GB2042489A

GB2042489A - Production of a substantially iron-free chromium (III) compound

Info

Publication number: GB2042489A
Application number: GB8003757A
Authority: GB
Original assignee: Outokumpu Oyj
Current assignee: Outokumpu Oyj
Priority date: 1979-02-06
Filing date: 1980-02-05
Publication date: 1980-09-24
Also published as: FI60381C; CA1151845A; FI60381B; SE8000918L; DE3004174A1; FI790391A; BR8000616A; GR66007B; AU5501580A; SE435492B; ZA80505B; GB2042489B; AT373857B; TR20612A; ATA55480A

Abstract

A chromium (III) compound containing very little iron compound contaminant is selectively precipitated from a mixture of iron and chromium compounds in aqueous acid solution by use of an elevated temperature and pressure, the acid aqueous solution of iron and chromium compounds containing at least 2 g/l of bivalent chromium.

Description

SPECIFICATION Process for the production of a substantially ironfree chromium (III) compound The present invention relates to a process for the precipitation, at an elevated temperature and pressure, of a substantially iron-free chromium (III) compound from an acidic aqueous solution which contains iron and chromium.

Chromium chemicals are currently produced industrially only from chromium or ferrochromium means of oxidative heating. Alkali metal chromate is separated from the product by dissolving the product in water, and after several purification and washing steps the chromate is converted to the dichromate by reaction with the sodium bisulfate derived from chromium trioxide production, with sulfuric acid, with carbon dioxide or the like. Also other chromium salts, e.g. ammonium, zinc and lead chromates, chromium trioxide, Cr203, Cr(OH)C12, CrO2, etc., and metallic chromium are obtained from the purified alkali metal chromate solution directly or by a suitable treatment of the alkali metal dichromate.

The prior known processes are both complicated and expensive. Since hexavalent chromium is toxic, they have always involved environmental and health hazards. The object of the present invention is to eliminate the above disadvantages and to provide a process for the precipitation, at an elevated temperature and pressure, of a substantially iron-free chromium (III) compound from an acid aqueous solution which contains iron and chromium, without the co-precipitation of iron. The raw material used in the process according to the invention can be chromium-poor or chromium-rich chromite, ferrochromium, chromium-bearing residues, etc.

The present invention provides a process for preparation of substantially iron-free chromium (Ill) compound(s) from an acidic aqueous solution which contains ions of iron and of trivalent chromium which process comprises precipitating the chromium (III) compound(s) at an elevated temperature and pressure from an acidic aqueous solution which also contains at least 2 grams per litre of ionic bivalent chromium and separating the precipitate from the mother liquor of the precipitation.

Percentages in this Specification, except yields, are by weight. Weight/volume proportions are expressed in grams per litre.

In the process according to the invention, the co-precipitation of iron is prevented by carrying out the precipitation from a solution which also contains bivalent chromium at minimum 2 g/l, preferably approx. 5-15 g/l. The pH of the solution is preferably at minimum 0.5, most preferably 1.5-2.2, and temperature approx. 110-300"C, preferably 180-250"C.

Many acids can be used. Those which are most interesting technically are, of course, primarily H2S04 and/or HCI.

Depending on the raw material used, the beginning of the process can vary greatly. For SiO2, calcium, sulfate, sulfite, bivalent nickel, trivalent aluminium, etc., can be separated during the pretreatment stages. The use of so-called low-valence processes has been prevented by the fact that the separation of iron and chromium from each other, in particular, has been difficult. This difficulty has now been overcome by the process according to the present invention and without using any external reagent. This is achieved by performing the precipitation from a solution which also contains ions of bivalent chromium art a minimum of 2 g/l and preferably at a minimum of approx. 70-90 g/l. of the trivalent chromium.In this manner it has been found possible to precipitate the chromium as chromium oxyhydrate and/or a basic sulfate without the coprecipitation of iron in the final product. At the same time the acid, which may contain chromium, can be regenerated or recirculated to the initial stages of the process, after separation therefrom of the precipitated chromium compound. The concentration of iron in the circulating acid can be maintained at a suitable level, for example within the range 40-70 gll, by causing iron compounds to precipitate and removing the precipitate. This can be done by crystallizing iron salts out from the solution, notably as the sulfate when the acid is sulfuric, or precipitating salts by a hydrolysis at an elevated temperature.

