GB2061326A - Improvements in and relating to the treatment of spent dyebaths - Google Patents

Abstract

In the process of dyeing with mordant dyes and a chromating agent, soluble thiosulphate is incorporated at the same time as, or after, the addition of the chromating agent, at pH 3-4 to leave the eventual spent bath depleted in chromium, and substantially free from Cr<6+>. The soluble thiosulphate may be an alkali metal or ammonium thiosulphate.

Description

SPECIFICATION Improvements in and relating to the treatment of spent dyebaths The present invention concerns the removal of chromium from spent dyebaths (which may form part of dye works effluent) of the mordant dye type.

It is known that chromium leaves residues in dyeing and chromating baths, mainly in the form of hexavalent chromium, and that the national regulations concerning water pollution put increasingly severe limits of acceptability on the concentration of chromium in effluent waters, in particular for chromium in its hexavalent form which is the most dangerous one.

Thus, for instance, an Italian 1 976 regulation establishes final limits of acceptability of 2 mg/lt. (as chromium) for trivalent chromium and of 0--2 mg/lt (as chromium) for hexavalent chromium for discharges into all superficial and underground waters, as well as into inland and marine waters, into public as well as into private waters, as well as into sewers, on the ground and under the ground.

It is an object of the invention to provide for the elimination, totally is required, of the more dangerous hexavelent chromium and to bring the total chromium of the spent baths to values that are below 7-8 mg/lt, thus reducing the load of total chromium in the spent baths to levels that are lower than or, at least, very close to those allowed by the Law. In the less favourable cases, a modest dilution will suffice to bring the chromium levels within the limits of acceptability for their discharge as works effluent.

In the dyeing of natural as well as synthetic polyamide fibres with chromium dyes, in the chromation phase there are used sodium, potassium or ammonium chromate and bichromate in order to form the complex of the dye with the chromium, which allows the attainment of a product dyed in the shade and with the fastnesses requested by the customer. At the end of the chromation there always remains in the spent bath as a residue considerable quantities of hexavalent and trivalent chromium (total chromium) which must be removed in order to make the effluent ecologically non-polluting.

The established purification techniques for these spent baths are complicated and costly and require special installations which remove the residual chromium in various degrees by precipitation reactions or by absorption with ion-exchanging resins; also known are methods of eliminating the chromium directly from the chromating bath by the addition of complexing or sequestering agents.

According to a method suggested by IWS, there is used in particular, as a complexing agent, lactic acid. With such a material it is quite possible to eliminate almost completely the hexavalent chromium, which is converted to trivalent chromium, but without any effect on the total chromium.

From the British Patent No. 1,487,539 there is also known a process for the elimination of the hexavalent chromium by the use of reducing agents such as sulphites, dithionites, sulph-oxylates, salts of hydroxymethanesulphonic acid, organic reducing agents such as 3,4-dihydroxy-benzoic acid, 3,4-dihydroxy-cyanamic acid, ascorbic acid, etc., in the presence of organic products capable of complexing the resulting trivalent chromium present, with its consequentia-absorption and fixation onto the dyed polyamide fibre.

Also with this method the elimination of the hexavalent chromium is practically complete, while, on the other hand, the fixation of the trivalent chromium on the fibre is less efficient and the quantity of total chromium detectable in the effluent waters generally exceeds the limits allowed by the environmental protection regulations.

It has now surprisingly been found that by adding suitable quantities of salts of thiosulphuric acid in the dyeing and chromating bath, without removing the goods to be dyed, and at a pH of 3-4, it is possible to eliminate the hexavalent chromium substantially completely and to reduce to within extremely low limits the value of the total chromium, without the need for recourse to organic complexing agents.

The most suitable salts of thiosulphuric acid are those soluble in the chromation bath and in particular those derived from the alkali- and alkaline earth metals and from ammonium hydroxide.

In comparison with the process of British Patent No. 1,487,539 the present invention allows, moreover, a considerable operational simplification and a reduction in costs, inasmuch as there is used one product both for eliminating the hexavalent chromium and for reducing at the same time of the appropriate degree the trivalent chromium.

Moreover there is obtained, compared to the above-described process, a shortening of the chromation phase, with the possibility of reducing the overall time required for the dyeing operation.

