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As he was obtaining over 4 oz. to the ton the process was paying very well, but it will be seen that the price would be prohibitive for poor ores unless they could be concentrated before calcination. The Pollok process is a newer, and stated to be a cheaper mode of lixiviation by chlorine. It is the invention of Mr. J. H. Pollok, of Glasgow University, and a strong Company was formed to work it.
The resulting "concentrates" are washed off from time to time and reserved for secondary treatment. To begin with, they are roasted to get rid of the sulphur, arsenic, etc., which would interfere with the amalgamation or lixiviation, and then either ground to impalpable fineness in one of the many triturating pans with mercury, or treated by chlorine or potassium cyanide.
A good deal of salt is made by lixiviation of the soil and evaporating by fire. The head woman had a tame khanga tolé or tufted guinea-fowl, with bluish instead of white spots.
Much also depends on the proportion of gold to the ton of rock under treatment, as the most scientific and perfect processes of lixiviation hitherto adopted will not pay, even when all other conditions are favourable, if the amount of gold is much under half an ounce to the ton and even then will leave but a very small profit.
Besides the mineral resources of the world in their original state, the displays embraced many devices of mining machinery; such as pumps and engines used in mining, moving, and delivering ores; apparatuses for breaking out ore and coal; for crushing and pulverizing; for reducing metals, for instance the extraction of gold and silver by milling, lixiviation, and fire; furthermore, boring and drilling tools; grinding and polishing substances, etc.
The object of calcining or roasting certain ores before treatment is to dissipate the sulphur or sulphides of arsenic, antimony, lead, etc., which are inimical to treatment, whether by ordinary mercuric amalgamation or lixiviation. The effect of the roasting is first to sublimate and drive off as fumes the sulphur and a proportion of the objectionable metals.
When the temperature has descended to 100 deg. in the boiler the cover, D, may be opened, after the liquid has been allowed to flow out through the cock, C. Next, lixiviation is effected by connecting the cock, C, with the steam pipe, P, and causing steam under pressure to enter the boiler, A. The action of the steam on the contents of the latter, which are now converted into cellulose, mixed with a large quantity of dissolved matters and of liquid, effects a complete washing and permits of the recovery of considerable quantities of useful chemical products.
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