GB2284806A

GB2284806A - Process for the working up of chromium-containing sodium hydrogen sulphate

Info

Publication number: GB2284806A
Application number: GB9423128A
Authority: GB
Inventors: Jost Halstenberg; Rainer Weber; Manfred Ernst; Andreas Ossko; Hans-Heinrich Moretto
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1993-12-16
Filing date: 1994-11-16
Publication date: 1995-06-21
Also published as: JPH07206444A; GB9423128D0; ITMI942453A0; DE4342909A1; ZA9410010B; ITMI942453A1

Description

2284806 Process for the Working-up of Chromium-Containing Sodium Hydrogen

Sulphate The invention relates to a process f or the working-up of chromium- containing sodium hydrogen sulphate that arises during the manufacture of chromic anhydride by reaction of sodium dichromate with sulphuric acid.

According to UllmannIs Encyclopedia of Industrial Chemistry, Fifth Edition, Volume A 7, pp. 79 to 80, chromic anhydride (Cr03) is produced by reaction of sodium dichromate with sulphuric acid. In this connection a distinction is made between the melt and the wet processes.

In the melt process, sodium dichromate crystals and concentrated sulphuric acid are reacted at about 2000C. Molten chromic anhydride formed is separated from the sodium hydrogen sulphate, which likewise is molten, and cooled.

In the wet process, sulphuric acid and sodium dichromate are reacted in concentrated aqueous solution and chromic anhydride contaminated with sodium hydrogen sulphate crystallizes out. This product is melted and then worked up as in the melt process.

The filtrate containing sodium hydrogen sulphate which remains after isolation of chromic anhydride is used in the conversion of sodium monochromate into sodium dichromate according to the equation 2Na2CrO4 + 2NaHS04 - Na2Cr207 + 2Na2S04 + H20 Sodium hydrogen sulphate contaminated with chromium(III) and chromium(VI) compounds arises as a by-product both in the melt and in the wet process. The proportions of chromium compounds depend on the reaction conditions of the chromic acid manufacture. The chromium contents amount on Le A 29 997-FC - 1 - average to 0.8 to 3 -0k chromium(VI) and 0.3 to 3 chromium (I I I).

Because of the chromium content, up to now contaminated NaHS04 could be used only in the manufacture of sodium dichromate from sodium monochromate, wherein it is necessary for the chromium(III) constituents to be largely removed by precipitation as chromium(III) chromate (DE-B 26 33 570) or as chromium(III) phosphate (EP-A 340 585). Both processes have proved to be technically realisable only with difficulty, especially because of filtration problems.

In view of the problems in the working-up of the residues from sodium dichromate manufacture, there was therefore a need to provide a process for the working-up of these residues. The objective was the almost quantitative isolation of the chromium ingredients from the sodium hydrogen sulphate, so that both fractions can be reused in chemical processes.

It has now been found that the chromium ingredients in the sodium hydrogen sulphate can be converted almost completely into a sparingly water-soluble complex compound containing exclusivelv chromium(III), of the stoichiometric compo- sition Na3Cr(S04)3, if the chromium- containing sodium hydrogen sulphate is heated for some time at temperatures of 220 to 550 OC.

on dissolution of this melt in water, followed by isolation of the chromium complex compound, there is obtained a sodium hydrogen sulphate solution which has only small re maining chromium concentrations.

At temperatures below 220 0C, soluble chromium (VI) 35 ingredients are still present. Above 550 OC a considerable decomposition of sodium hydrogen sulphate according to the equation Le A 29 997-FC - 2 - 2NaHS04 Na2S04 + S03 + H20 is observed, and a chromium complex compound that is more soluble in water is formed.

A solution obtained by the process according to the invention or sodium hydrogen sulphate obtained from it by evaporation of water can also be used in many other fields of application, in addition to the aforementioned use in the manufacture of sodium dichromate, as for example the neutralization of alkaline solutions and adjustment of acidity in the chemical industry, metal surface treatments, fluxes in metal production, and the bleaching and dyeing of textiles.

is The present invention provides a process for the working-up of chromium- containing sodium hydrogen sulphate which arises during the manufacture of chromic anhydride by reaction of sodium dichromate with sulphuric acid, that is characterized in that the chromium-containing sodium hydrogen sulphate is heated to temperatures of 220 to SSOOC, the melt formed is held for at least 5 minutes at this temperature, water is then added, and the chromiumcontaining insoluble ingredients are isolated from the solution formed.

