Study On Removal Of Calcium From Papermaking Wastewater By Chemical Precipitation

At present,calcium carbonate is used as the filler and coating in papermaking industry,and the water discharge per ton of paper is decreasing,resulting in a large amount of calcium ions accumulated in papermaking wastewater.The existence of a large number of calcium ions will lead to scaling of water delivery pipeline,calcification of anaerobic granular sludge,RO membrane pollution and other problems,which seriously affect the stable operation of papermaking wastewater treatment process.Therefore,it is of great significance to solve the problem of calcium accumulation in the papermaking system and to find an efficient calcium removal technology for papermaking wastewater.In this paper,two typical papermaking enterprises are selected to investigate and analyze the process and water quality of the papermaking wastewater treatment system,and the scaling of the reclaimed water reuse system is analyzed by Langley index and saturation index;On this basis,the removal performance of three dosing schemes for calcium ion was compared,and the optimal dosage was studied,and the settling performance of Ca CO₃was optimized by PAM;By comparing the consumption of carbonate and the removal of calcium ions,the composition of precipitated products was preliminarily determined.The morphology,structure and composition of precipitated products were analyzed by means of XRD,FTIR and TG.The mechanism of removing Ca²⁺by chemical precipitation was discussed;On the basis of the above work,the technical economy of three dosing schemes is compared and analyzed,and an optimal scheme is selected for practical engineering.The main conclusions are as follows:1. Characteristic analysis of calcium pollutants in the water reuse system of paper mill.According to the analysis of the wastewater treatment system of two papermaking enterprises in Jiaxing for four quarters,the p H,COD and SS of the effluent can meet the three-level standard of national comprehensive wastewater discharge,and NH₃-N and TP are lower than the national industrial wastewater discharge limit.The content of Ba,Cu,Fe and Mn in the influent of the reclaimed water reuse system is not more than 1mg·L^-1,the content of Ca and Mg is about400mg·L^-1,33mg·L^-1,the content of Na is about 310mg·L^-1;The content of phosphate does not exceed 1 mg·L^-1,and the content of bicarbonate,sulfate,and chloride ions are about 960mg·L^-1,340mg·L^-1,and 350mg·L^-1,respectively.By using Langley index I_Land saturation index I_S,it can be concluded that Ca CO₃precipitate will form in the influent water system of the two enterprises,but not Ca SO₄precipitate.2. Study on the removal of calcium from papermaking wastewater by chemical precipitation.Three different dosing schemes are adopted to remove Ca²⁺:one is to directly input Na₂CO₃;the other is to input Ca?OH?₂and Na₂CO₃;the third is to directly input Na OH.When the concentration of Ca²⁺is reduced from 400mg·L^-1to100mg·L^-1?reverse osmosis water intake requirement?,the simulated wastewater dosage is 636mg·L^-1Na₂CO₃or 592mg·L^-1Ca?OH?₂+106mg·L^-1Na₂CO₃or 320mg·L^-1Na OH;the actual wastewater dosage is 848mg·L^-1Na₂CO₃or 666mg·L^-1Ca?OH?₂+106mg·L^-1Na₂CO₃or 360mg·L^-1Na OH.According to the analysis of settlement volume,turbidity,zeta potential and particle size distribution,the settlement performance of Ca CO₃in scheme I and scheme II is quite poor,and the settlement performance of Ca CO₃in scheme III is far better than that in scheme I and scheme II.The use of PAM can significantly improve the settlement performance of Ca CO₃in Schemes I and II.The optimal dosage for Scheme II is 6mg·L^-1,and the dosage for Scheme II is 5mg·L^-1.3. Study on the mechanism of chemical precipitation to remove calcium pollutants from papermaking wastewater.The study of the effects of coexisting ions on Ca²⁺removal found that Mg²⁺had an inhibitory effect on Ca²⁺removal and SO₄^2-had almost no effect on Ca²⁺removal.The n(Ca²⁺):n(CO₃^2-)ratio of three solutions was calculated by measuring the amount of Ca²⁺removed and the consumption of CO₃^2-in the solution.Combined with XRD,FTIR,TG and other analysis results,it is judged that the precipitated product of Scheme I is Ca CO₃;the precipitated product of Scheme II includes Ca CO₃and calcium carbonate such as Ca₃?OH?₂?CO₃?₂,Ca₆?OH?₂?CO₃?₅,Ca₂?OH?₂CO₃,etc.;the precipitated product of scheme III is Ca CO₃.Based on the equilibrium distribution curves of carbonate and calcium hydroxide,combined with the results of characterization analysis,the reaction equations of the three schemes were predicted.4. Engineering application of chemical precipitation to remove calcium pollutants from papermaking wastewater.A papermaking enterprise with a processing capacity of about 25000m³·d^-1in Jiaxing was selected to build a final sinking pond between the second sinking pond and the reclaimed water reuse system.Ca?OH?₂,Na₂CO₃and PAM are added to the final sink to remove calcium ions from the wastewater.The dosage of Ca?OH?₂and Na₂CO₃is 666 and 106mg·L^-1,respectively,and the dosage of PAM is 5mg·L^-1.The estimated cost is 0.424 yuan per ton.