What is Salt Slag?
Salt slag is a waste material generated by secondary aluminium processing (the recycling and smelting of new aluminium from scrap, aluminium dross and other wastes).
Due to the nature of the contaminants it contains, salt slag (or salt cake as it sometimes known) is classed as a hazardous waste in most countries and banned from landfill. Disposing of the waste in an environmentally sustainable and cost effective way is a major challenge for the aluminium smelting industry.
How is it created?
The most common types of furnace used by the secondary aluminium industry are rotary or reverbatory furnaces. In these furnaces scrap aluminium is melted under a bath of molten salt flux which floats on the surface of the molten metal. The salt flux is a mixture of sodium chloride and potassium chloride, often with small amounts of fluorides (Cryolite).
Aluminium has a tendency to oxidize with the heat of the furnace so the salt provides an effective cover for the metallic aluminium, minimizing losses to oxidization and increasing metal recoveries. The fluoride in the salt helps break down oxide layers already on the surface of the scrap and so helps to improve separation between the aluminium and non-metal contaminants.
The molten metallic aluminium is tapped from the furnace and the last bit, where it meets the salt layer, contains some residual aluminium metal, plus various metal oxides (mainly aluminium oxide) and the salts. This mixture solidifies and cools to become salt slag; sometimes known as salt cake or black dross.
What to do with salt slag waste?
Historically, salt slag was land filled but this has caused a number of problems, including salt and other contaminants leaching into the water course and (being exothermic on contact with water) causing underground fires. As environmental awareness and regulations have developed, landfill of salt cake has been banned in many countries and the options for disposal have become a major issue for the industry. Salt slag processing and treatment for recycling is now the only sustainable option.
Ultromex treatment technology
Allows secondary smelters to process and recycle their own salt slag wastes as they arise, recovering the small amount of trapped metal to re-melt, recovering the salts to use again and with the residual aluminium oxides becoming a useful feedstock for other industries and – crucially – leaving zero waste for landfill.