Use of the total organic carbon (TOC) determination to estimate the yield and chemical oxygen demand (COD) in peroxide bleaching of fibers,Hochausbeute-Sulfitfaserstoffe für die Papierindustrie ; eine technologische und ökonomische Alternative?
A discussion covers an experimental study to develop a method to estimate the yield and chemical oxygen demand (COD) in peroxide bleaching of fibers by use of the total organic carbon (TOC) determination. Topics include introduction; material and methods; results (bleaching of the beech high yield cellulose; bleaching of the magnesium bisulfite cellulose of spruce/beech; bleaching of the pre-bleached pine wood); evaluating the experimental results in terms of the suitability of the TOC value to estimate the yield and COD value. The greatest value of the TOC versus the COD measurement is in the less time required per measurement. The use of costly chemicals such as silver and mercury sulfate is avoided. The disposal costs of chromium and mercury compounds are avoided. Using the Scan method, the overall cost (personnel, chemicals, and removals) of the COD is an estimated 7 to 8 times greater than for the TOC method. This does not include the cost for the TOC device and its amortization. The TOC measurements correlate well linearly with the gravimetrically determined yield losses. The effects of the various bleaching parameters on the yield and waste water loading are clearly determined. The TOC and COD values in the bleaching solution filtrate correlate linearly. For the peroxide bleaching solution filtrate the average COD/TOC ratio was found to be 2.7:1 for the various celluloses. This value is clearly less than the 3.3:1 that has been invoked in the modification draft of the waste-water regulation as conversion factor or modulus. For the various operating waters of a paper plant, COD/TOC ratios between 2.7:1 and 3.9:1 were determined. This indicates the variability of this ratio in the various waste water flows such that the ratio must be determined for each waste water flow individually if the TOC value is to be registered in a meaningful manner.