Study On Oxalate Scaling Behavior And Control During Pulping Process

Oxalate scaling is formed when oxalate ions reacts with various metal ions during pulping and paper-making process,which is one of the long-standing and urgent problems to be solved.Oxalate scaling tends to form porcelain-like deposit that may clog equipment such as pump,pipes and filtration,thus the normal production will be interfered.Widespread efforts have been made to study oxalate formation during pulping process in the past years.However,there are few reports on the distribution and controlling of oxalate during the whole process including cooking and bleaching sequence.Therefore,the formation,distribution and controlling of oxalate during the whole cooking and bleaching process of eucalyptus were investigated in this paper.Furthermore,new materials including layered double hydroxides?LDHs?and metal complex oxides?LDO?for adsorpting oxalate were prepared and applied to adsorb the oxalate in waste liquor.What is more,their adsorption properties were systematically studied,which provided a new idea for controlling content of the oxalate in waste liquor.In addition,the oxalate scaling was also explored,and an evaluation method for the performance of oxalate calcium scale inhibitors was presented.The above studies are significant to deepen the understanding of oxalate scaling,control oxalate scaling and enhance productivity.The oxalate concent in various plant fiber raw materials was analyzed and compared,and the formation rule of oxalate and other scalant during Eucalyptus cooking was revealed.It was found that the oxalate content in non-wood was the highest,e.g.bagasse,followed by it in eucalyptus,and the least appeared in masson pine?1.64 g/kg?.The formation of oxalate and other scalcant was promoted with the increase of time and H-factor during cooking process.During the early stages of the cooking,the content of oxalate and carbonate rapidly rose with the increase of time and H-factor,while dissolved lignin and calcium had a trend of rather slow increase.Higher hydroxide concentration led to more formation of oxalate,carbonate and dissolved lignin.Conversely,the dissolved calcium decreased.D₀ED₁,DE_PP and OD₁E_OPD₂ bleaching sequences were applied to treating with eucalyptus pulp.And the results showed that OD₁E_OPD₂ formed the most oxalate and DE_PP formed the least among these three representative bleaching processes,when 1kg pulp?80 ISO%?was produced.The oxalate which was present in spent liquor and pulp of each bleaching stage gradually decreased in sequence with bleaching process.Under different bleaching sequences,the change value of kappa number and brightness increased differently with oxalate forming.The oxalate formation in cooking process accounted for about 42%to 45%of all oxalate generated during the whole process.When the cooking and bleaching process were completed,about 98%of all oxalate was present in spent liquor.Types of LDHs and LDO were prepared by co-precipitation,urea hydrolysis and calcined method.It was found that LDHs had the anticipated layered structure,uniform particle sizes and good thermal stability.The layered structure of the LDO was completely disappeared.The LDHs and LDO were applied to remove oxalate.The highest oxalate adsorption capacity?about181.23 mg/g?was obtained by using the MgAl-CO₃-LDO.Therefore,the adsorption of oxalate by MgAl-CO₃-LDO and its mechanism were also investigated.It was found that the equilibrium adsorption quantities hardly changed with the initial pH value increased,while it raised obviously when increasing temperature,time or initial concentration of oxalate ions.The process of the adsorption of oxalate by MgAl-CO₃-LDO could be fitted by pseudo-second-order equation and Langmuir equation.The mechanism of adsorption was lied in the combination of"memory effect"and surface adsorption.After MgAl-CO₃-LDO was recycled 3 times,the adsorption effectiveness still maintained over 70%.Oxalate calcium scaling and its influencing factors were studied via conductivity method.The results showed that oxalate calcium crystallizing can be expressed as a first order reaction.When oxalate ions concentration,tempreture and rotating speed increased,the rate of crystal nucleation increased and while the rate of crystal growth had no obvious change.The presence of organic acid ions reduced the rate of crystal nucleation.The activation energy of crystal nucleation and crystal growth was 11.41 kJ/mol and 4.46 kJ/mol,respectively.Amongst these three kinds of crystallizing process,the rate of calcium oxalate crystal nucleation was the highest,and that of barium oxalate was the least.Additionally,the effect of scale inhibitors conditions on inhibiting calcium oxalate scale was studied.The results showed that the effect of polymaleic acid is the best,followed by polyaspartic acid,and polyacrylic acid is the worst,under the same conditions.The relative saturation increased with increasing temperature and concentration of scale inhibitors,and then it slowed down when it reached to a certain value.When the initial concentration of calcium ion was increased,the relative saturation decreased gradually.Becsides,evaluation method for the performance of scale inhibitors were established,which is proved to be fast,convenient and accurate.Therefore,it is able to evaluate the performance of oxalate calcium scale inhibitors.