0.—3

132

gold from its ores is chlorine. The special feature of the Pollok process is the method in which the chlorine is applied—a so-called hydraulic chlorination method— i.e., the chlorine after being liberated is liquefied by hydraulic pressure. This process, it is claimed by the patentees, is applicable to all ores or tailings containing gold, and especially to refractory and float-gold ores. Being an eminently cheap process, the estimated cost of treating a ton of ore under present circumstances being not more than Bs. or 95., it will be seen to be peculiarly suited for dealing with that large class of refractory ores which are unable to be treated by the older and more expensive amalgamation processes, which consequently have hitherto been allowed to accumulate in such large quantities in different parts of the world. As the process thus promises to be of great commercial importance, a more detailed account of its methods may not be without interest to our readers. The works at Keppochhill Eoad, Glasgow, have been in operation for the last two months. The gold-ore at present under treatment is from Chili. It contains, according to assay, 3oz. 6dwt. per ton. Out of this quantity of gold 3oz. ldwt. is extracted by means of the Pollok process. The following are the different operations which the ore is submitted to in this extraction process. The ore is first crushed by an ordinary crusher, and is rendered sufficiently fine to pass through a sieve containing thirty meshes to the linear inch. Too great fineness, it may be remarked in passing, is not desired in this process, as it is found to impede subsequent operations. In the case of very refractory ores a greater degree of fineness is desirable than in ordinary ores. In the case of what are known as float-gold ores, the ore is brought, immediately after being ground, to the chlorinating cylinder. In the case, however, of refractory ores containing sulphur, iron-pyrites, copper, lead, &c, the ore must be previously roasted. The object of roasting the ore in such a case is to remove the sulphur, oxidize the iron, &c, and thus to render the ore of a porous nature. The result of such previous roasting is to save the chemical reagents afterwards used, and to permit of the gold contained in the ore being more easily dissolved by the chlorine. In the case, however, of very poor refractory ores, the ore before being taken to the furnace is treated in a stamp-battery and concentrated by vanners. In this way a very poor ore, which it would be impossible to treat profitably otherwise, is concentrated some ten times. Thus, an ore containing no more than sdwt. of gold per ton , can, under favourable circumstances, be worked with profit. The furnace at present used by the Pollok Company for roasting their ores is the common hearth furnace, and holds some two and a half to three tons at a time. It is capable of roasting about 12 tons of ordinary ore in a day, and burns in the operation about one ton and a half of coal. In working abroad, however, the company purpose using a revolving-furnace holding eight tons at a charge. This furnace, it is calculated, would be capable of roasting thirty tons of ore in a day, and would burn from two to three tons of coal. The chief benefit of a revolving-furnace, it may be here pointed out, is that it feeds, stirs, and discharges automatically, thus saving labour—a most important consideration abroad. The time required for roasting the ores varies with the nature of the ore, some requiring a very much longer period than others. The average time is from four to six hours. Ores which have been previously concentrated in the manner above described require a considerably longer time. After being roasted the ore is raked out on to a cooling-floor. It is next conveyed to the chlorinating cylinders (Figs. 1 and 2), of which there are two at present in work. One of these (Kg. 1), designed by Mr. Stephen Alley, is made of light steel—it only weighs 25cwt.— and is lined with a thin coating of indiarubber, which protects the steel from the action of the chlorine. It has an automatic valve for charging and discharging, and a delivery-pipe. There is also an escape-valve on the top of the cylinder. The other cylinder, we may mention, is lined with wood. The rubber used for the purpose of lining the cylinder it about an eighth of an inch in thickness, and is made of pure rubber, and not vulcanized. It is found that the chlorine has no action on the rubber, and that the wear and tear from friction practically amounts to nothing. It will be seen that the value of the indiarubber lining is thus very great. The chlorinating cylinders are charged as follows : First, 801b. of nitre cake, forming about 2 per cent, of the whole charge, is dropped into the cylinder; then two tons of the ore; and lastly, 601b. of bleaching powder—or about 1J per cent, of the whole charge. The charging aperture is then closed, and a steam-injector is turned on, forcing in water till the pressure rises from 701b. to 1001b. It is important that all the air in the cylinder should be allowed to escape, as the presence of air is most detrimental to the action of the chlorine, as it mixes with the gas and prevents it liquefying. The cylinder is then revolved, thus mixing up the ore with the bleaching powder and the nitre cake. The chlorine which is thus liberated goes into solution, and the pressure forces the strong chlorine liquor into the pores of the ore, previously rendered porous by roasting. The period during which the cylinder is revolved varies from one hour to one hour and a half. At the end of this time the contents of the cylinder are discharged on to a filter-bed placed below. The waste chlorine is blown off, and largely recovered by being passed through slaked lime and thus absorded. The filter is made of steel-wire lined with indiarubber. , The charge after being decanted on to the filter-bed is 'filtered, the chlorine liquor, containing the gold, is drawn off by a specially-designed vacuum pump, by which it is pumped into the precipitating tank (Fig. 2), where the gold is precipitated by adding ferrous sulphate. The quantity of this reageant used is about 301b. to the ton, or 1-|- per cent. The precipitating-tank is fitted with a conical-shaped bottom, on which the gold precipitate settles. As soon as the liquor has become clear it is run off, and the gold paste is removed and fused, in Hessian crucibles with borax, into gold bars. The chief advantages of this process, as claimed by the patentees, are : (1) The rapid and complete extraction of the gold by the use and application of hydraulic pressure, a machine weighing 25cwt. being able to put through twenty-four tons in twelve charges in twenty-fours ; (2) The use of dry chemicals, which can all be safely and cheaply packed, shipped, and carried. The attached table embodies some of the results obtained by the treatment of ores by the Pollok process;—