Equipment Description:Cooking
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The aim of cooking is to break down the lignin bonds between the fibres using chemicals and heat whilst not destroying the cellulose and hemicellulose part. The cooking is continued until a certain amount of lignin bonds are dissolved. This can be done in either continuous digesters or batch digesters. In the continuous process, pre-steamed chips and chemicals are dosed into the top of a large digester vat and the cooked pulp is removed from the bottom. In batch systems different batches at different stages of cooking are simultaneously processed in smaller batches. The pulp produced is similar from each process. Batch systems offer the opportunity to simultaneously cook different pulps but there are some questions over efficiency versus continuous cooking and therefore neither system has a clear advantage. The two processes described are the sulphate and sulfite processes. The sulfite process cannot handle such a variation of raw material in terms of species and bark contamination when compared to the sulfate process and so is less widely utilised. |
White liquor sodium hydroxide and sodium sulfide as a cooking liquor Black liquor This is the chemical residue that has already been through the cooking process and is mostly made up of spent white liquor, dissolved wood parts and residues. Sulfate pulp Pulp produced using the cooking process often referred to as kraft pulping or Sulphate pulp Pulp produced using the cooking process often referred to as kraft pulping |
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Sulphate cooking is the most common process and uses sodium sulfide and sodium hydroxide as it’s main chemical components. The process produces a brown pulp often called kraft (from the German word for “strong”). The alkaline breaks down the lignin into small parts that are soluble in the chemical solution (liquor). Steam is used to heat the digester to a normal operating temperature of 150 to 180 deg C and the cooking is done under pressure. The temperature is critical to the rate of the cooking process. Cooking time is generally up to 4 hours. The fibres can be bleached if required. Unbleached kraft grades are normally used in packaging papers for their strength and have a higher yield and contain more lignin. Bleached grades have a lower yield and are made into white papers and are higher in cost. Sulfite cooking uses sulphurous acid and a bisulfite ion to break down and dissolve the lignin bonds into solution. The resulting fibre makes a weaker sheet but is easily bleached. Sulfite cooking takes longer but has a slightly higher yield than than the kraft process. |
Cooking was also mentioned in the following Fact Sheets (8): |
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