Water Use Reducation In BCTMP Pulping Process

Softwood BCMP production line from a pulp mill in Fujian province, was investigated as a target on the base of site inspection. Based on the actual operation parameters, the material consumption and fresh water usage were calculated according to pulp and white water equilibrium principle. After analyzing the pollution characteristics of seven kinds of process-water, the characteristics of them were compared, at the same time, organic types and metal elements were mainly compared. The whole process was divided into three independent water consuming units in the light of the fresh water distribution. Total solids (TS) was took as the key index of pollution chareacteristics, and the limited inflow and outflow concentration were defined. Combined above data, the whole water system of the process was optimized by process integration method(water pinch technology), and fresh water usage distribution reached its optimum condition while its consumption was minimized, which carried out the water reduction goal. The main contents of the study and conclusions are showed as follows:(1) Accoding to the operation parameters of BCTMP production line, the pulp and white water equilibrium calculated results testified: one ton production consumes 1.18t dry chips, and 45m~3 fresh water, of which 28.5 m~3 contributes to pulping process and 16.5 m~3 is consumed in papermaking process, and generates 40 m~3 wastewater.(2) The pollution characteristics of seven kinds of process-water were analyzed by ICP, GC/MS and other instruments. The contens of Fe, Mn, and Cu were tested as the main metal elements, which could result in hydrogen peroxide decomposition. Besides, K, Ca, and Mg concetrations were analyzed for which could cause equipment corrosion and pipes blocking. The results demostrated that: the contents of Fe and Mn in the seven process-waters vary in the range of 1.248 mg/L to 6.878 mg/L, and the range of 0.266 mg/L to 2.18 mg/L respectively. That of Cu changes from 0.206mg/L to 0.726mg/L, and the content is the least in chip washing wastewater and most in the discharge wastewater. Fe is the majority metallic element contained in these process-waters, whose amount is more than two times in extruding wastewater than that is in wood chip washing wastewater, and that of Mn in extrusion wastewater is 4.4 times as in chips washing wastewater. Besides, these water contains K from 3.762mg/L to 83.74mg/L, Ga from 20.72mg/L to 179.18mg/L, and Mg from 1.762mg/L to 35.66mg/L. The organic pollutant covers aromatic acid, aromaticalcohol, aromatic ketone and macromolecular heterocyclic substance, in which aromaticalcohol accounts for about 30%, aromatic ketone takes up about 10%, macromolecular heterocyclic substance contributes to 16% or so. The rest are macromolecular olefins and alkane.(3) The limit data for the three independent units are estimated, as TS is took for the main pollution index. The fresh water system is optimized by water pinch technology, and the fresh water consumption is minimized to 31.1 m~3/Adt when the concentration is 4006 mg/L. The comsumption of fresh water is reduced by 26.1%, from 41.1 m~3/Adt to 31.1 m~3/Adt.