White Liquor Preparation

White liquor preparation is the final part of the recovery process, referred to as the causticizing process. Here the sodium carbonate of the green liquor is converted into sodium hydroxide (NaOH) by reducing with burnt lime (CaO).

At the same time, lime mud (calcium carbonate CaCO3) is formed. This is then separated and washed in a lime mud washer with the filtrate being the weak liquor. This new cooking liquor is called white liquor (in fact it is slightly yellow). The washed lime mud is then burnt in a lime kiln at very high temperature to produce burnt lime once again.

The lime kiln is sometimes fuelled with oil or gas but more often with wood residues. These residues are first dried and then pulverised or degasified before being fed into the kiln. The lime kiln is a large, horizontal steel cylinder with a diameter of 2 to 4 meters and a length of 40 to 120 meters. The cylinder rotates at 1 to 3 revolutions per minute with a slope of 2 to 30 towards the burner.

Some of the most difficult applications for all valves are found in the white liquor plant. The green liquor is very erosive and has a strong tendency to crystallise and create “build up” on all of the wetted surfaces of a valve. Another difficult and very erosive application is the lime mud control.

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Some examples of NAF valves successfully used in this process

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