In order to succesfully be made into steel in a blast furnace, iron ore needs to be in a form that allows the free movement of oxygen around the particles. One way to achieve this is to agglomorate the ore into pellets, which are round balls of concentrated ore. I will attempt to describe the iron ore pelletizing process below.
In the palletisation plant, a suitable binder such as Bentonite is added to ground iron ore or iron ore concentrate which is mixed in a high intensity mixer. In order to harden the pellets, they are heated to temperatures between 1290C to 1340C. This treatment can sometimes cause certain chemical reactions to occur that change pellet’s properties. These reactions can include the dehydration of hematite or the oxidation of magnetite; in many cases “fluxed pellets” are produced by the addition of limestone or dolomite to the balling feed. These additions react with the gangue in the iron ore to enhance the performance of the pellets in certain downstream processing steps.
Iron ore pellets can be made by two different processes, the Straight Grate process and the Grate Kiln process. The Grate kiln system consists of three machines; a traveling grate, a rotary kiln and an annular cooler which heats, hardens and cools the ore respectively. These three different machines can be independently controlled, all operating at different speeds etc. This is not possible in a Straight Gate system as this consists of one continuous conveyor. This flexibility of the Grate Kiln process means that it can be switched from processing Magnetite to processing Hematite in just a few hours.
The travelling gate is for drying and pre-heating of the balls before they are transferred to the rotary kiln. The balls do not have the physical properties necessary to survive direct feeding to the rotary kiln and must be semi-hardened and to do this, the balls have to be substantially heated and any fluxstone carbonates need to be calcinated by removing any Carbon Dioxide.
The rotary kiln is a downwards-sloping cylinder from the traveling grate to the annular cooler. The rate of flow through the kiln is controlled by the speed of rotation and while they are traveling through the cylinder, there is a mixing action on the pellets. This action is important because it creates a uniform pellet size and all of the pellets are exposed to the burner flame for an equal amount of time. At the discharge end of the rotary kiln is a single burner, which uses the hot air from the annular cooler for combustion. Various different fuels such as oil, gas or coal can be used to start the combustion process.
The annular cooler is similar to the traveling grate. The hot pellets from the rotary kiln are fed into the annular cooler and air is forced upwards through the conveyors and the bed to avoid exposure of the machine parts to high temperatures. The pellets are levelled in the cooler to a depth of 660 mm and conveyed over several cooling zones. The various cooling zones are designed in such a way that they will recover the maximum heat from the hot pellets and various ducts carry this hot air to the other parts of the process. The cooled pellets are finally discharged through the discharge hopper at a controlled rate to a product load-out system.