, and, being stated to be partly present in the nascent state, attacks the gold vigorously in spite of the extreme dilution of the solution, while there is very little smell of the gas above the vats. It would appear, however, that the generation of chlorine would soon be completed, and the dissolving action would then be carried on by an aqueous solution containing about 0.1 per cent, by weight of chlorine, or about 0.3 volume of chlorine gas in one volume of water. The liquid is passed through the ore until gold ceases to be dissolved, after which the tailings are thrown away. The solution is heated to 160° F. by steam, and precipitated by ferrous sulphate. The collection of the gold is expedited and ensured by adding acetate of lead to the solution; by this device lead sulphate is precipitated with the gold, and, in settling to the bottom, carries the particles of precious metal with it. This process was in continuous operation at the Fahlun Copper Works from 1885 to 1888. In that period, 1,500 tons of gold ore and the tailings from 29,000 tons of copper ore were subjected to treatment. It is stated that in the year 1886, the tailings from 14,000 tons of copper ore were treated with the following results:—
The low cost assigned to labour is very remarkable. These tailings had, of course, been previously crushed and roasted before being treated as above.
In the same year 960 tons of gold ore were treated at a cost of 12s. 2¾d. per ton, the ore containing 523.62 grains of gold per ton before treatment, and 6.02 grains per ton after treatment, so that no less than 98.85 per cent, of the gold was extracted.
The following details concerning the practice at Brade, Hungary, may be added. In addition to gold, the ores contain iron pyrites, barytes, zinc-blende, antimonial minerals and argentiferous galena, and also in some cases calcite and carbonate of magnesia. The concentrates, which are subjected to treatment by the Munktell process, contain 0.86 oz. of gold and 4 ozs. of silver per ton, and 36 to 40 per cent, of sulphur.
The oxidising roasting takes twenty-eight hours, after which 5 per cent, of salt is thrown upon the ore and mixed thoroughly with it, and four hours later the charge is drawn from the furnace and allowed to cool slowly in a covered-in brick pit. The loss by volatilisation is, however, considerable, and it is proposed to build seven-storey furnaces. The leaching vats are made of wood, lined with lead, and are 3 metres broad, 5 metres long, and 0.75 metre deep, and hold 10 tons, the charge being 0.5 metre deep. There is a false bottom and filter-bed of quartz, as usual. The ore is leached with five different solutions in succession:
- Warm water, 2,500 gallons being required for the charge : Chlorides of copper and zinc and about 25 per cent, of the silver are removed in solution.
- A solution containing 2 per cent, of hyposulphite of soda, 1,600 gallons being used; this dissolves the rest of the silver.
- Dilute sulphuric acid, 2,700 gallons being used to remove the oxides of iron.
- Weak solutions of bleaching powder and sulphuric acid to dissolve the gold; and
- Cold water to wash the ore. The solutions are all preserved separately.
The leaching takes in all nine or ten days, the chlorination alone occupying three days. The gold and silver are precipitated from their solution by sodium sulphide, and the precipitate is dried, pressed, roasted, and melted down. The expenditure per ton of concentrates is as follows:
The percentage of extraction is not given. Somewhat similar treatment is followed at Bovisa, in N. Italy, where about 25 tons per day of pyritic ore, containing 34 per cent. of sulphur, 10 to 12 per cent, of arsenic, and 0.6 to 0.7 oz. of gold per ton, are roasted dead (the arsenic being recovered) and chlorinated in lead-lined wooden vats holding 10 tons each. Weak solutions of bleaching powder and sulphuric acid are allowed to pass slowly through the ore, and the total time of treatment is three days. From 85 to 87 per cent, of the gold is recovered.
The Cassel-Hinman Bromine Process
The ore at the Nellie Bly Mill, Magnolia, Colorado, was treated in open vats by nascent bromine generated in the interstices of the ore. A mixture of bromide and bromate of soda or lime is prepared by adding bromine to a nearly saturated solution of soda or milk of lime. A solution in water of these salts is said to yield the whole of its bromine in the free state, when treated with an acid. The reactions are expressed thus:
- 6NaOH + 3Br2 = 3NaBr + 3NaBrO + 3H2O
On heating or standing, this becomes
- 6NaOH + 6Br = 5NaBr + NaBrO3 + 3H2O
- 5NaBr + NaBrO3 + 3H2SO4 = 3Na2SO4 + 3Br2 + 3H2O
Under similar circumstances only part of the chlorine would be liberated.
The ore which contained from 10 to 15 dwts. of gold was crushed by rolls and roasted in a Brown Horseshoe Furnace, after which it was fed into open lead-lined vats, provided with the usual filter-bed at bottom. The solution was then run over the ores, and the filtrate containing the gold, bromine, and soluble bromides was run into a closed tank, where chlorine was added to liberate whatever bromine had combined with the bases of the ore. The loss of bromine was stated to be only about ½ lb. per ton of ore treated.
The gold solution, after the bromine had been separated from it by an air current, flowed into a precipitating tank, and was precipitated by sulphuretted hydrogen.
One of the advantages gained by using bromine instead of chlorine is that solutions are obtained much more free from base metals which interfere with precipitation and make the bullion base. However, the process was soon abandoned.
Permanganate Chlorination Process
This process differs from the ordinary treatment of ore in vats by chlorine water only in the use of a different solution. M. Etard originally proposed a solution containing 45 lbs. of strong commercial muriatic acid, and 12 ozs. of crystals of permanganate of potash. Professor Black, of Otago University, as an alternative suggested 12 lbs. of common salt, 14 lbs. of sulphuric acid, and 6 or 7 ozs. of permanganate crystals per 100 gallons of water. These solutions smell slightly of chlorine, but it is claimed that in the absence of gold or other material attacked by chlorine, practically no free chlorine is produced. Both solutions dissolve gold at less than one-tenth the rate of a saturated solution of chlorine in water (see above, p. 12).
