Process for reducing iron in phosphoric acid solution

Iron contained in phosphoric acid solution is reduced from the trivalent to the bivalent oxidation stage. To this end, red phosphorus is used as the reductant and the reduction is effected in the presence of copper.

The present invention relates to a process for reducing iron from the 
trivalent to the bivalent oxidation stage with the use of a reducing 
agent, the iron being contained in phosphoric acid solutions, especially 
in wet-processed phosphoric acid. 
Various reasons make it often necessary for phosphoric acid which is to be 
processed and purified to contain considerable proportions of iron-(II). 
As to wet-processed phosphoric acid which is normally purified by 
subjecting it to neutralizing treatment, it is possible for it to be 
quantitatively freed from vanadium contained therein only if important 
proportions of iron-(II)-phosphate are co-precipitated therewith. To 
ensure recovery of uranium from wet-processed phosphoric acid by 
liquid/liquid-extraction, it is also often necessary for uranium-(VI) to 
be reduced to uranium-(IV), by means of iron-(II)-ions. 
Various processes which permit the relatively high concentration of 
iron-(III)-ions normally present in commercial wet-processed phosphoric 
acid to be reduced have already been described. 
In the process described in DE-PS No. 10 68 388, for example, metallic iron 
or zinc is used as a reductant. This is, however, not fully satisfactory 
inasmuch as the metal added combines with considerable quantities of 
P.sub.2 O.sub.5 whereby the yield of desirable P.sub.2 O.sub.5 naturally 
becomes significantly impaired. In addition to this, hydrogen is set free 
during the reaction and additional steps have to be taken as a safeguard 
against this. 
As disclosed in AT-PS No. 151 948 and DE-PS No. 15 67 619, it is also 
possible to use hydrogen sulfide and dithionites for reducing 
iron-(III)-ions. The use of H.sub.2 S makes it necessary to employ 
expensive equipment, such as degassing columns and pressure vessels. As a 
result of the fact that hydrogen sulfide is extremely toxic and that 
SO.sub.2 is set free in the event of dithionites being used, it is an 
imperative requirement to observe special care. In addition to this, 
dithionites are very hygroscopic and readily decomposable salts of which 
the technical uses are naturally very limited. 
It is therefore an object of the present invention to provide a process 
which avoids the adverse effects of the reduction methods described 
heretofore and permits iron, which is contained in phosphoric acid 
solutions, especially in wet-processed phosphoric acid, to be reduced in 
simple fashion from oxidation stage III to stage II. 
To this end, the invention unexpectedly provides a process wherein red 
phosphorus is used as the reductant and the reduction is carried out in 
the presence of copper, the latter being dissolved in the phosphoric acid 
solution or added thereto prior to effecting the reduction reaction. 
It is sufficient for the phosphoric acid solution to contain 5 up to 100 
ppm copper, which incidentally behaves in the fashion of a catalyst, 
although larger proportions could not be found to affect successful 
operation of the present process. 
It is preferable for the reduction to be effected at temperatures within 
the range 10.degree. to 90.degree. C., more preferably 60.degree. to 
80.degree. C. It is also good practice to contact the iron-containing 
phosphoric acid solution with red phosphorus over a period of 1 to 30 
minutes, shorter contact periods, within the limits specified, being 
selected for an increasing temperature and/or decreasing phosphoric acid 
concentration, and longer contact periods, within the limits specified, 
being inversely selected for a decreasing temperature and/or increasing 
phosphoric acid concentration. Useful starting material is more 
particularly wet-processed phosphoric acid with the iron and copper 
contaminants therein. The solution which is to be reduced should 
preferably be passed through a fixed bed formed of ground red phosphorus 
particles with a preferred size within the range 0.3 to 2 mm. 
In this manner, it is easily possible for the present process to be carried 
out continuously. To this end, the quantity of red phosphorus consumed for 
reduction is continuously replaced and the flow velocity of the solution 
through the bed of red phosphorus is selected so that solution and 
phosphorus remain in contact over the period at least necessary to ensure 
the reduction of iron. 
Although it is known from "Gmelins Handbuch der anorganischen Chemie", vol. 
phosphorus, part B, 8th edition, page 328, that it is possible for 
Fe-(III) in hydrochloric acid solution to be reduced to Fe-(II) by 
subjecting it to boiling treatment with red phosphorus, the present 
process cannot reasonably been said to have been obvious to the skilled 
artisan, for the following reason. While iron is easy to reduce by means 
of red phosphorus in mineral acids other than phosphoric acid, it is 
indeed not reducible in the latter under standard conditions. 
In the event of pure thermally-processed phosphoric acid being admixed with 
Fe-(III)-ions and red phosphorus, the iron is initially reduced to a minor 
extent only. Only in the presence of minor proportions of copper ions is 
the reduction accelerated catalytically. The phosphorus undergoes 
oxidation to phosphate and/or phosphite anions so that between 3 and 5 
mols iron-(II)-ions are produced per mol P. 
The present process inter alia offers the following technically beneficial 
effects: 
(a) High reduction efficiency (reduction of an average of 4 mols Fe-(III) 
per mol P). 
(b) High reactivity (this permits short contact times, use of relatively 
small-dimensioned apparatus, and operation at low temperature). 
(c) Purity of reductant (the acid becomes not admixed with contaminants. 
Resulting oxidation products are comprised of phosphate and phosphite 
ions. These have no adverse effects and are oxidizable to phosphoric acid 
during the processing treatment).