In a preferred embodiment of the process according to the invention, the chromium-containing sodium hydrogen sulphate is heated for 5 to 120 minutes at 300 to 450 OC.

This time can be shortened by reducing agents, as for example Na2S03, Na2S204 and Na2S20.;.

The partial decomposition of the chromium-containing sodium hydrogen sulphate with release of sulphur trioxide occurring on heating to 220 to 550 OC can be suppressed to a large extent by addition of sodium sulphate. 5 to 15 Le A 29 997-FC - 3 - parts of sodium sulphate preferably are added per loo parts by weight of chromium- containing sodium hydrogen sulphate. In this process variant, apart from pure sodium sulphate or its hydrates, there can also be used dichromatecontaining sodium sulphate such as is formed in the conversion of sodium monochromate by sulphuric acid or sodium hydrogen sulphate to sodium dichromate according to the equations 2Na2Cr04 + 112S04 - Na2Cr207 + Na2S04 + H20 2Na2Cr04 + 2NaHS04 - Na,Cr207 + 2Na2S04 + H20.

At temperatures of 220 OC to 550 OC, the dichromate ingredients of the sodium sulphate also are converted to sparingly soluble Na3Cr(S04)3.

is By application of the process according to the invention, sodium hydrogen sulphate solutions can be obtained having chromium contents below 10 mg/1. The isolated chromium complex compound, which is insoluble in the sodium hydrogen sulphate solution, is preferably used in the manufacture of basic chromium sulphate, which is used in the tanning of leather. Examples of manufacturing routes to basic chromium sulphate (Cr2WH)2(S04)2) are:

(1) Na3Cr(S04)3 + WaHe03 - Cr(OW3 + 3Na2S04 + 3C02 2Cr(OW3 + 2H2S04 Cr2WH)2(S04)2 + 4H20 (2) 2Na3Cr(S04), + 2Cr(OH). + H2S04 2Cr2WH).(S04), + 3Na2S04 + 2H20 The invention will be further described with reference to the accompanying drawing which is a flow sheet of a process in accordance with the present invention.

Referring now more particularly to the drawing, sodium dichromate crystals (Na2Cr207-2H20) are intimately mixed with 96 96 sulphuric acid in a screw mixer, whereby a viscous paste Le A 29 997-FC -4 - of chromic anhydride, sodium hydrogen sulphate and water is formed (1). This paste is charged to a heated revolving tube and mel ted at about 2 0 0 0 C (2). The melt flows into a separating cell (3), wherein the denser chromic anhydride collects at the bottom. Chromic anhydride is discharged with the aid of a riser pipe and formed to flakes on cooling rolls (4). The upper layer of chromium-containing sodium hydrogen sulphate leaves the separating cell by an overflow and is fed to a heated melting tank (5). In this melting tank the chromium- containing sodium hydrogen sulphate is mixed with sodium sulphate, heated to 300 to 450 OC and left for 5 to 120 minutes at this temperature. In the course of this heating the chromium(VI) ingredients are reduced and Na3Cr(S04)3 'S formed. The molten Na3Cr (S04) 3 containing sodium hydrogen sulphate is cooled on flaking rolls (6) and then dissolved in water (7). There are formed in the process a solution of sodium hydrogen sulphate and a light-green precipitate of Na3Cr(S04)3, which is separated from the solution by means of a decanter (8). The Na3Cr(S04)3 isolated is used in the manufacture of chrome tanning agent (9).

The invention will be illustrated in more detail with the aid of the following examples.

For the experiments a chromium- containing sodium hydrogen sulphate was used that was obtained at 200 OC during the reaction of sodium dichromate crystals with 96 -. sulphuric acid. This sodium hydrogen sulphate had the following composition:

90.3 % NaHS04 3.7 OW H2S04 0.8 % Cr(VI) 1.0 % Cr(III) 3.2 % H20 Le A 29 997-FC - 5 - Examples

Example 1

100 g of chromium- containing sodium hydrogen sulphate in each case were heat-treated in a glass vessel for 1 hour at various temperatures and after cooling to room temperature dissolved in so much water that 250 ml of solution were obtained in each case. Undissolved Na3Cr(S04)3 was then filtered off. The 10 chromium content of the filtered sodium hydrogen sulphate solution was determined. The results of the experiments are shown in Table 1. For comparison, 100 g of sodium hydrogen sulphate were dissolved in water without heat treatment and filtered.

is The results are set forth in Table 1.

