Chromium oxide is safe, but the water that washes it is not
Green paint on roofs, glaze on tiles, refractory brick in the furnace of a steel plant: all of it is chromium oxide. Chromium(III) oxide is a dark green powder, one of the most durable pigments in the world: it does not fade in the sun, does not fear acids and withstands temperatures of hundreds of degrees. A tonne costs about $4,000: about $3,700 in China, about $5,500 in the US, about $4,000 in Japan. It is safe in itself, but the water used to wash it at the plant contains traces of hexavalent chromium, one of the most toxic metals in industry.
Washing the pigment creates an effluent that cannot be released
Chromium oxide is obtained by reducing sodium dichromate with sulfur or coal: the mixture is calcined in a furnace, and what comes out is chromium oxide and sodium sulfate. There is a lot of sulfate: for every tonne of oxide, almost as much salt. The oxide has to be washed free of this salt until only 0.1–0.5% remains, otherwise the pigment loses quality. All the salt ends up dissolved in the wash water, and along with it traces of chromate. It is toxic and carcinogenic, so the discharge limits for such water are among the strictest. It cannot be released into a river.
The classical answer is steam, and then a landfill
Usually the water is evaporated with steam to crystallization. The salt drops out as crystals, and the water is returned to washing. But the crystals contain traces of toxic chromium, so they are not sold but taken to special landfill. The plant pays twice: for steam to get the salt, and for burial to get rid of it. Take a site of medium capacity: 20,000 t of chromium oxide a year with revenue of about $80 million. It produces about 18,600 t of sulfate a year, and the effluent after washing is about 250,000 t a year, or about 680 t a day. The plant spends $10–23 million a year on this water: steam $6–14 million, chromium losses $0.2–0.8 million, carbon $0.6–2.3 million, burial $1.2–2 million, fines and upkeep $1.7–4.3 million.
Standard treatment methods remove one problem and bring another
Precipitating chromium with reagents gives chromium sludge, which also has to be buried. Ion exchange needs regeneration with acid and alkali, and these solutions become a new effluent. Membranes clog with sulfate. Every path removes one problem and brings another, and the salt and the water remain.
ARBOK-Cr evaporates the effluent in a cold vacuum and leaves nothing for a landfill
The ARBOK-Cr technology works in a deep vacuum of about 1 kPa, where water boils at 6–10 °C. There is boiling, but without heating: the solution stays cold, and a closed loop with leak control keeps it inside. The water returns as clean condensate to the washing of the oxide. The condensation heat returns to the cycle through recuperators, up to 98% of the energy goes round in a circle, and 0.7–1 kWh per tonne of effluent is left for electricity. Everything dissolved stays in the still and is removed by a screw as a dry and separated residue: chromium is separated from the salt, and the salt is polished by add-on equipment on the same unit and sold as dry sodium sulfate at $100–200 per tonne. There is no liquid sludge and no landfill: this is the ZWD technology, Zero Waste Discharge. A flow of 680 t a day is handled by 4 container modules of 200 t a day each, installed in 2–4 weeks without stopping production.
The site stops paying for steam and burial and starts selling salt
Steam disappears: electricity for the whole flow costs about $0.05 million a year instead of $6–14 million. Chromium losses fall several times over, burial disappears, carbon is not emitted at all and the freed quotas can be sold. In all the site spends about $0.3 million a year instead of $10–23 million, and the sale of dry sulfate adds $2–4 million. The total effect is $12–27 million a year, which is 15% to 34% of revenue, or $600–1,350 for every tonne of chromium oxide. Condensate returns to washing in the classical scheme too, but every cubic metre is paid for with steam; in a cold vacuum the same return costs about 1 kWh per tonne, so the pigment can be washed longer and more thoroughly.
On the off-take model the site gets the savings without investment
On the off-take model the operator pays back the set in 5–7 years, and the site gets the savings with no investment. If the world makes about 200,000 t of chromium oxide a year, that is about 10 such sites: their effluents cost $0.1–0.3 billion a year and 46,000–116,000 t of carbon dioxide, and this is how much profit a year ARBOK-Cr can bring the world industry. The most benefit is where chromium chemistry is largest: China, India, Kazakhstan, Turkey. Green is not always clean, but the water behind it can be.