The present invention relates to a new and useful method of preparing organophosphorous compounds. More particularly, the present invention relates to a method of preparing dialkyldithiophosphinates.
As is known, middling particles which result from either coarse grinding or from intimate association of the component minerals which make up an ore and frequently exist even after fine grinding are composed of two or more mineral species. In general, such polyphase particles are generally more difficult to concentrate by flotation than free mineral grains. In some free-milling ores, the values are readily separated from the gangue by crushing and grinding to permit the application of flotation techniques. In other ores a middling treatment problem is presented. The extractive metallurgist is thus compelled to compromise between obtaining a desired high recovery of mineral values and the grade of the concentrate produced. High recoveries in concentrates of low grade entail high treatment costs in subsequent upgrading and refining operations, which may force the rejection of larger percentages of the contained metal values so as to obtain a concentrate which is salable or amenable to further treatments. Further, in many cases, the middling particles are not recovered, even though amenable to standard refining processes, and thus represent high losses of the desired mineral values.
Dialkyldithiophosphinates are effective promotors in solving the middling treatment problem of sulfide ores (See U.S. Pat. No. 3,355,017). These promotors were reported therein to be more effective than known collectors usually employed in floating coarse mineral particles, particularly particles in the plus 150 Tyler mesh range.
Dialkyldithiophosphinates useful as sulfide mineral promoters conventionally are prepared by reacting dialkylphosphine with a mixture of water, sulfur and base, such as sodium hydroxide. Although the prior art discloses reaction temperatures up to about 100.degree. C., commercial considerations dictate that temperatures less than 30.degree. C. be employed (see U.S. Pat. No. 3,238,248). These low temperature reactions have resulted in yields in the range of about 75-80% and are extremely slow due to the depressed temperature at which the reaction proceeds. Elevation of the temperature of the conventional reaction produces a side reaction between the sodium hydroxide and the sulfur resulting in undesirable alkali sulfides. The presence of alkali sulfide has several detremental effects on the product and its application as a flotation promoter. In the product, alkali sulfides represent a yield loss of alkali and sulfur, produce an off-color solution and create a strongly disagreeable odor. In the application, alkali sulfides depress sulfide minerals during the flotation process and lead to a loss in recovery of those minerals.
Accordingly, there exists the need for an improved process for preparing dialkyldithiophosphinates. Such a new process should be capable of proceeding rapidly and yet maintain low levels of alkali sulfides so as to provide the highest yields and most effective flotation promoters.