Calcium Oxalate in Bleach Plant Filtrates, 1997 Minimum Effluent Mills Symposium Proceedings

One of the main problems seen today in a bleached kraft pulp mill with a high degree of system closure is the formation of deposits of sparingly soluble compounds of the alkaline earth metal ions Ca 2+ and Ba 2+ . This is a result of an accumulation of inorganic and organic substances due to a reduced water consumption resulting from counter-current washing in the bleach plant. It is mainly calcium oxalate, calcium carbonate and barium sulfate that precipitate under the conditions prevailing in a bleach plant for ECF (elemental chlorine free)- or TCF (total chlorine free)-bleaching. It was found that oxalate ions did not only form during the bleaching but also when acidic bleach plant filtrates were stored at a high temperature, e.g. in a filtrate tank. The amount of oxalate ions formed in a filtrate increased with increased amount of organic substance, temperature and residence time. The solubility of calcium oxalate in pure solutions and bleach plant filtrates increased with decreasing pH when pH < 4,="" with="" increasing="" temperature="" and="" with="" increasing="" ionic="" strength.="" in="" bleach="" plant="" filtrates="" the="" solubility="" also="" increased="" with="" increasing="" amount="" of="" organic="" substance="" present.="" the="" solubility="" of="" calcium="" oxalate="" was="" very="" high="" in="" bleach="" plant="" filtrates="" compared="" to="" pure="" solutions="" with="" no="" organic="" substance="" present.="" the="" increased="" solubility="" can="" be="" explained="" partly="" by="" the="" formation="" of="" a="" metastable="" colloidal="" solution="" of="" calcium="" oxalate="" (particle="" size="">< 0.01="" µm),="" and="" partly="" by="" a="" complex="" formation="" between="" calcium="" ions="" and="" organic="" anions="" with="" carboxylic="" groups.="" in="" this="" paper,="" data="" relating="" to="" the="" solubility="" of="" calcium="" oxalate="" in="" different="" types="" of="" filtrates="" and="" pure="" solutions="" are="" presented.="" the="" risk="" of="" formation="" of="" deposits="" of="" calcium="" oxalate="" in="" a="" bleach="" plant="" can="" be="" evaluated="" using="" a="" chemical="" steady="" state="" model="" developed="" from="" results="" of="" laboratory="" experiments,="" and="" the="" computer="" program="">