Continuous Process for the Removal of Chromium From Waste Waters and Valorization of the Recovered Chromium

This invention relates to a continuous process for the removal of chromium contained in waste waters, using it for valorising the same in that it is recycled to the process itself or it is directed to the production of other chromium compounds.

The problem of removing chromium from the processing waste waters, particularly from metallurgical processings, or tanneries, or processings of organic products using bichromate is well known. This problem presents several aspects: it is of essential importance from the ecological standpoint owing to toxicity of chromium even in small amounts; it is of considerable economical importance owing to the value of the discarded product; and at the same time its recovery involves technological difficulties, particularly for the difficult filtrability of the precipitate of trivalent chromium hydroxide, which is generally obtained.

Therefore, many researches have been carried out and realisations proposed in this field.

Chromium content in waste waters is about 1% (as Cr.sub.2 O.sub.3) in waters from tanneries, while even higher percentages are found in the residues of oxidation of organic material operations by bichromates.

Processes of recovery of trivalent chromium with alkaline agents of various nature are well known from the literature.

At a handicraftsman level, chromium is removed from the diluted chromium containing solutions by precipitation of chromium hydroxide by means of various alkaline agents, such as calcium oxide, sodium carbonate and bicarbonate, magnesium oxide and the like.

However, the slurries thus obtained which always have a low percentage content of solids, lead to filtering cakes having a high content of hydration water and accordingly being relatively poor in chromium, and of such a structure as to require long separation times.

Precipitation systems by the above mentioned alkaline agents have been the research object of an extensive study of J. C. de Wijs (see "Das Leder" (The Leather) 28 (1977) 1, pages 1-8) "Precipitation and reuse of trivalent chromium present in the tannery waste waters," in order to set up a simple and economical process for the recovery of such chromium residues and removing the same from the waters.

It has been found that the precipitation carried out with magnesium oxide leads to a chromium hydroxide higher in chromium content and having the best settling characteristics. Settling or sedimentation rates of 250 mm/hour are achieved and wet precipitates at about 10% chromium oxide (Cr.sub.2 O.sub.3) are obtained. The preferred pH for the precipitation of Cr.sub.2 O.sub.3 from the liquids to be treated is about 9 and it allows an almost quantitative precipitation of chromium without formation of chromites.

However, this process requires unduly long operation times (for example, 3 hours for the treatment of 300 liters of tannery waste liquid with an excess of 30% MgO over the theoretical value and an overnight settling, this leading to a filtered liquid containing 4.6 mg/l chromium.

Another recent proposal for the depuration of chromium-containing waste waters from tannery is described in the U.S. Pat. No. 3,950,131, issued Apr. 13, 1976 to H. H. Young, where an "automatic continuous process for regenerating chromium hydroxide from chromium containing liquids and the chromium recycle to tanning" is claimed.

The removal of chromium up to 1 mg/l Cr is obtained and the necessity of continuous analyses, as generally occurs, is failing.

The precipitation is carried out with alkaline agents (preferably sodium or ammonium hydroxides) in such an amount to raise the pH of the chromium containing solutions to at least 8.0-9.5, so as to obtain a tixotropic suspension of precipitated chromium hydroxide. The consistency of the suspension, thus obtained, inhibits the use of common press or vacuum filters, where it would accumulate making the filtering operation unduly long. Therefore, one has recourse to a particular multi-segment filter, on which a pressure higher than about 80 psi (about 5.62 kg/sq.cm) is exerted. Under continous stirring, the cake gradually passes on these segments and thickens. Thus, water is removed to obtain a cake containing 14-15% chromium hydroxide; the latter is discharged and acidified to solve it and to set it to the degree of basicity apt to the recycle to tanning.

Evidently, this process does not yet solve in this way the problem of filterability. A final cake containing up to 15% Cr(OH).sub.3, corresponding to about 10% Cr.sub.2 O.sub.3 is so obtained; the times required are still unduly long; filtering is bound to the use of a specific filter, which makes very troublesome the filtering operation, requires pressures higher than 80 psi and is accordingly expensive. Moreover, the operation requires highly diluted solutions and long times of decantation (see Table 1 of said patent). The treated and filtered liquid contains 0.2-1.6 mg/l Cr. The only foreseen outlet for the recovered chromium is a recycle to tannery.

