For some years past the principle enunciated in the discussion referred to has been known to the writer and demonstrated by him to be an efficient and economical way of separating water or solution from solids in the cyanidation, lixiviation or acid leaching of ore pulps. In utilizing this principle in commercial practice he has invented a method whereby not only the separation of solution from solids may be effected in any given tank, but also the counter migration of the solids and solution may be effected continuously through a series of tanks in which the hydrometallurgical treatment of ore pulps is being carried on.
In effect this method is analogous to the counter-current system, but is different physically in that it is carried on throughout a series of tanks on practically the same level, continuously and automatically, by hydrostatic pressure and gravity-flow, without the aid of intervening pumps or elevators, at the same time that agitation is being carried on in the same tanks.
In the Parral system, agitation is effected in tanks by a number of air lifts of comparatively small diameter through which the pulp is continuously transferred from the bottom of the tank and spouted horizontally on top of the charge. The spouting force of the several streams thus discharged maintains a rotary flow in the pulp charge which extends from top to bottom of the tank. This continuous transfer of pulp and the spiral flow from top to bottom of the tank maintains the pulp constituents in uniform proportions and meets the requirements of efficient treatment in a most economical manner. The quantity of compressed air required is very small, as the pulp is not lifted but simply transferred from the bottom to the top of the tank charge under hydrostatic balance.
To illustrate the apparatus and explain the results obtained, Fig. 2, 3, 4, and 5 are submitted. These figures represent two views each of two adjoining tanks taken from the middle of any series of tanks in which the continuous treatment of pulp is being carried on. Fig. 2 and 3 show a vertical cross-section through the center of two tanks equipped with my apparatus, along line AA in Fig. 4 and 5, which represent the top view of the same two tanks.
As shown in the figures, a diaphragm C is mounted concentrically in the upper portion of the tanks. This diaphragm is cylindrical in shape, open at the top and bottom, and is of such width and depth as required by the function it is to perform. The pulp is continuously agitated in the annular space between the diaphragm and the sides of the tank by transfer pipes to which compressed air is conveyed through pipes. At the bottom and below the lower edge of the diaphragm is the separation zone where the solids in the pulp settle by gravity and fall on the sloping sides of the cone b over which they gravitate to the intake ends of the transfer pipes through which they are continuously transferred and spouted on top of the tank charge in the agitation circle. Above the separation zone, the water or solution rises clear within the diaphragm, and by the hydrostatic pressure of the pulp undergoing agitation the clear solution is raised to a height above the level of the pulp commensurate with the depth of the sides of the diaphragm.
As an example, if the pulp being agitated contained 2 parts by weight of solution to 1 part by weight of solids, it would have the specific gravity of, say, 1.26, (depending on the specific gravity of the solids) or, about one and one-quarter times the weight of the same volume of water. Therefore, if the sides of the diaphragm extend, for example, 9 or 10 ft. down into the pulp, the weight of the pulp would raise and balance a
column of clear solution within the diaphragm to a height of more than 2 ft. above the level of pulp in the agitation circle. The separation of the solution from the solids takes place rapidly in the separation zone at the foot of the diaphragm, probably clue in a considerable degree to the centrifugal force set up by the rotary flow of the pulp in the agitation circle. At any rate, clear solution separates from the pulp mass in this region and rises rapidly and continuously within the diaphragm from the top of which it is taken off through pipes d and delivered into the pulp being agitated in the next adjoining tank, and so on until from the head tank of the series the solution goes to the clarifiers to remove any fine slime that may be carried over in suspension, and thence to precipitation.
The series of tanks in which the treatment is carried on is set so that each tank from head to tail of the series is 6 in. below the level of the tank preceding it. This height is quite sufficient to overcome the flow friction of the pulp as it goes from tank to tank through pipes e. The last two tanks of the series may be specially arranged so that the pulp may be thickened to a very high density before it is discharged to filter press or dump as conditions require. In this way, the counter migration of the solids and solution is brought about automatically and continuously through any number of tanks equipped as shown in the illustrations. The precipitated cyanide solution, which is added to the last tank of the series, flows from tank to tank toward the head of the series, dissolving the metals from the solids with which it is agitated as it goes. The solution thus becomes enriched as it goes from tank to tank until it flows from the head tank for precipitation, while the solids being carried from the head to the tail tank of the series become poorer and poorer until they are discharged barren from the tail tank.
The same principle may be used in the acid leaching of copper ores, but the tanks and the transfer pipes used for this purpose are made of wood in order to be acid proof. This method obviates the use of pumps between the tanks, which give great trouble in handling acid solution.
The agitation of the pulp may be carried on within the diaphragm, leaving the annular space outside the diaphragm for the rise of the clear solution. I have designed this latter method for a plant having Pachuca tanks, but the first described method may also be installed in Pachuca tanks to advantage because the clear solution may be lifted to any height which allows a series of tanks set on a level to be operated without the intervention of pumps.
Some time ago I suggested to the operator of a plant of Pachuca tanks that he equip the tanks with the apparatus I have described, but he was skeptical that water could be made to rise above its own level so as to flow from tank to tank as I described.
A 12-ft. length of stack was available and this was suspended in the tank to a depth of 10 ft. below the level of the pulp being agitated. In 2 hr. clear solution was overflowing the top of the pipe, which was 2 ft. above the pulp level, a continuous overflow as from an artesian well.
In the hydrometallurgical treatment of ore pulps, the principle developed as described and applied will be found to be an important forward step. The cost of installation is small and the cost of operation is nothing.