Recycling Zinc Phosphate Sludge

he cost of sludge disposal can contribute significantly to the cost of product manufacturing. An opportunity exists to eliminate this cost of land disposal of sludge resulting from the zinc phosphate pretreatment process. In addition, the cost of the phosphating process may be reduced by reclaiming this sludge to a zinc-phosphate replenishment solution and other useful products. For those not familiar with phosphating processes, a one-to-one relationship exists between the amount of phosphate deposited on the metal substrate and the amount of sludge generated. For many automotive and industrial processes, this sludge consists of iron, zinc, manganese, nickel, phosphate, and oil. The products, which result from the recycling process, are oil, which may be used as a heating fuel; a zinc phosphate concentrate; and a cake of iron phosphate. If desired, this iron phosphate can be further processed to an iron phosphate concentrate and iron oxide, which may be used as a pigment for paint. Several methods for reclaiming zinc containing sludge have been proposed previously. Certain companies offer reclamation services of zinc-bearing waste through a high-temperature metal recovery method however, the relatively low level of zinc contained in this sludge makes this process of little cost benefit for zinc phosphate sludge. On the contrary, the recovery process described in this article provides a benefit to the product manufacturer who can use the zinc phosphate replenishment solution reclaimed from the zinc phosphate sludge. U.S. Patent 5,376,342 was granted for a process very similar to the one described below; however, attempts to reproduce the patent examples resulted in a pH of 0.8, which is below the specified level of 1.5 to 2.5. If the amount of acid was reduced to achieve the minimum pH specified (pH IS) , the sludge slurry became extremely thixotropic and unworkable. Additionally, no mention is made of the removal of the oil prior to digestion of the sludge. This is an essential step, as the oil is typically 20 to 25% by weight of the sludge, and total digestion of the sludge is not possible without first removing this oil component. Additional patents have been issued for various other methods of reclaiming zinc phosphate sludge. U.S. Patents 4,986,977 and 3,653,875, and JP 03134181 discuss the recovery of products after an initial alkaline digestion. Patent EP 608874-A1 discusses the recovery of metals from such sludge using a strong acid cation exchange resin. US. Patent 5,273,667 discusses the conversion of the phosphate sludge to a lubricant additive. Finally, the U.S. National Technical Information Service, Public Bulletin # 211,933 (1972) describes a solvent extraction procedure for recovery of sodium phosphate, zinc, and iron. Each of these methods fails to discuss the removal of the oil present, and are of limited use for the production of products useful to the product manufacturer. The specific technical aspects of the process to be described may be segregated into three phases. Phase I is the separation of oil from the sludge. Phase I1 is the extraction of zinc, manganese, and nickel from the sludge following digestion with phosphoric acid. Phase I11 is the conversion of the iron phosphate residue from Phase I1 to pigment-quality iron oxide, and sodium phosphate that may be acidified to an iron phosphate concentrate.