The precipitation temperature of CrOOH depends on the concentration of hydrochloric acid in the solution. The upper temperature limit for pure sulfate is 250 C, but if hydrochloric acid is present this upper temperature limit is higher. The upper limit for temperature is due to the fact that above this temperature the precipitating iron compound is no longer soluble in water. The treatment temperature should not be too low because in that case the advantage of the regeneration of the acid would be lost, and the yield of the product reduced.

The solution containing iron and chromium, used in the process according to the invention, can be prepared from chromite. In the production of metallic chromium or chromium oxide to be used for dyes, the starting solution can also contain alumina, since the precipitating jarosite-type aluminium sulfate decomposes to the oxide during calcination. In this manner the color tone of the chromium oxide obtained can be regulated. Since metallic chromium can be produced from the chromium oxide by the aluminothermic process, the presence of this small amount of aluminium oxide in the chromium oxide starting material is not detrimental.

Bivalent chromium is best obtained directly by dissolving ferrochromium, but some other reducing agent or electrolysis can also be used. This, of course, also holds true when the initial raw material is something other than a metal-alloy type material, for example chromite. In this case a metal alloy similar to ferrochromium can simultaneously serve as a reducing agent and to neutralize the residual acid from the dissolving of the chromite, for example. Since the operation takes place, as regards Cr in a non-equilibrium system E" (Cr3+ < Cr2+) ~ -410 mV, as compared with the hydrogen electrode, it is self-evident that there should be no delay between the dissolving and the precipitation of a pure chromium compound.

Nevertheless, in industrial applications of the process it is often expedient, for example, in order to separate the dissolution residue or for temperature control, to carry out the dissolving and the precipitation of the pure chromium compound separately.

The separation is preferably carried out from a solution having a pH of 0.5 at minimum, but the pH of the initial solution must not rise to the precipitation range (pH over 3) of basic trivalent chromium, except in cases in which the dissolving and the precipitation are carried out simuitaneously, in which case the upper limit for the pH is determined, for example, on the basis of the precipitation of basic Fe2+ compounds and is in the order of 3.5-4.0.

An iron-free chromium (III) compound can be obtained by the process of the invention starting from many different raw materials, such as chromium-bearing scrap and waste, chromite and ferrochromium. Using the process according to the invention in combination with calcination it is possible to produce from them hydrous chromium oxyhydrate, calcinated chromium oxyhydrate, chromium oxide, or metallic chromium or a mixture or an alloy containing one of these.

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which Figures 1-3 depict three alternative flow diagrams for carrying out the process according to the invention.

In the embodiment according to Figure 1, chromium oxyhydrate is precipitated in accordance with the present invention, and thereafter the chromium oxyhydrate (CrOOH) or a basic salt of chromium is separated from the solution, which is sent to crystallization. The sulfates of nickel, cobalt and iron, having a chromium concentration of 0.5%, are thereafter separated from the solution, which contains bivalent iron 10-15 gull, trivalent chromium approx. 40 gll, and sulfuric acid over 100 g/l. The chromium-bearing alloy is mixed with this solution and possibly also electrolysed in order to increase the concentration of bivalent chromium in the solution to the level required by the subsequent precipitation of chromium oxyhydrate, if the initial material contains problematic oxidation catalysts of cr2+, In the embodiment according to Figure 2, the solution derived from chromium oxyhydrate precipitation is sent to an autoclave for the separation of iron in the presence of oxygen. From the filtration, a trivalent iron hydroxysulfate having a chromium concentration of 4-10% is obtained.Sulfuric acid is added to the solution, which contains approx. 20 g/l oftrivalent iron, at such a rate that the concentration of sulfuric acid is above 100 g/l, whereafter the solution is sent to a step of dissolving the chromiumbearing raw material in order to increase the concentration of bivalent chromium in the solution to a level sufficiently high for the subsequent precipitation of chromium oxyhydrate.

In the embodiment according to Figure 3, all or part of the solution derived from the cystallization and separation is sent to a step of dissolving chromite, whereafter the solution plus any subsidiary flow are subjected to electrolysis, or ferrochromium is dissolved in it in order to raise the concentration of bivalent chromium to the desired level.

