a Cottrell precipitator for treating the fume from the furnace though at the expense of increasing both installation and running costs. It would also be necessary to mill the ore dry, a method which is usually to be avoided whenever possible. Flotation has been tried for dealing with such ores with the result of producing a small quantity of high-grade concentrate, but a very low percentage recovery from the ore; moreover, generally speaking, the silver recovered by flotation is just that part of the silver content that is amenable to cyanide while the refractory silver remains in the tailing.

Probably the most hopeful chance of finding a solution of this problem lies in the process first suggested by the writer early in 1909 and mentioned in the Mining and Scientific Press of December 4th in that year. It consists in treating the ground ore in a water pulp with SO2 gas, or its solution in water by which the manganese is dissolved and washed out leaving the silver free to be acted on by cyanide. Extractions up to 95% of the silver contents have been made on refractory ores by this process and while the extra expense is considerable it will in most cases be a good deal lower than that involved in roasting processes, and might under favorable conditions be found of payable application to ore of average grade. At the present time (1919) experiments are being conducted with very promising results along these lines in Pachuca using the concentrates from the sulphide ore to furnish sulphur gas for the treatment of the refractory ore. Walter Neal1 appears to prefer the use of sulphuric acid and ferrous sulphate as a substitute for the sulphurous acid stating that he found the extraction due to these reagents somewhat superior to those from the SO2 but the method would seem a good deal more expensive unless the solutions could be regenerated electrolytically. The writer has made some tests along these lines, but without very encouraging results.

For those who wish to experiment with this process it may be said that the most satisfactory method for laboratory use consists in the addition of varying amounts of a solution of SO2 1 Private communication.

gas of known strength which may be prepared by boiling strips of metallic copper in concentrated sulphuric acid, passing the gas through a washing bottle and thence into distilled water until saturated. The method of determining the SO2 content of a solution is as follows:

N

Take from 50 to 200 cc of iodine solution according to the

10

strength of the SO2 solution and add to it from a pipette 5 to 10 cc of the SO2 solution. Should the brown color of the iodine be dispelled or nearly so, it will be necessary to make a new test using more iodine since the iodine must be in excess. Then N titrate the excess iodine with Na2S2O3. The original amount 10

iodine less the quantity shown by titration gives the iodine

consumed by the SO2.

N
10

1 cc I consumed

=

0.0032 gr. SO2

Arsenic and Antimony Compounds of Silver.-Ores containing silver in such combinations offer considerable difficulties in cyanidation and it has usually been considered that the removal of these compounds by concentration was the only way to deal with them. G. H. Clevenger, however, when working on the ores of the Nipissing Mining Company of Ontario, Canada, found that by grinding exceedingly fine and treating with strong solution for four or five days a high extraction could be obtained. The silver occurred partly as sulphide, sulphantimonide and sulpharsenide and partly as metallic silver and dyscrasite, an alloy of silver and antimony in varying proportions.

To accelerate the dissolution of the antimony and arsenic compounds J. J. Denny introduced a process that had for its object their decomposition in a preliminary treatment. This process consists in grinding the whole of the ore to a slime and subjecting it to the action of metallic aluminium in a caustic soda solution, after which it is filtered and subjected to cyanidation.

He says1

"By the preliminary treatment the silver, and in part at least, the antimony and arsenic, are reduced to the metallic state, and are so found. The reduction is accomplished by the nascent hydrogen resulting from the action of caustic soda on the aluminium according to the following equation:

(1)

2A1+2NaOH + 2H2O

=

Na 2Al2O4 + 6H."

"The probable reactions involved in complete reduction are indicated by the following equations:

(2) Argentite.

6H+3Ag2S + 6NaOH

=

3Na2S6H2O + 6Ag

(3) Pyrargyrite.

6H+Ag3SbS3 + 6NaOH = 3Na2S + 6H2O + 3Ag + Sb

(4) Proustite.