In the course of researches made to obviate the disadvantages pointed out above and to enable an improved utilization of the recovered chromium, it was surprisingly discovered that by adding--either preliminary or at the precipitation stage--to the liquid containing the chromium to be precipitated a suitable amount of acetic ion CH.sub.3 CO.sub.2.sup.-, from acetic acid and derivatives thereof, such as acetates, particularly alkaline acetates, chromium hydroxide precipitates are obtained which, maintaining the best physical characteristics, are distinguished by the highest solubility in mineral acids.

This offers the great advantage of allowing an easy preparation, from this chromium hydroxide of other highly pure chromium compounds or also of chromium tanning products of desired basicity.

At the present state of the research, such a phenomenon has not yet been sufficiently explained.

However, it was found that some parameters characterizing the reaction conditions, such as Cr.sub.2 O.sub.3, concentration, pH and percentage of CH.sub.3 CO.sub.2.sup.-, ion temperature, and staying time in the reactor, would contribute to give rise to it. Particularly, as regards the concentration of the acetic acid to be introduced into the solution to be treated, such a concentration may vary from 0.3% by weight and over, within limits of process economicity.

It was also observed that the ratio between the concentrations of CH.sub.3 CO.sub.2.sup.- and Cr.sub.2 O.sub.3, which allows to realize the present invention, is not a constant, but a function of the concentration of the liquid to be treated. For example, for initial concentrations in the order of 1% Cr.sub.2 O.sub.3, the percentage of acetic ion required to obtain a soluble precipitate is higher. However, it was observed that, if the solution pretreated with acetic ion is allowed to age for some time, the percentage of acetic ion required for such a purpose will decrease.

The primary object of the present invention is therefore to provide a continuous hot process to recovery, under a valuable form, the chromium present in waste liquids, particularly tannery waters, which process allows to obtain a complete and quick precipitation of the chromium hydroxide, while obtaining a chromium hydroxide very highly soluble in acids.

The present invention also relates to the acid soluble chromium hydroxide precipitate, thus obtained.

The method according to the present invention is essentially carried out in two stages:

1. Pretreatment of waste waters with acetic acid or alkaline acetates; and

2. continuous hot precipitation by alkaline hydroxides, at atmospheric pressure, of chromium hydroxide in a quite filterable form, which may be termed as "sandy" for distinguishing it from the tixotropic gelatinous form obtained in the above mentioned patent, and which is soluble in acids. The precipitate thus obtained is easily separable on an ordinary filter, such as a press filter or a vacuum filter and has a content of 40% Cr.sub.2 O.sub.3, while the discharged waters have a content lower than 2 mg/l Cr.

The process can also be carried out in a single stage by adding the CH.sub.3 CO.sub.2.sup.- ion during the precipitation with alkali.

Owing to its high solubility in acids, also in weak acids, such a precipitate distinguishes it self from those obtained by hitherto known processes, accordingly allowing to directly obtain chromium tanning liquids of desired basicity and titer, quite similar to the tanning materials available on the market, or chromium salts for different purposes.

Generally, alkaline hydroxides (sodium, potassium, ammonium) as well as magnesium oxide may be used as precipitating agents.

The process of the present invention is carried out in a reactor, maintained at a temperature in the range from 40.degree. C. to boiling point at atmospheric pressure, by continously introducing the waste waters (such as tannery waste waters) containing from 0.1% (1000 ppm) to about 10% of Cr.sub.2 O.sub.3, and an alkaline hydroxide, preferably sodium or potassium hydroxide, and automatically setting the liquid flow rate, so as to establish in the reaction medium a pH not lower than 6.5, preferably between 7 and 10, and more particularly about 7.5. Thus, a precipitate of Cr(OH).sub.3 is obtained in a (sandy) form, which immediately and completely filters on any ordinary filter. This analytical content of Cr.sub.2 O.sub.3 ranges between 40 and 43% related to the wet cake.

Unlike the cakes obtained by known methods, the so obtained cake is perfectly soluble in acids (higher than 99%).

The obtained solution, adjusting with sulphuric acid the basicity as desired (33-50 Sch), may be intended for tannery.

Otherwise, the washed cake may be supplied to the preparation of other chromium salts.

Finally, the chromium hydroxide can be also calcined to chromium oxide for metallurgical purpose.

It should be pointed out that the pre-treatment with acetic acid could also take place within the reactor.