The dyeing process may be carried out in a conventional manner. Thus, once the dyeing has been accomplished, the bath is cooled down suitably to below 700 C; the pH of the bath, if necessary, is brought back to 3-4, suitably with an organic acid, preferably formic acid; and there is then added a quantity of sodium or potassium chromate and/or bichromate, calculated by mutliplying the percentage of dye by the bichromate factor, which in general is from 0.1 5 to 0.6 (the bichromate factor of a mordant dye is an experimentally determined valued which, when multiplied by the percentage of dye used, provides for the greatest possible utilization of bichromate in order to achieve a satisfactory chromation). Such a value varies from dye to dye and is generally as indicated.

The dye bath is thereupon heated up to the boiling point and then treated with 0.52% of sodium or ammonium thiosulphate with respect to the weight of the fibre. Chromation is then carried on as usual-for 3045 minutes. Finally the bath is cooled down and unloaded.

The addition of thiosulphate may also be carried out simultaneously with the addition of the chromating medium. The percentage of thiosulphate used is substantially proportional to the quantity of bichromate present in the chromating bath, the ratio between thiosulphate and bichromate and/or chromate being maintained between 1 and 1.5.

An analysis of the spent dye baths reveals the absence of hexavalent chromium and the presence of slight quantities of total chromium, in general lower than 7-8 mg/lt.

It may be assumed that this result is achieved on the basis of the formation of a complex between the chromium and the thiosulphate anion, with its consequential re-absorption and irreversible fixation onto the polyamide fibre.

The Examples that will follow hereunder are given for a further illustration of the invention without limitation of the wider aspects thereof. Parts and percentages, if not otherwise specified, are expressed as values by weight.

EXAMPLE 1 200 kg of worsted wool top were dyed conventionally in a dyeing apparatus at atmospheric pressure, with 6%, on the weight of the goods, of concentrated Diacromo SB (C.l. Mordant Blue 1 3) at a gqods/bath ratio of 1:13.

After the dyeing, the bath was cooled down to 700 C; the pH brought up to 3.5 with formic acid - and there was then added 1.7% of potassium bichromate with respect to the weight of the goods.

The temperature of the bath was thereupon raised in the course of 15 minutes to 970C and, after 10 minutes, there were added 2% of solium thiosulphate based on the weight of the goods. Boiling was then carried out for another 35 minutes, and the reaction completed in the usual way.

A sample of the spent bath, drawn at the end of chromation, was analysed for the content of hexavalent chromium (using diphenylcarbazide) and the total chromium (by atomic absorption). The results of these determinations were: Cr6+: absent Total chromium:6.4 p.p.m (parts p. million = mg/lt) With the same batch of fresh wool the above indicated-dyeing was repeated, following the same procedures as before, but without the addition of sodium thiosulphate.

A sample of the spent dye bath gave by analysis the following results: Cr6+: 19.4 p.p.m.

Total Cr: 42.0 p.p.m.

EXAMPLE 2 100 kg of wool staple were dyed conventionally in a pressurized apparatus, with a goods/dye bath ratio of 1 5, at 1 C, with the following formulation (the percentages being referred to the weight of material): Yellow Diacromo RL (C.I. Mordant Yellow 8) 1.7% Red Diacrori o 3B (C.l. Mordant Red 26) 1.0% Grey Diacromo BLC (C.I. Mordant Black 81) 0.8% After dyeing, the bath was cooled down to 700C and the pH was brought to 3.5 with formic acid, whereupon there was added 1% of sodium bichromate. The temperature was then again brought up to 1020C.

After 10 minutes there was added 1.1% of ammonium thiosulphate, maintaining the temperature at the given level for another 20 minutes. The reaction was then finished in a conventional way.

A sample of the spent bath, drawn at the end of the chromation, gave the following analytical results: Cr6+: absent Total Cr: 1.7 p.p.m.

Proceeding in the same way, but without the addition of thiosulphate, the following data were found: Cr6+: 17.5 p.p.m.

Total Cr: 28.0 p.p.m.

EXAMPLE 3 50 kg of woollen fabric were dyed by the conventional technique of winch dyeing at 980C, at a bath ratio of 1 :40, with the following formulation (based on the fabric weight): Yellow Diacromo BN (C.l. Mordant Yellow 3) 0.9% Red Diacromo MW (C.I. Mordant Red 3) 0.6% Black Blue Diacromo B (C.l. Mordant Black 13) 0.7% After dyeing the bath was cooled down to 7O0C and the pH was brought to 3.0 with formic acid, after with it was treated with 0.73 of sodium bichromate (based on the fabric weight).

The temperature was then brought back to 980C and, after 10 minutes, there were added 0.7% of sodium thosulphate, again based on the weight of the fabric. The temperature was then maintained at 980C for another 35 minutes.

