1918.] The N.Z. Journal of Science and Technology. 
41 
froth-formation be continuous. In the working of the minerals-separation 
process the ore crushed to the necessary degree of fineness is violently 
agitated with water and a small percentage of a suitable oil. The more 
violent? the agitation the better the result. From the agitator the emulsified 
mass is run into a spitzkasten, where the froth, carrying with it the 
sulphides, is floated and removed, while the tailings sink and run to 
waste. 
In the pneumatic process as developed by Callow the ore, water, and oil 
are fed into a Pachuca tank, where they are emulsified by means of com¬ 
pressed air. From this tank the feed goes to the first roughing separatory 
cell. This cell consists of a tank about 9 ft. long and 2 ft. wide, with the 
bottom having a fall of from 3 in. to 4 in. per foot. It is 20 in. deep at 
the shallow end and 45 in. at the deepest. Through the bottom of the tank, 
which is porous, air is forced by means of a blower. Under this porous 
bottom are eight compartments, each of which is connected by a valve 
with the main air-line. By this means the air-pressure to each compart¬ 
ment can be regulated to correspond to the hydraulic head in the tank. 
A pressure of about 5 lb. is generally used. Each longitudinal cage of the 
tank is provided with a lip and launder to receive the froth. At the lower 
end of the tank a discharge-valve, operated by means of a float, is arranged 
in order that a uniform water-level may be maintained within the tank. 
The water-level is usually kept at about 9 in. below the overflow-lips. The 
tailings are discharged from the discharge-valve and run to waste, while 
the froth is run to a cleaner-cell. Usually one cleaner-cell serves for four 
roughing-cells. The tailing-discharge from the cleaner-cells is returned to 
the head of the roughing-cells. In later work the Pachuca tank has been 
dispensed with, and where possible the oil is fed into the tube mill or 
other crushing-machine, and from there directly to the first roughing-cell. 
Preferential Flotation. 
This is the flotation of one of the ordinary flotative minerals in the 
presence of another similar mineral. Thus a mixture of galena and blende 
can be floated collectively from a gangue of quartz and limestone, or other 
gangue material, while blende may be floated preferentially from a mixture 
of galena and blende. The Horwood method is the one most generally used, 
and depends for success upon the deadening of galena in a short roast at 
from 400° to 500° C., whereby the galena is coated with lead sulphate, while 
the blende is unchanged. This allows of the separation of the blende 
from the galena. 
Another process, known as the Lyster, has also been tried at Broken 
Hill. Lyster’s process is carried on in neutral or alkaline solutions of the 
sulphates, chlorides, or nitrates of sodium, calcium, potassium, and mag¬ 
nesium. Using eucalyptus oil, the agitation takes place in centrifugal 
pumps, discharging into spitzkastens with closed tops. It is claimed 
that a high lead concentrate can be obtained in the first spitz, and that 
by sending the tailings to a second spitz a high zinc concentration can be 
made. 
During the past few days particulars of the S0 2 method of preferential 
flotation as adopted by the Broken Hill Proprietary Company have come 
to hand. This method of flotation has been adopted to treat the lead- 
zinc slimes accumulated from gravity concentration that could not be 
dealt with by the Delprat process. The Lyster type of cell is used, the 
cells being connected in series. The S0 2 method as now adopted employs 
centrifugal pumps, connected in the series, one taking the original feed- 
