The liquor could be run away from the precipitating tanks in the copper works into a reservoir of suitable dimensions, where it would be allowed to remain some little time to permit of the solid impurities settling out, and also to allow the liquor to cool down to a certain extent. It could then be made to flow slowly and continuously along a shallow shoot, on the outside of which a current of cooling water ran in the opposite direction to the flow of liquor. In this way all the cooling effect of the water would be utilised, and the liquor would flow away at the end farthest from the reservoir at a temperature the same, or nearly so, as the cooling water, leaving behind it in the shoot all the soda sulphate crystals, which it was unable to hold in solution at that temperature. These could be fished out from time to time without stopping the flow of fresh quantities of liquor, and at once taken to a hydro-extractor, where they could be washed with a spray of saturated solution of soda sulphate, and dried. In this way they would be rendered almost entirely free from foreign bodies, and could then be furnaced and converted into salt cake.
This direct treatment could of course only be used when the temperature of the air and water was not much above 50° F., if a fair percentage of the crystals in the liquor were to be recovered, but as this temperature is considerably below the average for a great part of the year, the liquor would require at other times to undergo treatment before entering the reservoir, so that after such treatment it would be of a strength to yield per gallon at the particular temperature as much soda sulphate crystals as a gallon of the original liquor would yield at 50° F. This could of course be brought about by a partial evaporation. Let us suppose, for instance, that the temperature was at 59° F., and that the liquor was of such strength as to be capable of yielding at 50° F. 2 lb. of crystals from the gallon. From the table of solubilities at different temperatures, it is easily calculated that by evaporating away about 20 per cent. of the water from the liquor, or about 1*4 lb. per gal., a liquor would be obtained which would give per gallon the same yield at 59° F. as 1 gal. of the original liquor would give at 50° F., or if the temperature were as high as 68° F. an evaporation of 40 per cent. or about 2.8 lb. per gal., would again give a liquor which would yield the same result.
Thus by varying the amount of evaporation according to the temperature of the air and the yield of salt required, any required result could be arrived at, the limit of course being where the evaporation was carried to the extent of driving off all the water and leaving the dry salts. This was the case in the method of treatment which was formerly resorted to, and which, from the above, appears to be an expensive and, except in most exceptional circumstances, a useless method of procedure, for, by the method of cooling after partial evaporation, when the temperature is at 59° F. only 1.4 lb. of water require to be evaporated away in order to get 1.76 lb. of salt (that is 2 lb. less 12 per cent. for reduction in bulk of the liquor during the partial evaporation) in a fairly pure form, whereas by total evaporation, that is driving off about 7 lb. of water, only 3*75 lb. of a very impure salt is the result, or a little over twice the quantity of salt for 5 times the evaporation. Having evaporated the liquor in part, it could be run into the reservoir and be put through the same treatment as before stated.
If the liquor were run from the reservoir at a temperature 30° F. above that of the cooling water, theoretically there would be required less than twice as much cooling water as liquor to be treated, for from 1 gal. of liquor weighing 12.5 lb., 0 85 X 12.5 X 30=318.75 thermal units would have to be abstracted in addition to 200 thermal units for the 2 lb, of salt while crystallising out, and 2 gal. of water should be capable of abstracting 600 thermal units, though, of course, in practice, rather more than the theoretical quantity of cooling water would be required. If it is required that only a given quantity of salt is to be allowed to run away in the final waste liquor for each gallon of the original liquor, and we suppose that that quantity is fixed at that which would remain in solution after cooling the original liquor to 50° F.; when the temperature is at 59° F., in place of having to evaporate away 1*4 lb. per gal. we should have to drive off 2.5 lb. of water. Taking into consideration, however, that the liquor is a waste product of practically no value, this latter would not be such an economical way of working as the former, where a gallon of the partially evaporated liquor gave the same yield of salt as a gallon of the original liquor.
The plant required for working up this waste product in this way would be somewhat as follows: - Piping to run the waste liquor to an evaporating pan from the precipitating tanks in the copper works, an evaporating pan (one similar to those used in the evaporation of brine would be suitable), a reservoir, a cooling shoot, a small tank for making saturated solution of soda sulphate, a hydro-extractor for drying and washing the crystals in, with small gas engine to work the same, a furnace for driving off the water of crystallisation and converting the crystals into salt cake, and piping to run away the waste liquor when it left the cooling shoot, and to conduct the cooling water away to be used for any further purpose to which it might be applicable. Ellis concludes with a rough estimate of the cost of working per day of 24 hours, on the supposition that 20,000 gal. were to be treated in that time, and that the average temperature throughout the year is 59° F. For the partial evaporation, about 2 tons of coal would be required at 7s. per ton; say 60,000 gal. of cooling water at 4d. per thousand; 4 men at 4s. and 4 boys at 2s.; coal for the furnace, 2 tons at 7s,; 4 men at 4s, and i boys at 2s. to attend to the wash-ing and furnacing of the crystals, etc.; gas for the gas engine, etc, 4s.; rent of ground, 5s.; management, 15s; interest and depreciation on plant at 15 percent. say on 2000l., taking 300 working days per annum, 1l.; giving a total of 7l. The production of salt cake for this, taken at about 2 lb. of crystals per gallon, from about 17,000 gal. (after allowing for evaporation), should be over 7 tons, making its cost per ton about V.