=

3Na2S+6H2O + 3Ag + As."

6H + AgзAsS3 + 6NaOH "The reactions being reversible, probably the arsenic and antimony are not completely reduced to the metallic state in practice, and the investigation of the subject is rendered difficult by reason of secondary reactions by which the arsenic and antimony are possibly redissolved to form arsenates and antimonates by the excess caustic of the reducing solution, and the protective alkali of the cyaniding solution. The working solution shows the presence of these compounds, but in practice they are found to have no detrimental effect either in the reducing or the cyaniding treatments. The solution assays, antimony 0.0084% and arsenic 0.026 per cent."

In practice the difficulty has been to obtain adequate contact between the refractory mineral particles and the aluminium surface. Various expedients have been tried but so far the best results have been obtained by first passing the pulp through a tube mill loaded with small aluminium ingots instead of pebbles, and then sending it to an agitator tank lined with aluminium plates, in which it is kept agitating for about 12 hours. Even so the action is incomplete because if a sample of the pulp be taken after treatment is finished and divided into two parts, one of which is cyanided direct and the other subjected to further 1 Mining and Scientific Press, September 27th, 1913.

desulphurizing in a bottle and then cyanided, the latter will invariably yield a better silver extraction.

The dyscrasite is unaffected by this treatment, but will dissolve in the cyanide solution, given time and sufficiently fine comminution.

As regards costs the above mentioned article gives the following figures:

"In connection with the desulphurizing treatment, treating 7268 tons per month, the following data are available:

COLLECTING DESULPHURIZING and Transferring of Pulp

Labor....

Per ton

$0.050

Supplies (aluminium, 0.81 lb.; caustic soda, 1.46

"The desulphurizing treatment effects a saving of from one to four ounces per ton, depending on the amount of refractory minerals present, at a total cost of 54 c. per ton."

In the cyanidation of this ore, whether previously desulphurized or not, it was found that a serious deterioration occurred in the extractive power of the stock solution after repeated precipitation by zinc, an effect which was shown to be due to some obscure interaction between the zinc gradually accumulated in the solution and the arsenic and antimony derived from the ore. This was overcome by the substitution of aluminium for zinc as a precipitant, a method which proved so efficacious that the

stock solution after the plant had been running twelve months was more active in dissolving power than a freshly prepared cyanide solution.

Cyaniding of Concentrate.-The question as to whether concentration shall be included in the treatment of a given ore will often depend on the possibility or otherwise of recovering the precious metals from the concentrate at the mine. It may be suggested that if the concentrate can be cyanided after being separated from the gangue, why can it not be equally well cyanided if allowed to remain in the pulp going to the cyanide plant? The answer is that a small bulk of refractory high grade material can be treated by special methods which would not pay if applied to the whole of the ore. For instance, a few tons a day may be profitably roasted or slimed or treated with extra strong solution or for extra long periods, all of which might be prohibitive if applied to the entire ore.

There are two ways of dealing with a concentrate in a cyanide mill; one is to regrind to the degree of fineness found to be necessary by experiment and then to discharge the resulting pulp into the stream of mill pulp flowing to the cyanide plant, where the mixture goes through the usual routine of the process; and the other is to keep it apart and treat it in a small plant specially designed for the purpose. The latter has the advantages of greater flexibility and of affording an opportunity to ascertain exactly what results are being obtained.

Roasting prior to cyanidation is often the only way to get a satisfactory extraction from refractory gold concentrates, and the resulting material may be either leached, or reground and treated by agitation. It is seldom found profitable to treat raw concentrates by leaching though this was sometimes done before the development of processes for cyaniding slimed products.

It is to be noted that desulphurizing-roasting of concentrate containing silver has the effect of rendering a large proportion of that metal insoluble in cyanide, so that for silver or silver-gold concentrates some kind of raw treatment must be devised, unless there are reasons for preferring a preliminary chloridizing roast.