Dramatic water usage reduction with valuables recovery in pulp mill: Removal of non-process elements

Pulp industry is one of the most water consuming industry. Sustainable water use and the use of raw materials arises as a threshold issue when moving toward sustainable and material efficient pulp production. Closure of water circuits can significantly reduce the total water consumption of mill.

This study examines the closure of the water cycles in the bleaching plant of kraft pulp mill with application of tertiary treatment methods to remove non-process elements from the acid bleach filtrate. When closing the water cycle, harmful non-process elements (NPEs) can accumulate in the process as the possible purge points are eliminated. Especially earth metals can cause uncontrolled precipitation on the process equipment, causing process disturbances. Calcium is one of the non-process elements most harmful to the process, causing precipitates in bleaching. Additionally, some NPEs like cadmium can cause problems in wastewater treatment plant especially in sludge utilization.

The NPEs studied in the thesis were cadmium, calcium, and phosphorus. The selective separation of those from the process waters of acid bleaching stage to separate fractions was examined. Precipitation, filtration, adsorption, and ion exchange were chosen as the separation techniques to be examined. The overall and selective separation efficiency of the desired substances was examined by single separation techniques and by combinations of different separation techniques.

Results proved the overall separation efficiency of nanofiltration to be highest among other tested treatment methods for all elements measured. Permeate received can be re-used in the process whilst concentrate can be treated further. Sulphide and phosphate precipitation yielded promising results for the selective removal of cadmium from the acid bleach filtrate. Carbonate precipitation was found efficient but non-viable option in the separation of calcium due to the high chemical consumption.