Acetylene plant waste: every stream from a carbide-to-water plant how much there is, what makes it hazardous and where it must go

A carbide-to-water acetylene plant makes one product and one very large by-product. For every kilogram of carbide it gasifies it gives about 300 litres of acetylene (KIGL's published yield schedule) and about 1.17 kg of dry calcium hydroxide, which leaves the generator as a lime slurry. Everything else the plant discards is small by comparison, but several of those smaller streams are more hazardous than the lime: spent purifier mass, drier brine, used oil, solvent, vented gas, carbide dust, empty drums and old cylinders. This page lists every stream, how much there is where a figure can be sourced, what makes it hazardous, the Indian rule that applies and where it should go.

CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂ turns 64 g of carbide into 74 g of calcium hydroxide. For a real 300 l/kg carbide the mass balance compiled by S. A. Miller (1965) gives 1,106 g of Ca(OH)₂ and 60 g of other residue per kilogram, about 1.17 kg dry. It leaves the generator as a 12–20 % slurry; settled sludge is 2–3 kg per kg of carbide, or 6–9 tonnes per tonne of acetylene. At KIGL's published 300 l/kg, each cubic metre of acetylene means about 3.3 kg of carbide and 3.9 kg of dry lime.

The sludge cannot go to a sewer: it is strongly alkaline, carries sulphur, and its liquor deposits calcium carbonate in pipes. KIGL's plant manual sends it to a vented outdoor pit, to stand at least ten hours in not less than four times its bulk of water, and PESO places drying pits 9 m from the acetylene plant. Fresh sludge holds dissolved acetylene, about 1.0 % of the make at a 60 °C generator and 0.5–0.6 % at 75 °C (Hölemann and co-workers, 1959), which is why the pit must be open. Handling, the Schedule II pH question and by-product sale are covered in carbide lime sludge handling and disposal and uses of carbide lime.

The clear water above the settled lime typically carries about 500 mg/l of acetylene, 150 mg/l of H₂S and 0.8 mg/l of PH₃ (Strizhevskii, 1961). KIGL's manual returns this acetylene-enriched water to the generator with a flameproof diaphragm pump, closing the water loop. EIGA Doc 226 §5.3 adds that lime water must never be drained uncontrolled and must not be used as cooling water. Acidifying to about pH 5 and blowing air through strips the dissolved gases within an hour (Strizhevskii, 1961), but that releases them and is a controlled operation, not something to do in a drain.

The cooler-condenser condenses water and ammonia and catches lime; its condensate is drained hourly. The scrubber catches carried-over purifying mass and is drained and refilled weekly. Both waters carry lime, ammonia, fine dust and dissolved acetylene, so drain them slowly into the lime pit or the consented effluent system, never to a storm drain.

Cylinder acetylene is dried over calcium chloride in stages: as compiled by S. A. Miller (1965), a low-pressure drier of 75–85 % CaCl₂ takes out the bulk of the water and the high-pressure gas then passes through 95–99 % CaCl₂. The bottom of each bed absorbs the water and dissolves, and the liquor is drained off while fresh solid goes on top. That liquor is a strong chloride brine with dissolved acetylene and, on the high-pressure side, traces of oil. KIGL's plant operation manual also calls for the high-pressure drier interior to be flushed with kerosene or fuel oil, then water, after 200,000 m³ of production. Collect the brine, never send it to a sewer, send the oily flush with the used oil, and never fit brass drain valves. See acetylene drying.

The acetylene compressor runs on SAE 30 non-detergent oil, and its traps are drained hourly of water with emulsified oil; grey crankcase oil is changed. Because Schedule I of the 2016 Rules lists industrial operations using mineral or synthetic oil as a lubricant, used oil and oily residues are kept in closed, labelled drums on a bunded floor and handed to an SPCB-authorised recycler. Oil never goes to the lime pit, where it spoils a saleable by-product.

Acetone leaves with the gas on every use, on average about 0.0075 kg per litre of water capacity per fill against about 0.00025 kg/l for DMF (AIGA 037/16; see acetone vs DMF). EIGA Doc 225 says acetone and DMF must never be mixed and recovered solvent is generally not recommended for reuse. A retired cylinder's porous mass is a waste in its own right: modern monolithic calcium silicate is inert, but older masses can contain asbestos, which the US Chemical Safety Board's June 2006 bulletin on the Praxair St Louis fire reported being spread from ruptured acetylene cylinders. Scrapping an old fleet is an asbestos programme.

Acetylene leaves through safety valves, hopper vents at recharging, start-up purges and the outdoor venting of drained-back cylinders. KIGL's manual pipes every vent outside the building to at least 5 m above ground, ends covered with wire mesh. EIGA Doc 226 §6.3 names small amounts of acetylene, and arsine, as the gaseous emissions of a well-run lime pit, with no practical abatement for arsine, and notes that they are insignificant from a well-run generator at 70–85 °C. PESO's 90 m separation from air compressor intakes keeps vented gas out of compressed air; see layout and safety distances.

Carbide comes in moisture-proof steel drums, normally of 50 or 100 kg (IS 1040:1987 cl. 6.1; the standard has since been revised as IS 1040:2024), so each tonne used leaves 10 to 20 drums with dust in the bottom. EIGA Doc 226 §6.5 forbids putting carbide dust straight into the lime pit because of the explosion risk; slake it first, in small amounts, in the open and away from ignition sources. Wash out empty drums with plenty of water in the open, leave them open until no gas remains, and only then make them unusable for scrap; never cut, grind or weld a drum that has not been washed out.

Under the Gas Cylinders Rules 2016 a fire-damaged dissolved acetylene cylinder must be condemned and destroyed by a qualified person (Rule 24(2)), a weld-seam leak may not be repaired (Rule 12(5)), and a cylinder used to generate gas is deemed condemned (Rule 36(4)). PESO-recognised testing stations must have facilities for evacuation, neutralisation or scrubbing, and condemnation (Rule 35(3)). The gas is drained back, the solvent removed, the porous mass taken out as a separate tested waste, and the shell made unusable before scrap.