One of the processes in the mill which I have found to be most interesting is the recovery process. This process contains many different unit operations across various departments to lower the cost of production and ensure there is less waste being created. Recaust is a specific department within the recovery process in which the main goal is to recover a portion of the alkaline chemicals which are used in the Kraft Pulping process.
White liquor along with wood chips are transported through the digester. During this process the chemicals in the white liquor break down the various chemical structures in the wood and create pulp. From this process, weak black liquor is also created. The weak black liquor is sent through various concentrators and evaporator systems to remove excess water and create strong black liquor. This strong black liquor is then burned in the recovery boiler to create a substance called smelt. When the smelt exits the recovery boiler, it is transported to clarifiers and is turned into green liquor (sodium carbonate - Na2CO3) through the addition of water. Next, slaked lime, (C(OH)2) is added which turns the green liquor back into white liquor (caustic soda - NaOH).This particular addition of lime is known as causticizing. Once the white liquor is obtained, it can once again be used in the digester.
As one can imagine, the above process would become very expensive very quickly if the lime which is used to causticize the green liquor could only be used once. Fortunately, there are large pieces of equipment known as kilns which are able to regenerate the lime! The kiln is a large rotating cylindrical tube which is roughly 250 feet long and can reach temperatures of over 800 degrees Celsius. The kiln uses a combination of natural gas and other effluent exhausts to maintain the required temperature for the chemical reaction to occur (CaCO3→CaO+CO2).
Due to the specifics of the recaust process, large amounts of dust are created which must be removed from effluent exhausts. The dust can be removed either through the process of a wet-scrubber or an electrostatic precipitator. The electrostatic precipitator is more commonly used now as the efficiency of dust removal is significantly better.
This process is an integral component of the Kraft Pulping operation which happens here at the Thunder Bay Pulp and Paper Mill! It’s important to understand the necessity of the components of the recovery process to ensure a more cost-effective production of kraft pulp. The recovery process ensures optimal reuse of various chemicals which are needed for the process.
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