The accompanying Figure 4 depicts the percentage concentration of iron in the obtained chromium oxyhydrate precipitate as a function of the concentration of bivalent chromium (g/l). It can be seen in Figure 4 that the concentration of iron in the precipitate drops very sharply when the concentration of bivalent chromium in the solution increases.

The shape of the curve shown in Figure 4 remains approximately the same even under changed precipitation conditions. The curve can, however, move within the limits indicated by dotted lines in the figure, depending on the crystal nuclei used and on the other substances present in the solution, or on whether or notthe dissolving and precipitation are performed simultaneously.

The invention is illustrated by the following Examples.

Example 1 Ferrochromium was dissolved in sulfuric acid so that a solution was obtained directly which contained trivalent chromium 70 gull, bivalent iron 40 girl, and bivalent chromium 5 g/l. The pH of the solution was 2.0 in this case. The solution was heated in a pressurised vessel under a nitrogen atmosphere at 235do for 30 minutes. The obtained chromium oxyhydrate precipitate was washed with water and dried. The concentration of iron in the precipitate was 0.15%. The chromium yield of the precipitate was 65%.

Example 2 The starting aqueous acidic solution of Example 1 was oxygenated so that its concentration of bivalent chromium dropped to 2 girl. It was then heated in a pressure vessel as in Example 1, at 235"C for 30 minutes, to cause precipitation. The concentration of iron in the precipitate was 0.5% in this case. 1% of carbon was mixed with the product, and the product was kept in a chlorine gas flow at 1000"C for 30 minutes. The concentration of iron in the chromium oxide obtained was less than 0.1%.

Example 3 (comparative) The concentration of bivalent chromium in the starting solution of Example 1 was decreased to 0.5 gll and the solution was then heated as in Example 1 at 235"C for 30 minutes. The product obtained contained 4.3% iron.

Example 4 Ferrochromium was dissolved in a mixture of sulfuric acid and hydrochloric acid. A solution was obtained which had a pH of 2.0, a total chromium concentration of 100 gill. The chromium was mainly in trivalent form but the 100 g/l. included 6.7/9/1.

bivalent chromium, an iron concentration of 63 g/l and a chloride concentration of 50 gull. The solution was heated in a pressure vessel under a nitrogen atmosphere at 2300C for 60 minutes. Chromium oxyhydrate was obtained in a yield of 40%. The concentration of iron in the product was 0.08%.

Example 5 The concentration of bivalent chromiun in the starting solution of Example 4 was increased to 8.9 g/l by dissolving additional ferrochromium in the acid. The pH of the solution rose to 2.1, and chromium oxyhydrate nuclei were added at a rate of 2 g/l. The solution was heated at 230"C in a pressure vessel and under a nitrogen atmosphere for 60 minutes, whereby 48% of the chromium precipitated as chromium oxyhydrate from the solution. The concentration of iron in the product was 0.03%.

Example 6 The pH of the starting solution of Example 4 was reduced to 1.5 by means of sulfuric acid, and the concentration of bivalent chromium at the same time was reduced to 3.4 g/l. The solution was heated in a pressure vessel and under a nitrogen atmos phere at 230"C for 60 minutes, and the yield of chromium oxyhydrate was 35%. The concentration of iron in the product was estimated as about 0.95%.

Example 7 Chromate was dissolved in sulfuric acid. Part of the bivalent iron, bivalent magnesium and trivalent aluminum was removed by crystallization from the solution obtained. Ferrochromium was added to the solution in order to increase the concentration of bivalent chromium, and so the product was a solution in which the total concentration of chro mium including trivalent chromium was 92 g/l, the concentration of bivalent chromium 5.2 gll, of bivalent iron 43 gIl, of aluminum 1 g/l and of magnesium 1.5 g/l; its pH was 2.2 The solution was heated at 240or in a pressure vessel and under a nitrogen atmosphere for 30 minutes, whereby a chromium oxyhydrate was obtained in which the concentration of iron was 0.15%, of aluminum 1.8%, of magnesium 0.04% and of chromium 53%.