The method was tested by the treatment of 2,000 tons of ore at the Bethanga Mill, Victoria, in 1900. The ore consisted mainly of pyrites, mispickel, and chalcopyrite with some blende containing an average of 25 per cent, of sulphur and 9 per cent, of arsenic. It was crushed dry by a ball mill to 20 mesh and roasted in hand-worked reverberatory furnaces, 40 lbs. of salt per ton being added a few minutes before withdrawal. Wooden vats, 5 feet deep, holding 17 tons of roasted ore were used for chlorination. The solution contained 110 lbs. of commercial sulphuric acid, 90 lbs. of common salt, and 12 lbs. of potassium permanganate (per 1,000 gallons ?) The average time of treatment was 140 hours, the tailings being tested by panning, and finally leached with hot water. The ruby colour of the solution was used to gauge its solvent strength. The average contents of the roasted ore during one month were gold 2.313 ozs., silver 1.63 ozs. per ton, and the tailings contained an average of 0.147 oz. gold per ton, or nearly 3 dwts. This corresponds to an extraction of 93.7 per cent, of gold. About 500 tons of roasted ore were treated per month at a cost for chlorination, according to E. Harris, of 5s. 11.6d. per ton (?of the original ore). The cost of roasting was 12s, 7d. per ton. According to D. D. Rosewarne, the cost of chlorination, using permanganate, during four months averaged 11s. 5d. per ton, and using chlorine at the same time the average cost was only 7s. 4d. per ton. On this exhaustive test the permanganate process was abandoned.
Chlorination
This is the largest and most important chlorination mill in the world. The method of treatment is the original Munktell process, adapted and improved. It consists in subjecting the ore in open vats to the action of a solution of chlorine in water. It was adopted, after chlorination in 1-ton barrels had been used for about a year, in 1887-1888.
The ore from the upper levels, now chiefly in the form of mine tips, consists of sinter, kaolin, quartz, and ironstone, with a small percentage of sulphur. The ore from the lower levels consists of hard quartz, heavily charged with pyrites. The gold in the latter is believed to be mainly in the form of tellurides, and after roasting all the ores contain only finely-divided gold. The ores are treated in different plants by different methods.
At the “West Works” plant, a plan of which is shown in Fig. 67, over 100,000 tons of low-grade ore from the upper levels are treated annually. The ore is crushed dry in Krupp ball-mills, to pass a 20-mesh screen, roasted in revolving cylindrical furnaces and stored in main hoppers higher than the vats. The roasting is mainly to dehydrate the ore, so as to improve the speed of leaching, which is otherwise excessively slow. A small percentage of sulphur is also removed. The vats are 60 feet long, 12 feet 6 inches wide, and 5 feet deep, and hold 100 tons of ore. They are made of concrete lined with neat cement, which in turn is covered with pitch and tar applied hot. There are sixteen of these vats. The filter bed is 20 inches thick, consisting of 16 inches of sand and gravel resting on perforated planks supported by wooden bearers. The vat has a fall of 3 inches from one end to the other for draining off the solution. The depth of the bed of ore is little more than 3 feet. The ore is tipped in from trucks and roughly levelled.
The chlorine is generated in ordinary flagstone stills from sulphuric acid, manganese dioxide, and common salt. The acid is made on the spot from imported sulphur. The manganese and salt are crushed and thoroughly mixed before charging into the still. The chlorine gas is passed into scrubbing towers, see Fig. 68, each 2 feet 3 inches in diameter and 20 feet high, filled with glass bottles and old assay crucibles, always kept wet by trickling water. The solution of chlorine contains about 600 grains of chlorine to the cubic foot, or 0.14 per cent, of chlorine. The weight of chlorine used is about 2½ lbs. per ton of ore, which corresponds to about 30 cubic feet of solution per ton. The solution is stored in solution tanks (see Fig. 69), closed chambers with concrete domes, whence it flows through a 6-inch earthenware main to the vats. There are four stills, four towers, and four solution tanks, the latter having a capacity of about 950 cubic feet each.
The solution is allowed to run on the charge, from 6-inch earthenware mains (see Fig. 70), until the vat is full, when the valve below the false bottom is opened and the vacuum in the pump main of about 5 lbs. per square inch draws the liquor through. Fresh solution is run on to keep the ore covered, and this is continued until the gold is dissolved, which appears to take about thirty-six hours. During the first few hours all the chlorine is absorbed by the ore which becomes hot, doubtless from the attack on ferrous and basic salts as well as sulphides left in the ore after roasting. After about fifteen hours, the liquor issuing from the vat is found to contain gold, which gradually increases in amount.
When the liquor gives a good black precipitate with ferrous sulphate, and smells strongly of chlorine gas, it is assumed that all the gold is dissolved, and the vat is then said to be “ chlorinated,” and wash-water is turned on and passed through the ore until no gold can be detected in it by means of ferrous sulphate. The ore is then discharged by means of shovelling into trucks, though it is intended to replace this by a steam dredge. The solution of gold is passed through charcoal filters as described on p. 268.
At the “Top Works,” the ore treated is hard compact quartz heavily charged with iron pyrites. It contains an average of 4 ozs. gold per ton and 11 per cent, sulphur, which after roasting is reduced to 0.15 per cent. The ore is roasted in a Richards’ shaft furnace (see p. 230) at a cost of about 5s. per ton. The amount of solution required is larger than that at the West Works, but the time of treatment is about the same, as the ore is more porous and the washing rapid. The following are the chief items in connection with the chlorination:
