Patchy reactive dyeing caused by hard water and uncontrolled chemical addition
A jet dyeing machine processes 600 kg of cotton interlock at a 1:8 liquor ratio. The recipe uses 4% owf reactive dye, 70 g/L salt and 20 g/L soda ash at 60°C. The water is borewell water of 380 mg/L hardness as CaCO3 and no sequestrant is used. The lab dip matches, but bulk is about 5% lighter and patchy at the selvedge. pH after soda addition is 10.8. Salt and soda are added in one shot through a single port, the machine load is tight, and the fabric rope speed is uneven. The buyer allows a maximum 2% shade variation and delivery is in 10 days. A soft-water line exists but is connected to another department. The ETP consent is COD 250 mg/L; the dyehouse effluent is currently 420 mg/L. The production manager wants to add 2% shading dye to the whole batch.
1. Calculate the liquor volume, dye, salt and soda quantities. Calculate the hardness load in kg CaCO3 and explain its effect on reactive dyeing.
Read the worked answer
Liquor volume = 600 kg × 8 = 4,800 L.
Dye = 4% owf = 0.04 × 600 = 24 kg. Salt = 70 g/L × 4,800 L = 336,000 g = 336 kg. Soda ash = 20 g/L × 4,800 L = 96,000 g = 96 kg.
Hardness load = 380 mg/L × 4,800 L = 1,824,000 mg = 1.824 kg as CaCO3. At a sequestrant dose of 2 g/L, the addition would be 9.6 kg.
Calcium and magnesium ions complex with reactive dye, reduce dye solubility and can precipitate dye or form metal-dye aggregates. The result is lower colour yield, a lighter shade and uneven deposition, especially where water flow is poor. Hardness also consumes alkali and can shift the pH profile. A lab dip made with soft water will not reproduce in a bath with 380 mg/L hardness and no sequestrant. The calculation also shows that most of the bath is water and salt, not dye, so controlling water quality is a major lever on both shade and effluent load.
2. Diagnose the patchiness and the 5% light shade. Why is adding 2% shading dye to the whole batch a poor decision? Recommend a correction route.
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The patchiness has three mutually reinforcing causes: hard water without sequestrant, one-shot salt and soda addition, and a tight load with uneven rope speed. One-shot addition creates a high local concentration at the injection port; the dye strikes quickly before it is distributed. A tight load causes channelling and poor interchange between liquor and fabric. Hard water then precipitates part of the dye, so the bath is both lighter and less level.
A 2% shading addition is the wrong magnitude. The existing depth is 4% owf. A shade that is 5% light needs roughly 4% × 1.05 = 4.2% owf, an increase of about 0.2% owf, not 2% owf. The proposed addition is ten times the approximate requirement and would overshoot badly. Shading also does not remove patches; it adds dye unevenly over an already unlevel substrate and increases effluent COD.
Correction route: do not shade the whole batch. Run a migration test on a sample. If the patch is mainly surface unlevelness, re-dye with soft water, sequestrant and controlled salt/soda addition at a higher liquor ratio. If the patch is fixed, strip and re-dye rather than shade. Use the soft-water line for the correction and segregate the first bath from the ETP.
3. Propose a right-first-time recipe and calculate the water and salt reduction if the liquor ratio is reduced to 1:6 and salt to 50 g/L. State one risk of the lower liquor ratio.
Read the worked answer
Right-first-time changes: use soft water or a sequestrant, add salt in two portions, add soda in at least three portions after the dye has exhausted, control pH at the recommended value for the dye class, reduce the load to allow rope movement, and check the rope speed before adding chemicals.
At 1:6, liquor volume = 600 × 6 = 3,600 L. Water saving = 4,800 − 3,600 = 1,200 L per batch, a 25% reduction.
Salt at 50 g/L = 50 × 3,600 = 180,000 g = 180 kg, against 336 kg. Saving = 156 kg of salt per batch, a 46% reduction. Soda at 20 g/L becomes 72 kg, against 96 kg. Dye remains 24 kg.
The lower liquor ratio reduces water, salt and steam, and it lowers the hydraulic load on the ETP. The risk is crowding: a tighter ratio can cause creases, poor rope movement and channelling, especially with a tight load. The batch size should be reduced or the pump and nozzle performance checked before adopting 1:6 routinely.