Uses of carbide lime: which outlets are proven which are possible, and which do not work

The calcium hydroxide that comes out of an acetylene generator is not a weak or contaminated lime. A peer-reviewed side-by-side comparison measured more calcium hydroxide in carbide lime than in commercial hydrated lime, 88.4 % against 86.1 %, with less magnesia and the same iron and alumina. The things that limit its use are physical: about 1.8 % graphite that turns it grey, a coarser and less reactive surface, variability from one carbide source to another, and the residual sulphide, phosphide and ammonia it carries from the generator. This page sets out which outlets the evidence supports, with the source for each, and which it does not. How the sludge is handled in the plant is covered in carbide lime sludge handling and disposal.

The residue from a carbide-to-water generator is about 1.17 kg of dry solids per kg of carbide gasified, mostly Ca(OH)₂ with the carbide's silica, alumina, iron, ferrosilicon and free carbon carried through. As discharged it is a slurry of 25–30 % solids at pH 12.6–12.8 (EIGA Doc 143, Table 1). The best measured comparison with commercial lime is Cardoso, Fernandes, Pileggi, Cincotto and John, Powder Technology 195(2):143–149, 2009:

The same paper found carbide lime agglomerating in water after about an hour, its coarse fraction growing from 78 to 174 µm over 90 minutes while commercial lime got finer. EIGA Doc 143 states that carbide lime has identical properties to calcium hydroxide from natural limestone and warns that composition, especially of the impurities, varies by plant with the carbide source. That warning is the most useful sentence in the guide: no published analysis substitutes for an analysis of your own lime.

Neutralisation and water treatment

EIGA Doc 143 lists slurry use for neutralising effluent acidity, precipitating and flocculating impurities, treating wastewater-plant sludge, neutralising acid surface-treatment baths and stabilising and sanitising sludge, and states that recovery as a neutralising agent is about 70 % of the European market. Ramasamy, Agamuthu and Ibrahim (Waste Management & Research, 2002) report Malaysian sludge sold as slurry to industry for the same purpose, and found that a rotary-drum vacuum filter gave a clear filtrate at pH 7 with 98 % of the metals retained in the cake. This is the most forgiving market: colour and carbon do not matter, and the trace metals in the lime are the metals the process is removing anyway. Miller's compilation of 1960s practice records pickling-acid neutralisation in steelworks and sewage-sludge conditioning at named American and British works with very satisfactory results.

The one thing the buyer will discover is the impurities. EIGA Doc 226 §6.3 states that ammonia, hydrogen sulphide and phosphine remain in solution in the lime slurry and are shipped off site with it; a customer who acidifies the slurry liberates them at the point of mixing, and ammonia is toxic to fish. Declare them on the safety data sheet.

Cement

Calcium hydroxide does not need to be decarbonated, so a cement kiln burning carbide lime instead of limestone saves calcination heat and emits no process CO₂ from that part of the feed. Liu Yaojun and co-authors (ZKG International 5–6/2020) describe a Chinese works running 100 % carbide slag as its calcareous raw material on two 3,000 t/day clinker lines, drying it to 1.5 % moisture at a dryer outlet of 100–130 °C, and record the handling problems: stickiness and clogging, fines blocking cyclones, tramp iron jamming feeders, and dry slag that can explode in storage. In India, Rule 9(3) of the Hazardous and Other Wastes Rules 2016 exempts co-processing in a cement plant from a trial run where CPCB guidelines exist, which would give cement the simplest route if a Board classed the lime as hazardous; whether it is hazardous under those Rules has still to be settled with the State Pollution Control Board, so the route should not be treated as cleared. The obstacle is scale and distance: a single acetylene plant produces a few thousand tonnes a year against a cement plant's million.

Soil stabilisation, bricks and mortar

Under the geotechnical name calcium carbide residue, carbide lime has a substantial recent peer-reviewed literature as a clay and expansive-soil stabiliser, generally combined with a pozzolan such as fly ash, rice husk ash or slag; EIGA Doc 143 lists clay stabilisation and load-bearing improvement for road construction as a paste or slurry application. Indian road work follows MoRTH and IRC specifications, and whether these admit a non-quarried lime has not been established, so the route in is a specific project with its own test data rather than a general product.

Miller's 1965 compilation is specific on building uses. Carbide sludge with three parts of sand gave a mortar stronger after 28 days than the corresponding product from commercial lime; carbide lime with less than 11 % or more than 40 % water was usable in mortar, with difficulty at intermediate water contents; lime-sludge filter cake was confirmed as suitable by the Chalk, Lime and Allied Industries Research Association, but unsettled slurry was not, because its sulphide and soluble salts could be excessive. Furnace slag with 10 % carbide lime, compressed with 10–12 % water and autoclaved for eight hours at 8 atm, gave a crushing strength of 170 kg/cm². Lime-fly-ash bricks are grey anyway and strength-driven, which makes them the most natural building outlet; one Indian acetylene producer advertises its sludge to that trade.

AAC blocks and IS 712

IS 2185 (Part 3):1984, the Indian standard for autoclaved aerated concrete blocks, requires at cl 5.2 that the lime satisfy Class C of IS 712, the building-lime specification. The clause specifies lime by quality, not by geological origin. On composition, carbide lime should pass with a margin: on the Cardoso analysis its calcium oxide on the ignited basis is about 96–97 % and its magnesia 0.06 % against a 6 % maximum. The binding limits are elsewhere. IS 712:1984 caps CO₂ at 5 % and free moisture at 2 %, so the lime must be dried, and pit-aged carbide lime carbonates on standing: EIGA's slurry analysis shows 16.3–22 % CaCO₃ on dry matter, equivalent to 7–10 % CO₂, roughly double the limit. An IS 712 product needs fresh residue, dewatered and dried under cover and kept sealed, which no European acetylene producer does.

Agriculture

EIGA Doc 143 gives real dosing: maintenance liming about every three years at roughly 500 units of CaO, or two tonnes of 25 % CaO slurry per hectare, and corrective liming about every ten years at about four tonnes per hectare, with spreading subject to local regulation. Miller's compilation calls agricultural use a valuable outlet for the dry product but notes that wet material is difficult to spread. Acid soils are common across Assam and the North East, which makes the use attractive in KIGL's region, but the basis for selling a liming material under the Fertiliser (Control) Order 1985 has not been established, and spreading wet slurry on smallholdings needs equipment Indian farmers do not have.

Whitewash and plaster finishing coats need whiteness, and 1.8 % graphite is a defect that cannot be economically processed out. Sugar mills use lime in food contact and buy quicklime to slake themselves; the European beet-sugar use EIGA records runs under a food-safety regime with a registered, analysed product, and the sulphide, phosphide and ammonia content argue against it in India. Paper mills recover their causticising lime in their own kilns and would be contaminated by foreign carbon and silica. Flue-gas scrubbing at a power station uses 25,000–75,000 tonnes a year and, in EIGA's words, needs guaranteed uniform quality and continuous large supply, one to two orders of magnitude beyond a single acetylene plant.

Returning lime to a carbide furnace is real industrial practice; the US Geological Survey (Mineral Commodity Summaries 2026) notes that quicklime is regenerated from waste hydrated lime in the carbide industry. But the literature compiled by S. A. Miller (1965) records that recycle lime can be used only in limited proportion to fresh lime, because the impurities recirculate and carbide output falls, and it has to be dried and calcined first. That is an option for an integrated carbide works, not for a plant that buys its carbide.