4. SOLUTION AND CLASSIFICATION OF THE COPPER SULPHATE CRYSTALS.

As the crystals contain all the insoluble as well as the soluble materials, they must be dissolved and recrystalized in order to separate them. The solution-tanks and crystallization-launders are all situated on the upper story. Between this upper story and the lower ones the boilers for making the steam are placed. Across a passage, about two meters wide and above the boilers, are three tanks 3.20 meters square and 0.60 meter deep (Fig. 4), heated by furnaces for dissolving the fine crystals. The solution is 0.50 meter deep in the vats, and they are covered. The vats are lined with lead and are heated by an arched furnace, the fire of which is made to circulate twice on each side of the fire place by flues which join behind it. The fire-place itself is arched with brick and covered with iron plates. These plates are 0.96 meter by 1.15 meters by 0.015 meter thick. They are the same plates which are used round the works for flooring.

These vats connect at the bottom by a short spout with a launder on the outside, in which there are two pipes, closed with conical stoppers. One of these is on the level with the bottom, and serves to empty the mud collected there. The other, raised a little above it, is used for drawing off the clear mother liquor. This mother liquor from the last crystallization in the tanks below is pumped up into them so as to be about 0.40 meter deep and heated to from 70° to 76° R. It is diluted with water until it is at from 150 to 180 B. Only about half of the mother liquor is used for this purpose. The other half is pumped up to be used in dissolving the granulated copper. The crystals are then added. It is not allowed to concentrate beyond 260 B., as the crystals which are produced from a more concentrated liquor are too small to be easily sold. The fire is drawn when the solution is made. While the solution of the crystals is going on the liquor is kept constantly stirred with wooden rabbles 0.30 meter long by 0.20 meter wide, with a handle 4 meters long. As a very small quantity of silver goes into solution with the copper crystals, it is necessary to precipitate it. This is done with the very fine shot copper which is produced when the copper is granulated for solution, which precipitates all the silver. In order to prevent any loss of heat the tank is covered with a hood and the surface of the liquor protected with boards.

When the solution has taken place, the liquor is left until it settles, which will generally take about twelve hours. During this time it cools down to about 65° R., and falls to 29° B. It must not be allowed to go below this temperature, as there would be danger of small crystals forming. It must therefore be made to cool as slowly as possible, so as to prevent its being too cold when it comes to the crystallization tanks. When the liquor is quite clear, it is run off by the pipe to the crystallization tanks.

5. TREATMENT OF THE SILVER AND GOLD RESIDUES.

The mud which settles, is composed of some sub-salts and both silver and gold in a fine state of division. This mud, after two or three operations, is washed out through the pipe into two vats below, 1 meter square and 1.50 meters deep.

When enough is collected it is pumped by steam through a filter press with six filters in each press, and washed with water in the press. The cakes are dried and sent to the silver works. They contain most of the gold, from 2 to 4 per cent. of silver, with some gypsum, and the oxides of lead, arsenic, and antimony.

6. CRYSTALLIZATION OF THE COPPER SULPHATE.

As soon as the liquor has settled it is siphoned off to the crystallization vats on the floor below. There are twelve of these vats to each solution tank. They are all lined with lead. One tank full fills a vat, and one vat is emptied of its crystals each day, so that the liquor remains 12 days in each vat. There are 36 of these vats. They are built two together, and supported on the sides and bottom by braces 0.15 meter square and held by iron-rods at the top. They are 3.75 meters square and 1.05 meters deep.

On the top of each of these vats are 25 poles with five strips of lead, .004 meter thick and 0.02 meter wide, hung on them and reaching to the bottom of the vat. Here the liquor is allowed slowly to crystallize. The largest crystals collect on these strips, the next on the sides of the vat. On the bottom small crystals often collect, which are taken out with a scoop and re-crystallized. When the crystallization is complete the mother liquor is drawn off to be used again in the solution of the copper and of the first copper crystals. The crystals are collected from the strips first, and then from the sides. When taken off they are put down on one corner of the vat until they are all collected. They are washed with water and are dried in a large room which is kept as dark as possible to prevent the decomposition of the crystals. This room is kept heated at 200 R. This is considered essential, as copper sulphate easily loses part of its water, and then becomes opaque. As the only judgment of the quality is by the eye, this makes them look badly, so that they are not easily sold. There are three shelves around the room for drying the crystals, each of which is 2 meters wide. The highest shelf is 2 meters above the ground. The crystals are first carefully sorted. The large ones are sold, the small ones kept for use at the works. They are arranged in doubly inclined rows at right angles to the edge of the shelves, so that they can be easily turned over to dry them. Three categories of crystals are made, two of which are sold. They are arranged mostly as to size. They are packed in barrels which weigh 250 kilograms and kegs which weigh 50.

In the course of the process it is found that every kilogram of granulated copper produces 3.80 kilograms of blue vitriol. It requires for its solution 2.40 kilograms of sulphuric acid at 50° B. Six solution tanks, two boiling vats, and twenty-four crystallization vats can produce from two and a half to three tons of blue vitriol in twenty-four hours. The injectors are all of lead with the points made of hard lead.

To use this process it is necessary that the copper should be nearly free from iron and nickel, as these metals concentrate in the process and would at last injure the quality of the metals unless there is enough of them to make a special treatment for them with the concentrated liquors; that sulphuric acid should be abundant and cheap, and that there should be a ready sale for the copper sulphate crystals. The presence of small quantities of arsenic, antimony, and lead does not interfere with the process, and it is therefore applicable to very impure black coppers, or to those which are very rich in silver and gold.

As the acid mother liquors are constantly used over again there is but little loss of sulphuric acid. Most of what does not go into the copper sulphate is used again. These mother liquors can, except in the cases mentioned, be used indefinitely. The process is simple, requiring but little intelligent labor.