Chrome tanning agent (basic chromium sulphate)

01

Set Cr2O3 content, basicity and the chrome offer

UNIDO's chrome-management report for tanneries in South-East Asia says chrome tannins are used in powder form at about 25% Cr2O3 and 33% basicity, and that basic chrome sulphate liquors, prepared by reducing sodium or potassium dichromates in the presence of sulphuric acid with sulphur dioxide or technical sugars, are also used. It gives the chrome offer in tanning as 1.5–2.5% Cr2O3 on pelt weight in 60–80% float, with basicity rising from 33% to up to 66% by the end of tanning; sodium hydrogen carbonate at 0.8–1.2% on pelt weight, or magnesia containing 60% MgO at 0.6–0.8%, brings the final float to pH 3.8–4.2, finishing at 35–40 °C after at least 6–8 hours of drumming. [1] Balmer Lawrie's BALSYN CR 25, a spray-dried basic chromium sulphate, is listed at 24% Cr2O3 minimum, 33 ± 2% basicity and pH 2.8–3.2 in 10% solution, as a dark-green fine powder easily soluble in water, with 24 months' shelf life in sealed packing. [4]

Basicity counts hydroxide groups per chromium: a LANXESS patent on chrome tanning agents states that at 33.3% basicity there is one hydroxide ion per chromium ion and at 66.6% there are two, and cites 20.9 mg of Na2CO3 per 1000 mg of Cr2O3 as the base needed to raise basicity by 1%. [5] UNIDO adds that “a chrome complex solution of 33 % basicity has pH 2.8 and contains mostly binuclear species”, while one of 50% basicity has pH 3.5 and contains more astringent trinuclear and larger ions. [1]

UNIDO's chrome-balance model for shoe-upper grain leather offers 8% basic chrome sulphate at 25% Cr2O3, equal to 2% Cr2O3 on pelt weight, of which about 75% reacts with the collagen; a high-exhaustion variant uses 4.6% of the same powder plus 2.6% of a self-basifying chrome tannin at 7% Cr2O3, 1.4% Cr2O3 in total, with about 98% taken up. [2] About 2% Cr2O3 gives a shrinkage temperature of 110 °C or more, leather tanned at that offer contains 4–5% Cr2O3, and an offer below 1.6–1.7% Cr2O3 may make boil fastness and even chrome distribution hard to reach. [1]

Editorial calculation: 1,000 kg of pelt at 2% Cr2O3 needs 20 kg of Cr2O3, which is 20 ÷ 0.25 = 80 kg of a 25% powder or 20 ÷ 0.24 ≈ 83 kg of a 24%-minimum powder; raising the basicity of that 20 kg of Cr2O3 from 33% to 50% would take about 17 × 20.9 mg × 20,000 ≈ 7.1 kg of Na2CO3.

Give the supplier the form (powder, self-basifying powder or liquor), Cr2O3 content and basicity with tolerance, pH of a stated solution, solubility, moisture and insolubles, iron and other impurities, reducing agent used in manufacture, Cr(VI) content of the tanning agent, the leather type and offer you plan, package size, and quantity.

02

Control Cr(VI) in leather and chrome in effluent

Commission Regulation (EU) No 301/2014 added to REACH Annex XVII entry 47 that leather articles coming into contact with the skin, and articles with such leather parts, shall not be placed on the market where they contain chromium VI at 3 mg/kg or more of the total dry weight of the leather, applying from 1 May 2015. Its recitals explain that chromium VI can form in leather by oxidation of the chromium III compounds used in tanning, and that 3 mg/kg is the determination limit of the EN ISO 17075 test method. [3]

UNIDO reports limits on total chrome discharge of 0.05–10 mg/l for direct discharge to water bodies and 1–50 mg/l for discharge to sewers, and spent conventional tanning floats at 3.5–7.0 g Cr2O3/l. Recovery precipitates chrome from collected floats with sodium hydroxide or carbonate at pH 8.5–9.0, not above 10.0, or with MgO at pH 8.0–9.0, at 35–40 °C; with MgO about 5 mg/l chrome remains in the supernatant. The filter cake, at least 25–30% dry substance, is redissolved with 2 kg or more concentrated sulphuric acid per kg of chrome oxide to pH 2.5, giving recovered liquor at 100–150 g Cr2O3/l after conventional tanning. [1]

UNIDO cites Indian experience that leather tanned with 70% fresh and 30% recovered chrome matches leather tanned with fresh chrome only, German trials showing quality effects at 50% recovered chrome, and advises a fat content below 45 mg/l in recovered liquor to prevent discolouring. [1] In its balance, conventional tanning leaves 25% of the Cr2O3 offer in effluent, high-exhaustion tanning 2%. [2]

Editorial calculation: at 100–150 g Cr2O3/l, the 20 kg Cr2O3 offer for 1,000 kg of pelt corresponds to about 133–200 litres of recovered liquor (20,000 g ÷ 150 or ÷ 100 g/l).

03

Powder, self-basifying and liquor forms of basic chrome sulphate

The reviewed sources describe three forms: powder basic chrome sulphate at 24–26% Cr2O3 and 33% basicity, self-basifying chrome tannins with an MgO or dolomite component, and chrome liquors made by dichromate reduction or recovered from spent floats. [1] [4] When requesting quotations, ask each supplier for a certificate of analysis per lot and the Cr(VI) result for the tanning agent.

04

Basic chromium sulphate, reducing agent, self-basifying component, basification and masking agents

Only the first row is the tanning agent itself; the others are either set during its manufacture, such as the sulphur dioxide or sugar used to reduce dichromate, or added in the drum, such as the 0.8–1.2% sodium bicarbonate or 0.6–0.8% MgO for basification. [1]

05

Cr2O3 content, basicity, pH, impurities, solubility and shelf life for a chrome tanning agent order

Cr2O3 content sets how much powder the offer needs: 2% Cr2O3 on 1,000 kg of pelt is 80 kg of a 25% powder but about 83 kg of a 24%-minimum grade (editorial calculation), so ask for the lot value, not only the minimum. [1] [4]

06

Send the grade, offer and limits together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Form: [powder; self-basifying; liquor]
Cr2O3: [%]
Basicity: [%; tolerance]
pH: [solution]
Impurities: [Fe; insolubles; Cr(VI)]
Offer: [% Cr2O3 on pelt weight; leather type]
Packaging: [bag; IBC]
Quantity: [tonnes]

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