|The iron for the process came in two forms, firstly as clean scrap steel and secondly as Copperas which is the biproduct of another important pigment industry. Copperas is the compound iron sulphate. It was dissolved to make a very strong solution which was used in the process.
During the process the pigment, a yellow slurry, was produced in vats of a varying sizes. These process vats were filled with the strong solution of iron sulphate made with the Copperas and were charged with the scrap steel. Air was bubbled through and the temperature was raised to 70 degrees C using hot water or steam.
|Click on image to see video clip: Adding the Copperas (200K)|
|Two chemical reactions now took place. First the oxygen in the air oxidised the iron sulphate to form the yellow iron oxide pigment and generated sulphuric acid. In the second reaction this sulphuric acid attacked and dissolved the steel, producing iron sulphate solution again and hydrogen gas. These reactions took place at the same time and the iron sulphate solution remained unchanged. The two reactions can be seen as one reaction which was basically the controlled rusting of iron.
Iron + Air -> Iron Oxide solids (pigment) + Hydrogen gas
Iron oxide solids formed as needle-like crystals. In order to produce a clean bright yellow colour, the crystals needed to be all the same size. To make the crystals uniform some very fine iron oxide crystals were added to the vat when it was filled. They are called seed and were made in another part of the process. Scroll to end of page for more details on seed making.
|Click on image to see it larger Looking down into an aeration vat, where the seed was made. Here it had already been taken out of service and the pipes had been cut off.|
|Click on image to see video: Aeration Vat (204K)|
|The seed crystals gave a base for the iron oxide to grow onto and kept the product uniform in crystal size. For a lighter yellow more seed was added. As the crystals were more numerous, they were therefore smaller. So for a deeper yellow, less seed was added and the crystals grew larger.
The mixture remained for around 4 days in the process vats. The vats were emptied when the required colour was attained.
The yellow slurry was now separated into iron oxide solids and the iron sulphate solution in which the product grew. This was done by filtering the product slurry in large multi-compartment, vertical pressure filters.
First the slurry was sieved to remove any debris washed out of the scrap steel. The slurry was heated to make filtration easier and was pumped into the filter. In the filter the product was partially compressed, washed, further compressed and dried. It was discharged from the press in cake form, but was still in fact 50% water.
|Click on image to see video: Press and Conveyer Belt (936K)|
|The iron oxide "cake" then had to be dried or calcined depending on which colour was to be made.
|Click on image to see video: Drying Yellow Iron Oxide (848K)||Click on image to see video: Milling Yellow Iron Oxide (240K)|
Yellow cake was taken to a higher temperature in a rotary kiln called a calciner. The kiln was a long inclined tube which rotated slowly causing the product to slowly tumble down through the kiln. At the lower discharge end was a gas or oil burner whose hot combustion gases passed up the kiln to meet the cake coming down. The hot gases leaving the kiln were quite dusty. They therefore passed through a high temperature fabric filter unit, so that discharge leaving the premises was dust-free.
The yellow cake tumbling down the inside of the kiln was first dried. The heat then removed the tightly bound water causing it to turn red. Finally the high temperature sintered the red making it deeper in colour. The kiln hot zone was between 650 and 950 degrees Celsius.
The calcined iron oxide was milled to a fine powder to improve the pigment strength and colour.
|Click on image to see video: The Calciner (396K)||Click on image to see video: Red Packing Area (624K)|
To make seeds weaks were passed through a pit of steel turnings. This iron pit process reduced the iron sulphate to the right state for seed making. A small addition of acid was made to activate the reaction. The treated weaks were pumped into large stirred tanks. Caustic was added to make the solution alkaline. All the iron sulphate was converted to iron hydroxide solids and no iron was left in the solution.
|Return to Oxide Works: Contents Page|