THE DE BAVAY PROCESS

THE DE BAVAY PROCESS.

This process is the result of the researches of A. J. De Bavay. The patents are owned by the De Bavay Sulphide Process Company, which lets the use of the process on a royalty. By a recent combination with the Minerals Separation, Ltd., these patents have been merged with those of the latter company.

De Bavay, as reflected in his early patents, was developing a process in which the surface tension of the liquid, without the extraneous aids of oil and acid, was the idea involved. The process works on the principle of the ' greased needle,' and there is none of the frothing which characterizes the Potter-Delprat, Minerals Separation, and Elmore processes. During the operation of the process there is simply a film of sulphides floating just as indicated in Fig. 10, Chapter IV. De Bavay, in the course of the development of his process, recognized the value of oil and acid as accelerating factors in surface-tension processes, and now uses oil for the same purpose as the others. The similarity in many ways of the early De Bavay patents to that of Bradford is apparent.

The first plant was erected on the mine of the North Broken Hill Mining company. This plant was altered and re-modelled many times while the process was being developed, and no complete story of these will probably ever be written, nor is it necessary. The plant did much good work all through the period of development from 1905 to 1908, when a measure of finality was reached in the methods. The process, as worked here in the later stages, is as follows : The tailing from the lead mill is first de-slimed, leaving practically a clean sandy product. The slime is discarded. The clean sand, crushed to about 4O-mesh, is then fed into a mixing device, known locally as the A.Z. agitator, and mixed with cold acid solution of 0.19 strength. A photograph of the machine is shown in Fig. 28. This agitator is provided with a central tube worked with a rack and pinion for raising and lowering the upper end of the tube. The proportion of acid solution added is four or five of solution to one of material. Thorough agitation is imparted to the mixture for a considerable time. The agitator is run at high speed for the purpose, as explained by the millman, of cleaning the surfaces of the mineral particles. This operation being completed, the speed of the agitator is reduced and the centre tube lowered so as to draw off the bulk of the solution, upon which is floating some metallic scum. When the excess of the solution has been removed, the centre tube is raised, clean wash-water added, and agitation renewed at high speed. After thorough washing, the speed of the agitator is again reduced, the excess of liquor removed as before, and a second washing effected to free the tailing of any acidity remaining. The scum of metallic particles drawn off is treated on vanners for the recovery of lead concentrate. The cleaned tailing is next conveyed to the ' oiling ' vat. In this vat, which is provided with an agitator, the material is mixed with water, to which a definite proportion of oil has been added, and the whole mass thoroughly agitated to bring the metallic particles into contact with the oil. The oil is composed of one part of castor oil and four parts of low-grade kerosene ; two to three pounds of oil to one ton of ore is used, and also some chlorine gas. From this oil-mixer the oiled pulp is elevated by means of a montejus and compressed air to the first tier of eighty-eight flotation cones, one of which is shown in section in Fig. 29, and in plan in Fig. 30 The material is fed to the cone at the top, and runs down over a series of corrugations to the water level A, where the sulphides float at the surface of the water (Fig. 30), and pass over the lip B into the launder C. The tailing, on the other hand, sinks through the surface of the water at B, and passes down the box and out at D. It is then fed to another similar cone and box with some further recovery of sulphides, and so on to a third and fourth cone and box. Throughout this manipulation, including the acid washing, the oiling, the raising with compressed air, and the flowing over the corrugated cone, the sulphide particles are repeatedly aerated, and as a result, especially after the oiling, take up their adhesive air films and float at B in exactly the manner shown in Fig. 2, Chapter IV. The function of the chlorine may be that of gasification of the sulphide particles, or, through the formation of hydrochloric acid, it might indirectly assist by acidulating the solution ; or it may have some other obscure and secondary uses. The subjecting of the oiled pulp to compressed air may be an essential part of the operation. If so, the analogy to other oil-and-gas processes is close.

NORTH MINE PLANT. The De Bavay plant on this mine was erected in 1905. and began operation in 1906. The first unit was estimated to treat 10 tons per day, and this was increased during the latter year until the plant was treating 300 tons per week. During 1907 more or less experimental work was done, and by the close of the year 65,000 had been expended on the plant. It was then considered capable of treating 1,700 tons per week, and during the year 17,000 tons of tailing was treated. In 1908 the capacity of the plant was further augmented, and a total of 74,200 tons was treated during the year. The 22,590 tons of concentrate produced this year averaged 47.5% zinc, 7% lead, and 6oz. silver per ton. This was the best concentrate produced by any flotation process at Broken Hill, and in this regard the De Bavay process continues to hold the lead. This factor is due to the careful de-sliming of the material before treatment. In 1909, 30,000 tons of concentrate was produced, assaying 48.5% zinc, 6.4% lead, and 5-9OZ. silver. This high-grade concentrate was a noteworthy product. During the first half of 1910 the plant continued operations, but in August of that year it was closed because of the new phase of operations under the ownership of the Amalgamated Zinc, Ltd.

AMALGAMATED ZINC. This company took over the Australian rights to the De Bavay process, the mill on the North mine, and all the tailing contracts of the De Bavay Treatment Company. It purchased further dumps, and in 1909 began the construction of a large new mill at the extreme north end of the Broken Hill district. The first section of the new mill was started in March, 1910 ; the second section in August, and the third in October. The capacity of the whole plant is 1,800 tons per day. A re-treatment plant, consisting of tables, was installed, and the whole or a part of the flotation concentrate is treated thereby for the recovery of lead concentrate.

The tailings which are available for treatment by this plant are as follows : From the South mine 150,000 tons from the dump, and the current production of the lead-concentrating mill for ten years, a total of 1,750,000 tons, assaying 19% zinc, 3.5% lead, and 3-5oz. silver. This material contains 5 to 6% of carbonates, and gave the other processes which tried to treat it no end of trouble. From the North mine 100,000 tons of dump tailing, and the current production for ten years, a total of 1,700,000 tons, assaying 18.5% zinc, 4% lead, and 5oz. silver. From the Block 10 mine 370,000 tons of dump tailing, assaying 18.7% zinc, 4.8% lead, and 8.7oz. silver. In comparing this plant and process with others it should be kept in mind that this material is the cream of the district. It is of high grade, and free from slime, and for this reason is the most easily treated of any zinkiferous material now being treated.

Owing to the small capacity of each of the cone units, this plant covers a large area. The cost of the plant exceeds, per unit of tonnage, that of any of the other flotation plants, being 200,000, or 111 per ton d'aily capacity. Some auxiliary works were included in these figures, such as sulphuric acid plant, but it is certain that the De Bavay plant proper cost in excess of 100 per ton daily capacity for a plant of 1,800 tons per day.

RECOVERY.

If one of the best month's operations is taken and the content of the material treated is calculated from the figures given in the prospectuses and Government reports and from what is known otherwise of the district, the recovery in the De Bavay process works out as given in the following tables. We are, however, in the dark as to how much material was discarded as slime, which generally is considerably richer than the original material :