THE AMOUNT OF AIR
WORKING COST.
The working cost must vary with the locality and the cost of supplies and labour. For the flotation-concentration, at Broken Hill, exclusive of crushing, sizing, and any after-treatment of the concentrate, the working cost has been given as, roughly, 33. per ton. As these figures were derived from a plant where sulphuric acid was comparatively cheap, it would be safe to take a round figure of four shillings per ton. In detail the figures referred to are as follow :
MEN EMPLOYED FOR SIXTEEN MACHINES.
Vacuum floor . . . . . . . . 1
Mixer floor . . . . . . . . 1
Discharging floor . . . . . . 1
THE AMOUNT OF AIR.
If the pulp was made up in the ratio of 4 tons of water to I ton of ore, and if the ore was of the Broken Hill type, this would be 4 tons of water to about half a ton of sulphides. The water contains 2.2% by volume of air in solution, and it may be assumed that the vacuum releases 2% by volume. Four tons of water is 142 cubic feet ; 2% of this is 3 cubic feet of air released for the purpose of floating half a ton of sulphides. Three cubic feet of air loaded with sulphides to an extent just sufficient to remain in suspension would require 3 by 63 Ib. of sulphides =189 Ib. of sulphides, or considerably less than half a ton. There is enough air in a pulp of I ton of water to i ton of ore to lift 60 Ib. of concentrate. The following table gives the amounts of air for pulps of different proportions
As the proportion of water to ore rarely exceeds 6 to i, and as the ores treated generally yield more than 16% of concentrate, it will be seen that some gas other than that naturally dissolved in the water must be found to effect flotation. This is generally secured by adding limestone to the ore, and then acid at the point where the pulp enters the vacuum chamber. It would be interesting to investigate whether, by limiting the quantity of gas available for froth production, it would be possible to secure differential flotation between lead and zinc sulphides. This machine seems especially adapted to the development of this idea.
