Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Patent
1993-04-19
1994-09-27
Spear, Frank
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
210197, B01D 1104
Patent
active
053505174
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a process for recycling the phosphating sludge which accumulates in the Zn phosphating of metal surfaces, more particularly steel surfaces, by treatment of the phosphating sludge with mineral acid, removal of constituents and their reuse in the preparation of phosphating solutions.
STATEMENT OF RELATED ART
Before lacquering, metal workpieces (mainly steel sheets) are subjected to a number of pretreatments, comprising cleaning and degreasing steps and subsequent phosphating. In the phosphating process, a conversion layer consisting, for example, of Zn phosphate (hopeite) or Zn/Fe phosphate (phosphophyllite), depending on the external conditions, is produced on the metal surface by immersion in a zinc phosphate bath which may additionally contain other dissolved metals, such as Ca, Mn, Ni, and the like. The conversion layer formed serves on the one hand as protection against corrosion and on the other hand as a basis for better lacquer adhesion. Phosphate coatings are also applied as lubricants for the cold forming of workpieces.
In the phosphating process, small quantities of iron from the steel surface are also inevitably dissolved. To keep the Fe concentration constant, an oxidizing agent (accelerator) is added to the phosphating bath to oxidize Fe.sup.2+ to Fe.sup.3+, which then precipitates from the phosphating solution as iron phosphate sludge. Other constituents of the phosphating bath are entrained in the process.
After drying at 110.degree. C., the phosphating sludge generally has the following composition in % by weight: iron about 20, zinc about 10, manganese about 1-3, nickel <1, phosphate 50-55.
It is known that the phosphating sludge can be processed and utilized in various ways, namely: as an additive for coating compositions which also have certain corrosion-inhibiting properties; for solidifying lacquer coagulates from spray lacquering; by pyrometallurgical processing, for example by reduction sintering processes, for recovering the metals zinc, iron and nickel in addition to sodium phosphate; and by wet-chemical processing by leaching constituents with acids and bases and reusing them in the preparation of phosphating solutions; and, finally, by digestion with mineral acid, removal of constituents and their reuse.
DE-OS 26 29 776 describes a process for recycling zinc phosphating solutions for steel in which zinc and phosphate ions are leached from the sludge with mineral acid at a pH value of 1.4 to 1.6 and the acidic liquor obtained is used as a phosphating solution (see claims 1, 3, 4 and 12). The phosphate is leached from the undissolved residue containing iron and phosphate ions with alkali so that, ultimately, an iron-containing residue remains behind. The phosphate in the alkaline liquor is precipitated as zinc phosphate and may also be used for the preparation of phosphating solutions. This process has the following disadvantage: the leaching with mineral acid is incomplete. Alkali metal ions are entrained in the subsequent leaching of the FePO.sub.4 filter cake, increase in concentration and make repeated recycling impossible.
Powell, Smith and Cochran describe a process for liquid/liquid extraction from a sludge digestion solution in hydrochloric acid. The iron is separated as FeCl.sub.3 with isopropyl ether. Zinc is then extracted with di-2-ethylhexyl phosphoric acid (DEHPA) in kerosine. Finally, the phosphoric acid is recovered by crystallizing out the raffinate as Na.sub.3 PO.sub.4. The constituents are recycled. This known process has the following disadvantage: the liquid/liquid extraction uses two different extractants and is relatively expensive. After crystallization of the Na.sub.3 PO.sub.4, Na.sup.+ Cl.sup.- and Mn and Ni ions remain in the raffinate; recycling is not complete.
DESCRIPTION OF THE INVENTION
1. Object of the Invention
The problem addressed by the present invention was largely to avoid the disadvantages mentioned above and to find a simple and complete method of separating the phosphating sludge int
REFERENCES:
patent: 3989607 (1976-11-01), Bush et al.
patent: 5198020 (1993-03-01), Fennemann et al.
Buchmeier Willi
Roland Wolf A.
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Spear Frank
Szoke Ernest G.
Wisdom, Jr. Norvell E.
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