The effect of grinding on the acidulation of phosphate rock

An investigation was undertaken to determine the effect of grinding on the acidulation of phosphate rock. Bench-scale work was carried out in a one quart, laboratory ball mill made of stainless steel. The mill was equipped with a heating chamber so that the material could also be dried in the mill;The results from this work indicated that a normal superphosphate product suitable for commercial use could be obtained within an hour after the addition of the first acid. The product had low moisture and low free acid contents, and under suitable conditions, granular form. The mixing action of the balls resulted in good heat transfer during drying with little danger of overheating the product. Consequently, the drying air temperature had little effect on the product except in the rate of production. The reaction was sufficiently rapid in the mill so that no preliminary grinding period was necessary before the drying operation was started. Low acidulation ratios resulted in low conversions as in any normal superphosphate process. Low acid concentrations were more conducive to a rapid reaction between the rock and acid. Below an acid strength of about 55 percent, however, there was no further advantage in dilution;The favorable results of the bench-scale work led to the construction of a pilot plant to determine whether the process could be carried out on a larger scale and on a continuous basis. The process was built around a heated tube mill with a stainless steel lining and a feeding mechanism. Heating was indirect. The acid and rock were fed into one end of the mill and product taken from the other;Successful pilot plant operation showed that the bench-scale results could be duplicated on a larger scale and on a continuous or semicontinuous basis. Furthermore, it was demonstrated that the materials could be handled satisfactorily in the tube mill. Plugging and other difficulties were overcome;Finally, an economic comparison of the process with an equivalent, standard, normal superphosphate process indicated that the quick-curing process was favored. Fixed capital, production cost and working capital were estimated to be less than for a conventional, normal superphosphate process. The estimated return on investment for a conventional plant was 3.8 percent while the equivalent quick-curing plant gave an estimated 5.6 percent