Reactive Dye Shade Failures After Salt Supplier Change
A dyehouse batch-dyes 100% cotton single jersey, 220 g/m2, in 600 kg jet machines using exhaust reactive dyes. In the last 20 lots, 8 have failed shade approval with Delta E 1.1-2.1 against a tolerance of 1.0. The failures are mainly in medium and dark tertiary colours. Exhaustion curves show slower and less complete strike than the historical curve; initial dyebath pH is 10.9 instead of 11.2, and salt is dosed on a weighed basis. The sodium sulphate supplier was changed six weeks ago; the certificate shows 8% moisture and 1.5% calcium/magnesium as received, but the old supplier's salt was 2% moisture and 0.2% hardness. Effluent COD and TDS are close to the consent limit, and the ETP has little spare capacity. Right-first-time is now 60% against a customer target of 95%. Reverting to the old supplier would add 12% to salt cost, about PKR 1.8 million per year. The dyehouse has 14 days of new salt and 4 days of old salt left.
1. Interpret the shade failures and exhaustion curves. Identify the most probable root cause and explain why the problem appears mainly in medium and dark tertiary colours.
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The timing, the drop in initial pH, and the slower strike point to the salt change rather than to the dye recipe. The new salt contains 8% moisture, so each weighed kilogram delivers about 8% less active sodium sulphate. That lowers ionic strength and reduces dye migration to the fibre. The 1.5% calcium/magnesium is more serious: hardness ions consume alkali and can form insoluble complexes that block dye sites. With less active salt and a pH of 10.9 instead of 11.2, the reaction between reactive dye and cellulose is slower, so exhaustion is incomplete and the dyebath tails.
Medium and dark tertiary colours are most affected because they use higher salt and alkali concentrations and depend on a precise strike for shade reproducibility. A small loss of ionic strength shifts the equilibrium enough to change the final shade. Light shades have more recipe tolerance; dark tertiary shades do not. The effluent link is that more salt or alkali to correct the problem raises TDS and COD, so the fix must be dose-based, not simply more chemicals.
2. Describe a laboratory verification programme to prove or disprove the salt hypothesis, and state the production correction you would make if it is confirmed.
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Laboratory verification Run a paired dyeing series with old and new salt at equal anhydrous sodium sulphate dose, same dye, alkali, temperature, and liquor ratio. Test at least three colours, including one failing tertiary shade. Measure exhaustion at 15, 30, and 60 minutes, final pH, and Delta E against the standard. Independently test the salt for moisture, calcium/magnesium hardness, iron, and insolubles. If the new salt dosed on an anhydrous basis gives Delta E at or below 0.6 and an exhaustion curve within 5% of the old salt, the salt is not the sole cause and the alkali dosing system should be checked.
Production correction Dose salt on an anhydrous basis by dividing the weighed requirement by 0.92; add 0.5 g/L sequestrant for hardness; adjust the initial pH to 11.2 with alkali; and run a three-lot pilot in medium and dark tertiary shades. Monitor pH at the start and at 30 minutes, and measure final shade. Do not increase salt blindly, because the ETP has little TDS headroom.
3. Recommend a supplier and inventory decision for the next four weeks, balancing right-first-time, effluent limits, and the 12% salt cost increase.
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Do not make a binary choice between the two suppliers. Use the remaining four days of old salt for the most critical dark tertiary shades while the laboratory verification and three-lot pilot run. For the new salt, use the anhydrous-dose correction plus sequestrant and pH control. Continue only if right-first-time reaches at least 90% in the pilot and the effluent TDS/COD remain within consent after the corrected dose.
The 12% salt cost increase, about PKR 1.8 million/year, must be compared with the cost of re-dyeing 40% of lots. Re-dyeing consumes dye, energy, water, and machine time and also raises effluent load; it is usually more expensive than the salt premium. However, if the new salt still gives Delta E above 1.0 after correction, revert for critical shades and negotiate a lower-moisture, lower-hardness specification with the new supplier. The decision rule is: accept the new salt only with a certificate of analysis, anhydrous dosing, and a demonstrated 90% right-first-time over 20 lots.