What is Slag?
Slag is essentially a waste product of ore smelting. Mined ores are impure collections of oxidized metals, silicates and other unwanted compounds. When the ores are heated to purify the desired metal, slag is left behind.
However, slag is also deliberately introduced into some smelting reactions in order to control the chemistry of the reaction. The introduced slag is sometimes called synthetic slag. For example, when making steel, quicklime and magnesium may be introduced to assist in neutralizing impurities removed from the ore, which helps protect the furnace and also helps remove further impurities from the ore. The introduced synthetic slag combines with the impurities to form even more waste slag, which needs to be removed.
The exact chemical composition of slag varies depending on what type of smelting was being done, and on whether synthetic slag was added to the reaction. However, all waste slag looks kind of the same, like coarse pebbles. That is because the waste slag is pumped out of the furnace, cooled with water and passed through several filters. The pebbled slag is then dried and ground up into a very fine powder. And then it needs to be taken away from the refinery before the refinery fills up to the brim with it.
Who Uses Slag?
Fortunately, a use for waste slag has been found: as a component of high-performance cements. So the waste slag from the ore refineries needs to be transported to the cement mixing factories.
How is Slag Transported?
Transporting very fine powders requires specialized equipment. In small quantities it can be packaged into plastic bags and carried around by regular trucks and trains, but transporting bulk powder in this fashion is cost-prohibitive.
Special tanker trailers for both railways and roads have been developed for transporting fine powders. They look quite similar to tankers designed to transport liquids such as oil and milk. Essentially, the powders are sort of poured into the special trailers, hooked up to a regular truck, and driven to their destination, where they are then poured out into a storage hopper or silo.