Chemical waste is treated by turning each hazard into something harmless before it leaves the site: acids are neutralised with lime, alkalis with dilute acid or carbon dioxide, chlorine gas is absorbed in caustic soda, and dissolved metals are precipitated as hydroxides and buried in a lined pit. The reagent mass follows from the equation: 2 kg of sulfuric acid needs 1.51 kg of slaked lime, and 1 kg of chlorine needs 1.13 kg of sodium hydroxide. Never discharge waste into a water source, and never mix waste streams that can react.
Part of Industrial Chemistry
Neutralising, absorbing and disposing of acid, alkali, chlorine, metal and solvent waste, with the reagent arithmetic.
The Carrier’s Treatment Table
The carrier lists treatments for six waste types.
| Waste | Treatment | Method |
|---|---|---|
| Acid waste | Neutralise with lime (CaO or Ca(OH)2) | Add slowly, stir, test pH until neutral |
| Alkaline waste | Neutralise with dilute acid or CO2 | Bubble CO2 through or add vinegar |
| Chlorine gas | Absorb in NaOH solution | Bubble through caustic soda |
| Heavy metal solutions | Precipitate as hydroxides | Add NaOH, filter solids, bury in a lined pit |
| Organic solvents | Evaporate and burn vapours | Controlled incineration with good airflow |
| Calcium chloride (Solvay waste) | Dilute and discharge to land | Spread thinly on non-agricultural land |
The carrier’s warning: never dump chemical waste into water sources, because even neutralised waste may carry toxic metals or residual chemicals; dispose on land, downhill and downstream of the water supply, in pits lined with clay.
Four Notes On That Table
- Neutralisation gives off heat and, for sulfuric acid, a solid. Add the neutraliser slowly, stir, and check the pH by indicator (pH Testing); stop at neutral. Calcium sulfate formed is only slightly soluble and settles as sludge.
- Metals dissolve again in too much alkali. Some metals, notably zinc and lead, form hydroxides that redissolve if sodium hydroxide is overdosed, a general chemistry fact. Add alkali until the precipitate forms and no further; do not pour in excess.
- Solvents are best reused, by redistilling (Distillation Apparatus). Burning, the carrier’s last-resort route, must be done outdoors at a distance from buildings and people. Never burn chlorinated solvents, which make toxic gases when burned (a general fact). Solvent stores follow Storage Containers.
- Calcium chloride is a salt that kills plants. Spread thinly on non-agricultural land, as the carrier says, and away from drains.
What You Need
- Neutralisers: slaked lime (Slaking), caustic soda (Caustic Soda) and vinegar or CO2.
- pH indicator (red cabbage juice).
- A chlorine scrubber: a tank of caustic soda solution with the gas bubbled through, vented outdoors.
- A lined disposal pit with clay lining, sited downhill and downstream of any water supply.
- A waste log: what, how much, when, how treated, where placed.
Method: Neutralising Acid Waste
Step 1: Estimate the acid in the waste.
Step 2: Compute the neutraliser mass from the equation (below), add a little more for margin.
Step 3: Add the lime in small portions to the waste (not the reverse), stirring.
Step 4: Check the pH, continue to neutral.
Step 5: Let the solids settle, decant and place the sludge in the lined pit.
Worked Examples
Neutralising sulfuric acid. H2SO4 + Ca(OH)2 → CaSO4 + 2H2O. Molar masses (H 1, O 16, S 32, Ca 40): H2SO4 = 98; Ca(OH)2 = 40 + 34 = 74; CaO = 56.
- 2 kg of pure sulfuric acid = 2,000 ÷ 98 = 20.4 mol.
- Slaked lime needed: 20.4 × 74 = 1,510 g = 1.51 kg. As quicklime: 20.4 × 56 = 1,143 g = 1.14 kg.
- For waste at 10% acid by weight (illustrative), 20 kg of waste holds 2 kg of acid, so it needs 1.51 kg of slaked lime.
Absorbing chlorine. Cl2 + 2NaOH → NaCl + NaClO + H2O. Cl2 = 71; NaOH = 40.
- 1 kg of chlorine = 1,000 ÷ 71 = 14.1 mol.
- Sodium hydroxide: 2 × 14.1 = 28.2 mol × 40 = 1,127 g = 1.13 kg of pure NaOH.
- As a 10% solution (illustrative): 1.13 ÷ 0.10 = 11.3 kg of solution per kilogram of chlorine.
- The product contains sodium hypochlorite, which is bleach. Treat it as a bleach: never add acid or ammonia to it. See Chlorination.
Solvay waste. For each tonne of soda ash, theory gives 111 ÷ 106 = 1.05 tonnes of calcium chloride (Yield and Efficiency). A plant making a tonne a day must dispose of about a tonne a day, hence the carrier’s instruction to dilute and spread it thinly, and the need to plan the ground for it.
Common Mistakes
- Adding acid to the lime slurry or water to acid: spatter and boiling.
- Overdosing alkali and redissolving metals.
- Disposing of waste upstream of the well.
- Mixing waste streams, for example bleach liquor with acid waste, which releases chlorine.
- Not logging. A pit nobody remembers is a future hazard.
Safety
Waste is still hazardous until it is treated, and treatment makes heat and gas
Neutralisation warms the mixture; chlorine absorbers fill with bleach that releases chlorine on contact with acid; metal sludges are poisonous. Wear eye and skin protection, work outdoors, keep streams separate, and keep all waste away from drinking water and from fields where food grows. See Toxic Gases and Safety and Waste.
What’s Next
- Learn next: Safety and Waste — the wider hazard picture
- Combine with: Solvay Chemistry — where the biggest waste stream is made