Carbide lime sludge: how much there is what it still holds, and how to handle and dispose of it safely

For every tonne of carbide an acetylene plant gasifies, about 1.17 tonnes of dry calcium hydroxide leaves the generator, carrying with it most of the carbide's sulphur, a tenth of its phosphorus and dissolved acetylene, at a pH of 12.2–12.9. Nobody buys an acetylene plant for the lime, but the lime pit is part of the generator's hazard envelope and the by-product is the plant's largest effluent stream. This page sets out the quantities, what the sludge still holds, how the pit should be sited and run, and where Indian waste law currently stands.

The arithmetic starts from the generator reaction, CaC₂ + 2 H₂O → Ca(OH)₂ + C₂H₂, applied to a commercial carbide of 300 l/kg gas yield, about 82 % CaC₂ with 12 % free lime and 6 % inert impurities. The mass balance compiled by S. A. Miller (1965), checked against the page image, is:

The dry residue is therefore about 95 % Ca(OH)₂ (1,106 of 1,166 g), with the balance being the carbide's silica, alumina, iron oxide, ferrosilicon, silicon carbide, free carbon and magnesia, carried through essentially unchanged. Older KIGL material quotes 1.27 tonnes per tonne; that figure came from a misread digit and 1.166 is the corrected value.

Two chemical facts about the residue follow from the impurity chemistry set out on the phosphine page. Under 3 % of the carbide's sulphur leaves as hydrogen sulphide, so over 97 % of it, about 4 g of S per kg of carbide, stays in the sludge as calcium sulphide; about 10 % of the phosphorus and most of the nitrogen, as ammonia, stay with it too. EIGA Doc 226 makes the same point from the operating side: ammonia, hydrogen sulphide and phosphine remain in solution in the lime slurry unless the pH changes significantly, and are recycled with return water or shipped off site with the lime. Ammonia is toxic to fish. Any buyer who acidifies the slurry, which is what a neutralisation customer does, will drive those gases out at the point of mixing.

As to physical form, EIGA Doc 143 gives the slurry from a wet generator at 25–30 % lime solids by weight, pumpable above about 65 % water, needing a mechanical digger at 60–65 % water, and reducible to a paste of 50 % water or less by filter press. EIGA Doc 123 gives 10–12 % solids from the wet process, 30–40 % after decanting and 50 % or more from a pond, with a filter press at 8–14 bar giving a cake at about 15 % moisture. Slurry density after simple decanting is about 1.2 and the pH is 12.6–12.8 (EIGA Doc 143 Table 1), independently confirmed at 12.2–12.9 across three Malaysian plants by Ramasamy, Agamuthu and Ibrahim (2002).

This is the part of carbide lime handling that the trade under-rates. Acetylene dissolves in lime slurry essentially as it does in water: the figures of Strizhevskii and Strebulaeva (1955), as compiled by S. A. Miller (1965), are 0.685 g per litre at 60 °C and 0.438 g per litre at 80 °C. The liquor leaves the generator saturated at generator temperature and gives the gas back as it cools and stands. The loss is quantified: dissolved acetylene in the sludge is about 1 % of a wet generator's make at 60 °C and 0.5–0.6 % at 75 °C (Hölemann et al., as compiled by S. A. Miller, 1965), which is 70–150 times the loss from a wet gasholder. Running the generator near the top of its temperature band nearly halves the largest single loss in the plant, and EIGA Doc 226 adds that at 70–85 °C in a well-run generator the lime pit emissions are insignificant. The temperature ceiling itself is discussed under generator overheating.

The second source of gas is aftergeneration. Carbide that gasifies too slowly, or lumps that became coated in lime before they finished reacting, carry on generating in the pit; the C.P.I. (1968) required settling basins to hold residues long enough for intact pieces of carbide coated in lime to decompose completely, and both the German regulations and the American IAA standard of that era carried clauses on acetylene evolved by the sludge after discharge. The same compilation states the hazard in one sentence: if the acetylene dissolved in generator slurry is allowed to evolve into a semi-confined space, an explosive mixture may form.

The effluent liquor typically carries 500 mg/l of acetylene, 150 mg/l of hydrogen sulphide and 0.8 mg/l of phosphine. A published treatment clears all three: acidify to pH 5 and bubble air through at 11 litres per hour per litre of liquor, which removes the acetylene in 60 minutes, the H₂S in 45 and the PH₃ in 30. That is a controlled stripping operation to be done outdoors with the off-gas vented safely, never in a tanker or a buyer's pit. Acidifying a material whose value is its alkalinity is self-defeating if the lime is to be sold. The simpler control is heat and time.

The question every State Pollution Control Board will eventually ask is whether carbide lime is a hazardous waste under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules 2016. The position, as checked against the gazette text, has two parts.

Schedule I, the list of 38 processes generating hazardous waste, contains no entry for acetylene production, industrial gases, calcium carbide or lime, and the words do not appear anywhere in it. On the process route, carbide lime is not listed.

Schedule II is a different matter. Class C2 (Corrosive) captures a waste that is aqueous and has a pH less than or equal to 2 or greater than or equal to 12.5, together with a steel-corrosion limb of 6.35 mm per year at 55 °C; these are the US RCRA criteria transposed into Indian law. Carbide lime slurry sits at about 12.2–12.9 (12.6–12.8 in EIGA Doc 143). The route into hazardous-waste classification is therefore open on characteristics, not closed. Three things still help: the limb applies to an aqueous waste as presented, so a dewatered paste is a different material from a slurry; pH falls as the lime carbonates on standing in air; and a laboratory pH on the material as actually disposed of is determinative. The practical step is a NABL-accredited analysis of your own residue, fresh and pit-aged, for pH, sulphide, phosphate, ammoniacal nitrogen and the Schedule II metals, followed by a written question to the State Board with the analysis attached.

If the material is non-hazardous, it is managed as industrial solid waste under the plant's Consent to Operate, and selling it requires the Consent to recognise it as a by-product. If a Board treats it as hazardous, utilisation runs through the CPCB procedure under Rule 9 (December 2021), with one useful exception: co-processing in a cement plant needs no trial run where CPCB guidelines already exist. EIGA Doc 226 states that lime slurry should be considered a by-product and not a waste, on four conditions: it is intended for marketing rather than disposal, produced to a specification, supplied with a safety data sheet, and usable without further processing, with regular analysis to support the claim. Those four conditions are also the sensible internal standard for any Indian plant that wants the pit to stop being an effluent. What the lime can be used for is set out in uses of carbide lime.