Example 8 A solution which contained bivalent iron 43 gull, trivalent chromium 70 g/l and bivalent chromium 3 g/l and which had a pH of 2, was heated in a pressure vessel and under a nitrogen atmosphere at a temperature of 200"C.

(a) for an hour, in which case the yield of chromium oxyhydrate was 10% and the concentration of iron 0.4% and (b) for 16 minutes, in which case the yield of chromium oxyhydrate was 6.5% and the concentration of iron in the precipitate 0.2%.

Example 9 Asolution which contained bivalent iron 71 gull, trivalent chromium 90 gill, and bivalent chromium 5 gll and which had a pH of 2 was heated in a pressure vessel under a nitrogen atmosphere at 250"C for 30 minutes, in which case the yield of chromium oxyhydrate was 74% and the concentration of iron in the precipitate was 4.5%.

Example 10 A solution which contained bivalent iron 43 gull, trivalent chromium 70 gull, bivalent chromium 15 girl, magnesium 10 gel and which had a pH of 2, was heated for an hour at 235"C, in a pressure vessel and under a nitrogen atmosphere, whereby a chromium hydroxide precipitate having an iron concentration of 0.03% was obtained.

Example 17 Afinely-divided ferrochromium, with an extremely low silicon content enclosed, together with H2SO4, N2 and CrOOH nuclei, was heated in a pressure vessel under a nitrogen atmosphere with good mixing. The autogenous pressure was reduced periodically by discharging gas from the vessel. The temperature was allowed to rise to 21 00C. At its best, approx. 1/4 of the chromium was in Cr2+ form. After four hours the solution and the precipitate were removed and filtered. The pH of the FeSO4solution was 1.4 and its Fe concentration 83 g/l. After a slight grinding, magnetic separation, an acid wash, and drying, the concentration of Fe in the CrOOH product was 0.18%.

Claims

1. A process for the preparation of substantially iron-free chromium (III) compound(s) from an acidic aqueous solution which contains ions of iron and of trivalent chromium which process comprises precipitating the chromium (III) compound(s) at an elevated temperature and pressure from an acidic aqueous solution which also contains at least 2 grams per litre of ionic bivalent chromium and separating the precipitate from the mother liquor of the precipitation.

2. A process according to claim 1, wherein the aqueous acidic solution contains 5-15 grams per litre of ionic bivalent chromium.

3. A process according to claim 1 or 2, wherein the acidic aqueous solution has a pH of 0.5 or greater, and the precipitation is effected at a temperature of 1100 to 300"C.

4. A process according to claim 3, wherein the acidic aqueous solution has a pH of from 1.5 to 2.2.

5. A process according to claim 3 or 4, wherein the precipitation temperature is from 180 to 250"C.

6. A process according to claim 5, wherein the precipitation temperature is from 200 to 250"C.

7. A process according to any one of claims 1 to 6, wherein the acidic aqueous solution contains at least 70 grams/litre of ionic trivalent chromium and at least 40 grams/litre of ionic iron.

8. A process according to any one of claims 1 to 7, wherein the acidic aqueous solution is obtained by dissolving an iron-bearing chromium raw material in an acid and, when necessary, adding bivalent chromium to this acidic aqueous solution and/or generating bivalent chromium by reducing the trivalent chromium already present in it.

9. A process according to claim 8, wherein the iron-bearing chromium raw material isferrochromium or chromite.

10. A process according to claim 8 or 9, wherein the acid comprises sulfuric or hydrochloric acid.

11. A process according to claim 8,9or10, wherein the acid comprises an acid regenerated or recirculated from a previous operation of the process after the chromium (III) compound has been precipitated.

12. A process according to claim 11, wherein the acid has been regenerated or recirculated after removal therefrom of at least part of the iron ions therein by crystallisation of an iron salt.

13. A process for the precipitation, at an elevated temperature and pressure, of a substantially ironfree chromium (Ill) compound from an acid aqueous solution which contains iron and chromium, characterized in that, in order to prevent the coprecipitation of iron, the precipitation is performed from a solution which also contains bivalent dhromium 2 gll at minimum.

14. A process according to Claim 1 substantially as described in any one of Examples 1,2 and 4 to 11.

15. Achromium (III) compound substantiallyfree of iron when obtained by a process claimed in any preceding claim.