The dyeing was finished conventionally. A sample of the spent bath, drawn at the end of chromation, gave the following analytical results: Cr6+: absent Total Cr: 0.5 p.p.m.

Proceeding in the same way, but without the addition of thiosulphate, the following data were obtained: Cr6+: 2.5 p.p.m.

Total Cr: 3.4 p.p.m.

EXAMPLE 4 100 kg of wool yarn were dyed as in Example 1, but with the variation that there were used 5% on the weight of the yarn of Black Diacromo R (Mordant Black 11),1.2% weight on the yarn of potassium bichromate and 1.2% by weight on the yarn of sodium thiosulphate. A sample of spent bath, drawn at the end of the dyeing, gave the following analytical results: Cr6+: absent Total Cr: 1.5 p.p.m.

Proceeding in the same way, but without the addition of thiosulphate, the following data were obtained: Cr6+: 12 p.p.m.

Total Cr: 22 p.p.m.

Operating under conditions similar to those of Example 1, but with the indicated mordant dyes and chromate additions, and with the addition of thiosulphate as in Example 1, the following analytical results for Cr6+ and total Cr were obtained:

Dye % K Blchromate Total on weight on weight of Cr+ Cr Diacromo Dye Colour index of goods goods mg/lt mg/lt Yellow BN, extra concentrad Mordant Yellow 3 0.6 0.24 0 0.4 Yellow MRM Mordant Yellow 44 1.5 0.37 0 0.4 Yellow CE Mordant Yellow 31 1.4 0.28 0 0.4 Yellow RL, extra concentr,d Mordant Yellow 3 1.4 0.56 0 0.4 Orange MR Mordant Orange 1 0.9 0.36 0 0.4 Orange GR Mordant Orange 6 1 0.35 0 0.5 Orange R Mordant Orange 3:1 1 0.35 0 0.5 Red G Mordant Red 19 2 0.30 0 0.5 Red 3BG no C.I. 2 0.50 0 0.6 Red 3B Mordant Red 26 2 0.80 0 0.7 Red MW extra Mordant Red 3 2 0.54 0 0.6 Garnet-red A Mordant Red 5 2 1.00 0 1.1 Violet R, extra concentr. Mordant Violet 34 1.5 0.64 0 0.7 Cyanina R Mordant Blue 3 0.6 0.3 0 0.4 Blue heaven BN, extr. conc. Mordant Blue 1 0.6 0.15 0 0.3 Indigo Red R, concentr. no C.I. 2 0.30 0 0.5 Blue BL Mordant Blue 7 3.5 0.42 0 0.7

Dye % K Blchromate Total on weight on weight of Cr+ Cr Diacromo Dye Colour index of goods goods mg/lt mg/lt Blue SB, high concentr. Mordant Blue 13 2.4 0.96 0 0.9 Blue B, high concentr. Mordant Black 17 2.5 0.52 0 1.8 Green B Mordant Green 17 1.5 0.37 0 0.9 Brown KE Mordant Brown 15 2 0.80 0 0.9 Brown MRH, extr. conc. Mordant Brown 33 2 0.30 0 0.9 Black blue B, high concentr. Mordant Black 13 2 0.80 0 0.9 Gray BLC Mordant Black 81 2 0.56 0 1.0 Black P2BN, concentr no C.I. 6 1.5 0 1.3 Black PV2A, high concentr. Mordant Black 9 5 1.0 0 0.8 Black NPCL, concentr. Mordant Black 9 5 1.5 0 1.4 Black PL8, extra concentr. no C.I. 6 1.2 0 0.9 Black PS8, concentr. Mordant Black 11 4 1.0 0 4.0 Black A8 extra Mordant Black 1 5 1.0 0 1.2

Claims (5)

1. A process of dyeing with mordant dyes with use of a chromating agent in which chromium is exhausted from the bath, to leave a spent bath of low chromium content, by incorporating a soluble thiosulphate at a pH of 3 1, at the same time as or after the addition of the chromating agent.

2. A process according to Claim 1, in which an alkali metal or ammonium thiosulphate is used.

3. A process according to Claim 1 or 2, in which the ratio between the thiosulphate and the chromate and/or bichromate chromating agent in the dye bath is from 1:1 to 1.5:1.

4. A process according to Claim 1, carried out substantially as herein exemplified.

5. A process in which chromium is fixed onto dyed fibre in the chromating bath of a mordant dyeing process by the presence of thiosulphate.