Table 1

Temperature during heat Cr content of the sodium treatment PC) hydrogen sulphate solution after filtration (mg/1) no heat treatment 6830 1800 250 118 300 13 350 2.1 400 3.3 450 12 500 76 Le A 29 997-FC -6 - Example 2 g of chromium- containing sodium hydrogen sulphate in each case were heattreated in a glass vessel at various tempera- tures for various times and after cooling to room temperature dissolved in so much water that 250 ml of solution were obtained in each case. After isolation of undissolved Na3Cr(S04)3 the chromium contents of the filtered solutions were determined. The results are set forth in Table 2. 10 Table 2 Heat treatment Cr content of the sodium hydrogen Temperature PC) During (min) sulphate solution after filtration 250 60 118 250 120 44 250 240 45 300 60 13 300 120 2.5 300 240 1.5 350 60 2.1 350 120 2.0 350 240 4.3 400 60 3.3 400 120 21 is Le A 29 997-FC - 7 - Examiple 3 g of chromium- containing sodium hydrogen sulphate in each case were heat-treated in a glass vessel at 350, 400, 450 and 500 OC for 15, 30 and 60 minutes in each case and after cooling to room temperature dissolved in so much water that 250 ml of solution were obtained in each case. After isolation of undissolved Na3Cr(S04)3 the chromium contents of the filtered solutions were determined. The results are set 10 forth in Table 3.

Heat treatment Cr content of the sodium hydrogen Temperature Duration sulphate solution (OC) (min) after filtration (mg/1) 350 is 72 350 30 12 350 60 2.1 400 is 8.9 400 30 1.3 400 60 3.3 450 is 2.5 450 30 4.2 450 60 12 500 is 5.1 500 30 16 500 60 76 is Le A 29 997-FC -8 Examiple 4 g of chromium- containing sodium hydrogen sulphate in each case were mixed with 0, 5.6, 10, 25 and 50 g of sodium sulphate and heat-treated in a glass vessel at 350 OC for 60 minutes. After cooling to room temperature the solidified melt was dissolved in so much water that 250 ml of solution were obtained in each case. After isolation of undissolved Na3Cr(S04)3, the chromium contents and acid contents of the filtered solutions were determined. The results are set forth in Table 4. The acid contents are a measure of the degree of decomposition of the material with SO, release.

Table 4 is Sodium sulphate Chromium content Acid content addition (g) of the sodium of the sodium hydrogen sulphate hydrogen sulphate solution after solution after filtration filtration (mg/1) (g/1 H2S04) 0 2.1 117 5.6 1.3 134 10 2.1 152 28 146 so 197 152 As is apparent from Table 4, distinctly higher acid contents are obtained in the sodium hydrogen sulphate solutions through the addition of sodium sulphate, which proves that the decomposition with S03 release is suppressed. With the addition of 25 g or more sodium sulphate per 100 g of chromium- containing sodium hydrogen sulphate, the chromium content of the sodium hydrogen sulphate solution after filtration rises.

Le A 29 997-FC - 9 - It is understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.

Le A 29 997-FC -10 -

Claims

Claims 1. A process for the working-up of chromium- containing sodium

hydrogen sulphate produced during the manufacture of 5 chromic anhydride by reaction of sodium dichromate with sulphuric acid, comprising heating the chromium- containing sodium hydrogen sulphate to a temperature from about 220 to 5SOOC, thereby forming a melt, holding the melt for at least 5 minutes at such temperature, adding water to form 10 a solution, and isolating chromiumcontaining insoluble ingredients from the solution.
2. A process according to Claim 1, wherein the chromiumcontaining sodium hydrogen sulphate is heated to about 300 is to 4500C.
3. A process according to Claim 2, wherein the melt is held for about 5 to 120 minutes at about 300 to 4500C.
4. A process according to Claim 1, including adding to 100 parts by weight of the chromium- containing sodium hydrogen sulphate about 5 to 15 parts by weight of sodium sulphate.
5. Chromium-containing insoluble matter produced by the process of Claim 1.
6. A method of tanning of leather wherein the leather is contacted with a chromium-containing tanning agent, wherein said tanning agent is made f rom the product of Claim 5.

Le A 29 997-FC