The process according to the present invention is continuously carried out and on industrial base, and--by allowing the removal of chromium below 2 mg/l from the waste waters containing the same--also solves the ecological problem of the disposal of these liquids.

For a better illustration of the foregoing description, some examples will now be given, examples which should be considered only as illustrative and unrestrictive of the present invention.

A solution, prepared in laboratory for comparison, containing 5.27% Cr.sub.2 O.sub.3, was continously treated with 25% sodium hydrate at 95.degree. C. up to pH 7.6 in reactor having a useful volume of 7 liters, at atmospheric pressure, by automatically setting the flow rate of the liquid to be treated or of the precipitating agent to allow an average reaction time of about 60 minutes.

Thus, a slurry containing precipitated chromium hydroxide was obtained, having such filterability characteristics as to enable filtering on buckner (.phi.=150 mm) of 4 kg of slurry in less than 1 minute, at a vacuum of 300 mm Hg, using a paper disc as filter.

The analysis of the obtained cake gave:

Cr.sub.2 O.sub.3 %=33.38 on wet cake

H.sub.2 O%=43.0 on the cake at 110.degree. C.

Cr.sub.2 O.sub.3 %=58.56 on dry at 110.degree. C.

100 g of this wet cake were treated for 3 hours at the boiling point with such an amount (44 g) of commercial 98% H.sub.2 SO.sub.4 to obtain a solution of a tanning material having a basicity of 33 Sch and 113 g water.

About 30 g solid remain undissolved in the final slurry, so that the yield of the final dissolution corresponds to about 70% and the solution obtained is not suitable for use as a tanning material.

A waste liquid containing in solution 5.27% by weight Cr.sub.2 O.sub.3 was added with 80% acetic acid to obtain a 3.30% by weight solution; then the chromium hydroxide was precipitated from the same in a 7 liter reactor, maintained under stirring, at 95.degree. C. at a constant pH of 7.8, with 25% sodium hydrate, adjusting the flow rate to have an average reaction time of 60 minutes.

After filtering on buckner, the characteristics of the obtained chromium hydroxide were as follows:

Cr.sub.2 O.sub.3 by weight=41.52 on wet cake

H.sub.2 O% by weight=29.0 on wet cake

Cr.sub.2 O.sub.3 % by weight=58.47 on dry cake

This cake was then treated with 98% H.sub.2 SO.sub.4 and water in the following ratios:

cake=400 g

98% H.sub.2 SO.sub.4 =172 g

H.sub.2 O=649 g

in less than 40 minutes, at the boiling temperature; a tanning liquid was so obtained having the following characteristics:

Cr.sub.2 O.sub.3 % by weight=14.17

SO.sub.3 % by weight=11.58

CH.sub.3 COOH% by weight=0.26

Na.sub.2 SO.sub.4 % by weight=2.90

Sch=48.2

Any insoluble residue is absent, that is the dissolution yield is higher than 99%.

From a liquid, containing 1% Cr.sub.2 O.sub.3 by weight and 0.6% CH.sub.3 COOH by weight, treated under the conditions of Example 2, at 80.degree. C., a cake of chromium hydroxide having the following characteristics was obtained:

Cr.sub.2 O.sub.3 by weight=23,23 on wet cake

H.sub.2 O% by weight=61.50 on wet cake

Cr.sub.2 O.sub.3 % by weight=60.34 on dry cake to 110.degree. C.

This cake, treated with the stoichiometrical amount of conc. sulphuric acid and with an amount of water to lead to a titer of 13.5% in Cr.sub.2 O.sub.3 provided within a time less than 60 minutes a tanning liquid with a basicity of 35 Sch. In this case, the dissolution yield was higher than 99%.

From the foregoing the considerable advantages of the process according to the present invention over the prior art are readily apparent, which advantages being essentially summarized as follows:

possibility of directly using waste waters even much diluted (0.1%);

easiness and rapidity of a separation and isolation of the chromium hydroxide in an easily filterable form and provision of a precipitate perfectly soluble in acids;

processing at atmospheric pressure;

short working times;

saving in overall size, machinery and labour;

power saving in filtering;

high percentage of the waste chromium recovered (higher than 99.9%).

chromium recovery for recycle to tanning operation; and

chromium recovery in the form of a valuable chromium compound.