Zinc neutral-leach iron carry-over and goethite residue loss
A zinc plant leaches 200 t/day of calcine assaying 50% Zn, 9% Fe, 0.4% Cu and 0.8% Pb. Neutral leach operates at pH 5.0–5.2 and 70 C with 2 h residence time. Solution leaving neutral leach carries 3.5 g/L Fe to purification. Goethite residue is produced at 90 C and pH 3.5 and assays 10% Zn and 22.5% Fe at 80 t/day; overall zinc recovery is 92%. The cellhouse current efficiency has fallen from 90% to 84%, and cobalt in electrolyte has risen from 0.3 to 1.2 mg/L. The acid plant is at capacity, so increasing neutral-leach acidity would send more iron to purification. The refinery target is 92 t/day of zinc cathode, and the residue is currently washed only with a small volume of fresh water.
1. Using the assays, calculate the zinc loss in the goethite residue and assess whether the 3.5 g/L Fe carry-over is the dominant cause of the recovery and current-efficiency problems. Show your reasoning.
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The residue mass is 80 t/day at 10% Zn, so zinc in residue = 80 × 0.10 = 8 t/day. Feed zinc = 200 × 0.50 = 100 t/day, so the residue loss is 8% of feed zinc and the 92% overall recovery is consistent with the numbers. Iron in residue = 80 × 0.225 = 18 t/day, which matches the 9% Fe in 200 t/day of calcine, so essentially all iron is reporting to residue. The 3.5 g/L Fe carry-over is therefore not iron that failed to precipitate as goethite; it is iron that was never precipitated, most likely ferrous iron. Goethite precipitation requires Fe3+, and at pH 5.0 ferrous iron remains soluble and passes to purification.
That ferrous iron oxidizes and cycles at the cathode, lowering current efficiency; the rising cobalt is a separate but related symptom because cobalt is not being removed effectively and also causes cathode corrosion. The 6-point CE drop is large, so Fe carry-over is a credible dominant cause, but the 8% residue zinc loss is also significant and should not be ignored. The first check is oxidation conditions-air or oxygen supply, ORP, and ferrous/ferric ratio-not simply pH.
2. Recommend a control strategy for the neutral leach and goethite circuit, and explain the trade-offs between goethite, jarosite and hematite routes in this flowsheet.
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The immediate control strategy is to oxidize ferrous iron before or during neutral leach, verify by ORP and Fe2+/Fe3+ analysis, and avoid operating neutral leach at a pH that precipitates zinc hydroxide while leaving ferrous iron soluble. If iron is still high, use a separate goethite stage with seed recycle, controlled oxygen addition, pH 3–4, and 80–90 C, then wash the residue in countercurrent stages to displace entrained zinc-bearing solution. Increasing acid is not an option because the acid plant is at capacity.
Route trade-offs: goethite gives a lower residue volume and lower sulfate demand than jarosite, but it needs good oxidation control and seed handling. Jarosite is more robust and easier to settle but produces a larger residue, consumes alkali, and can co-precipitate zinc. Hematite gives a small, clean residue but requires high temperature and pressure, high capital cost, and careful sealing. For this plant, improving goethite oxidation and washing is the lowest-capital route; a hematite conversion would be justified only if residue volume or zinc loss remained unacceptable after that.
3. The residue at 10% Zn is too high for the recovery target. Propose two ways to reduce zinc loss without expanding the acid plant, and state the main risk of each.
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First, improve residue washing and displacement. The goethite residue is washed with only a small volume of fresh water, so much of the 10% Zn may be entrained pregnant solution rather than chemically bound zinc. A countercurrent wash with low-zinc water, plus better filtration and wash ratio control, can reduce soluble zinc without adding acid. The risk is that washing also returns more iron and sulfate to the circuit, and poor wash efficiency can simply move the loss elsewhere.
Second, modify the goethite precipitation step to grow denser, more filterable particles and reduce zinc co-precipitation: recycle goethite seed, control pH and oxygen addition more tightly, and avoid over-neutralization. A weak acid wash of the residue could dissolve zinc hydroxide or basic zinc sulfate without dissolving much goethite, but the acid load must be balanced against the acid-plant limit. The main risk is re-dissolving iron and creating a recycle loop that increases purification loading. Pilot testing should compare zinc in washed residue, wash-water consumption, and iron return before full-scale change.