THE POTTER-DELPRAT PROCESS
The inventions which form the basis of this method were the work principally of two men working entirely independently Charles V. Potter and Guillaume D. Delprat. It is another in a long line of instances where two minds arrived at practically the same scientific point simultaneously. The ideas are so similar that litigation soon developed ; but, unlike many engaged in the development and application of metallurgical patents, these people soon wisely compromised. Potter had priority of patent date, and the slight differences between their processes, it must be admitted, would put a judge in a quandary. The basis of compromise was that Delprat was free to use the process in Australia, while the Potter Sulphide Company acquired all the patents and the right to license the use of the process. The amount asked for royalty in the past was 2s. 6d. per ton of concentrate, which amounts to about one shilling per ton of ore of the Broken Hill type.
After what has been said of the general theory of flotation processes, it is not necessary to give in extenso the whole of the principles set forth in Chapter IV. 'The Potter-Delprat is one of the processes making use of surface-tension phenomena. The act of flotation is accomplished by virtue of the natural adhesion of carbon-dioxide gas bubbles to particles of sulphide, and the result of the manipulation to which the material is subjected is the formation of a dense froth of bubbles and mineral, which floats at the surface of the apparatus. No substance other than acid is used in the process, as in most of the other processes for the purpose of enhancing the adhesion of gas films, but there may be organic substances in the ore which, upon the addition of acid, yield gummy organic compounds that selectively adhere to the ore. It is difficult to understand the working of this process without some such theory to account for the gummosity of the concentrate. It is worth noting that this process is the only one that secures flotation by the buoyancy of carbon dioxide gas, all the others claiming to use ir alone as the active flotation gas. To differentiate from the other flotation processes this one may be called an ' acid-flotation process,' or more correctly a ' carbon-dioxide gas- flotation process.' The gas is generated in the pulp by the action of hot dilute sulphuric acid, or acid salt-cake, on the carbonates which occur naturally in the gangue.
This process, as in use at the Broken Hill Proprietary mine, treats the tailing from the lead-concentrating mills. The tailing assays roughly 20% zinc, 6% lead, and 6 oz. silver. The gangue is a mixture of quartz, rhodonite, and garnet, with about 3% of calcium and iron carbonates. The last-named are of prime importance in the process. The material to be treated is taken from the dumps and re-crushed to about 4o-mesh in grinding-pans. After removal of the water used in crushing, the material is hauled to the treatment plant, where it is delivered into bins (see Fig. 22)*.
From these bins it is fed automatically into flotation vats, where the separation takes place. These vats are inverted wooden pyramids (see Fig. 23), lined with sheet lead. It will be noticed that these vats have two pockets at the bottom one with and one without an outlet. Iron pipes introduce the solution into the vat close to the bottom, and so aid in stirring the material. One of these pipes discharges into the blind pocket A, and the other into the pocket with an outlet. The blind pocket collects all heavy stones, bolts, nails, etc., which otherwise would obstruct the outlet. A 2 to 3% solution of sulphuric acid is introduced into the vat at a temperature of 82 C., but as the quantity introduced is more than can run through the bottom outlet, a large quantity passes over the top by an overflow specially arranged for this purpose. The tailing is then fed mechanically from the bin into the vat, above the blind pocket. As the material falls into the pocket it is washed over the partition B, which separates the two pockets, and the gangue passes away through the outlet. On its way down the vat, however, the acid acts on the carbonates contained in the ore, and produces bubbles of carbon dioxide gas. This gas shows a selective action, and adheres to the particles of blende and galena, but not to the particles of gangue. The adhering bubbles cause the particles of blende and galena to float ; they come to the surface in the form of a dense froth, and are washed away by the current over the lip C.
The overflows of all the vats pass to one launder, which discharges into a set of wooden tanks, where on cooling the particles of blende and galena lose their bubbles of gas, and fall to the bottom.
When the tank is full, another tank is filled while the contents of the first is being drained, washed with clean water, drained again, and finally loaded into railway trucks. A flow-sheet for the process with the re-treatment of the concentrate is shown in Fig. 24.
It is found that each flotation vat can treat from 10 to 12 tons of material per hour, but three vats are generally used for this amount. The speed depends greatly on the fineness of the material. About 200 tons of concentrate is the daily product of three units, but as much as 300 tons per 24 hours has been produced by each flotation vat. Another form of vat for the same process is shown in Fig. 25.
The bottom outflow from the vats carries all the gangue with it. The solution and gangue together fall on a 3-ft. india-rubber travelling belt, with side rollers to make it into a trough. The belt runs uphill, by which means the solution is separated from the tailing and run into a sump, while the solids settle on the belt. On the arrival of the tailing at the end pulley of the belt it is sufficiently dry to be used as filling for the underground workings of the mine.
The temperature most suitable is 80 C. ; in order tor each this, superheated steam at 260 C. is introduced into the solution vats, which stand on a high enough level to command the whole plant. Air-lifts are used to return the spent solution from the drained tailing and concentrate to the solution vat. One treatment only is given the material in this process, and in this regard the process has an advantage over several of the others which require repeated treatments to secure commercial recoveries.
As there are no moving parts in the plant, except the mechanical feeders and travelling belt, the cost of repairs is small. Two engines of 10 h.p. each supply all the power required, except that required for the air jets.
It was found in the experiments conducted by the Zinc Corporation that the addition of some substance that would produce a coagulated appearance to the pulp was of material benefit. Among other substances tried was resin, but the best substances for the purpose of producing this effect would be oils. This brings us back to the question of the enhancement of the inherent quality of the sulphide particles to adhere to gas films by coating the sulphide particles with films of oil. and shows the near relationship of all the flotation processes. Recent Potter patents have sought to cover this idea. It has been said that the present users of this process do not use any oil in their operations, and when I visited the plant on the Proprietary mine none was being used. At first glance it would appear that the concentrate from this process would be readily amenable to re-treatment by concentrating tables ; but the Zinc Corporation found that the concentrate had a ' gummy ' or sticky ' quality, which rendered such an operation extremely difficult. This is surprising, as the sulphuric acid or any of the salts derived from its use are apparently sufficiently soluble in the table water to render the sulphides loose and sandy. The explanation may be that the hot sulphuric acid forms with the organic matter, which is always present in tailing heaps, insoluble ' gummy ' or ' sticky ' compounds which exercise a preferential adhesion to the sulphides.
The apparatus and equipment of this method has arrived at a state of finality that some of the others have not reached, the reason being the great simplicity of the idea involved. The cost of the plant is obviously low. The equipment is such that it can be built on the spot from easily procured materials. Ordinary timber, with sheet lead for a lining of the flotation-boxes, is all that is needed. The cost per unit should not exceed 200, including installation.
In the early development of the process, difficulty was encountered in finding a machine or flotation-box that would save the concentrate, the trouble being that the bubbles would break and drop their load. This was overcome by Goyder & Laughton in a machine having a series of traps. Later, when it was discovered that a froth prevented the bubbles breaking, it was an easy step to see that if the flotation-boxes were crowded to a high capacity the froth itself, if produced in sufficient quantity, would act as a blanket, and so prevent the troublesome breaking of the bubbles.