Process for reducing metal ions in aqueous solutions

Abstract

Metal ions are reduced from a higher to a lower oxidation stage using ferrophosphorus as the reductant.

Description

The present invention relates to a process for reducing metal ions from a higher to a lower oxidation stage with the use of a reductant in an aqueous solution.

It is generally known that metals which in aqueous solution may exist in form of ions of various oxidation stages can be reduced to a lower oxidation stage by means of a reductant.

The reductants which are commonly used include iron, e.g. in form of iron turnings or borings, zinc, aluminum, sulfides, sulfites, hydrazine etc.

A disadvantage of these known reductants resides in the fact that their efficiency depends on the acidity of the respective solution and that the metals give rise to the evolution of hydrogen.

We have now unexpectedly found that these adverse effects do not appear when ferrophosphorus is used as the reductant.

In order to increase the reduction velocity, it is good practice to use the ferrophosphorus in form of particles with a size smaller than 100 .mu.m, preferably smaller than 10 .mu.m. It is also possible to increase the reaction velocity by increasing the reaction temperature to more than 60.degree. C.

A further technically beneficial effect resides in that the reduction can be carried out not only in an acid but also in a neutral or alkaline medium, even at a pH of 9 or more.

It is also an unexpected result which would not have been foreseen that ferrophosphorus permits a low redox potential of less than -150 mv to be established in crude phosphoric acid (45-50% P.sub.2 O.sub.5), and that hydrogen is practically not evolved within the acid range where commonly employed standard reductants, e.g. iron turnings or borings or hydroxymethanesulfinate, have a redox potential of +170 mv as the end point.

The reduction in accordance with this invention can be effected, for example, in a hydrochloric, sulfuric or phosphoric acid medium.

The process of this invention can be used for reducing higher valence metal ions, e.g. ions of iron, vanadium, chromium or uranium alone or in admixture with each other or further ions.

The following Examples are intended to illustrate the process of this invention which is naturally not limited thereto.

__________________________________________________________________________ Initial redox Conc. of metal ions Aqueous Reductant in Redox potential Conc. of reducedEx. potential in feed material solution wgt % after reduction metal ions__________________________________________________________________________1 +830 mV Fe.sup.3+ 0.331% HCl 12% Ferrophosphorus -130 mV Fe.sup.2+ 0.607% V.sup.5+ 426 ppm 0.8% Fe.sup.3+ 0.013% Cr.sup.6+ 389 ppm V.sup.5+ not detectable V.sup.4/3 439 ppm Cr.sup.3+ 457 ppm2 +618 mV V.sup.5+ 329 ppm H.sub.2 SO.sub.4 75% Ferrophosphorus +280 mV V.sup.5+ not detectable 0.8% V.sup.3/4 314 ppm3 +490 mV V.sup.5+ 640 ppm crude H.sub.3 PO.sub.4 Ferrophosphorus -180 mV V.sup.3+ 620 ppm Cr.sup.6/3 589 ppm 65% 0.8% V.sup.5+ not detectable Fe.sup.3+ 0.28% Cr.sup.6+ not detectable Fe.sup.2+ 0.61% Fe.sup.3+ < 0.003%4 -384 mV V.sup.5+ 200 ppm Na.sub.2 HPO.sub.4 Ferrophosphorus -780 mV Fe.sup.3+ V.sup.5+ Fe.sup.3+ 200 ppm 36% pH.about.8.9 0.05% < 10 .mu.m not detectable5 +475 mV Fe.sup.3+ 0.225% + crude H.sub.3 PO.sub.4 Na--hydroxymethane- +220 mV Fe.sup.3+ 0.013% V + Cr 66% sulfinate 0.8%__________________________________________________________________________

Claims

1. A process for reducing metal ions from a higher to a lower oxidation stage by means of a reductant in an aqueous solution, which comprises using ferrophosphorus as the reductant.

2. The process as claimed in claim 1, wherein the ferrophosphorus substantially consists of particles having a size of less than 100 .mu.m.

3. The process as claimed in claim 2, wherein the ferrophosphorus substantially consists of particles having a size of less than 10 .mu.m.

4. The process as claimed in claim 1, wherein the reduction is effected at a temperature higher than 60.degree. C.

5. The process as claimed in claim 1, wherein the reduction is effected in an acid, alkaline or neutral medium.