Study On Calcium Removal By Seed Mediated Enhanced Chemical Precipitation

Waste paper is the main raw material for papermaking in our country.The high recycling rate of papermaking wastewater results in a high concentration of calcium ions in wastewater.Membrane treatment is the key to the recycling of waste water from papermaking,but high calcium in wastewater can easily cause RO membrane pollution,restricting the recycling of wastewater.This paper investigates the wastewater from paper-making enterprises in Jiaxing,analyzes the wastewater characteristics and fouling tendency.Combining the calcium ion removal mechanism,proposes three chemical precipitation methods and screens them.Aiming at the problem of large dosage of chemical precipitation method,a seed-mediated enhanced chemical precipitation method for calcium removal technology has been developed.The seed-mediated mechanism is discussed from three aspects:critical supersaturation,apparent activation energy,and precipitate morphology.Based on this,engineering application research is carried out.The main conclusions are as follows:1.Investigation on the status of wastewater treatment in paper-making enterprises in Jiaxing.In 2017,the wastewater discharge of 24 paper-making enterprises in Jiaxing is 83323 m³·d^-1,and the average discharge per ton of paper is5.5 m³.The papermaking enterprise adopts the treatment process of"primary sedimentation tank+anaerobic+aerobic+final sedimentation tank+advanced treatment".The calcium ion concentration of the effluent from the final sedimentation tank is above 9mmol·L^-1,beyond the RO membrane requirement(Ca²⁺<2.5mmol·L^-1).Judging by the Langelier index formula,the effluent of the final sedimentation tank has a tendency to scale calcium carbonate.2.Research on calcium removal by chemical precipitation.Calcium carbonate solution consists of 9 substances whose concentration changes with p H.Theoretically,when the p H is 11-12,the calcium ion concentration is the lowest.Based on the water characteristics of wastewater from paper-making enterprises,three precipitant dosage schemes of Na OH,Na₂CO₃and Ca（OH）₂+Na₂CO₃ are determined.According to the calcium ion effluent requirements and the cost of the dosage of the medicament,the dosage plan of the medicament is determined to be 8 mmol·L^-1Ca（OH）₂ and 1mmol·L^-1Na₂CO₃.3.Study on calcium removal by seed mediated enhanced chemical precipitation.The seed crystal mediation can promote calcium carbonate precipitation by increasing the number of crystal nuclei and increasing the crystal surface area,thereby enhancing the removal effect of calcium ions.When the dosage of seed crystals is 3.1g(2.0 g·g^-1),the enhancement effect is the strongest.The optimum dosage of calcium hydroxide and sodium carbonate for seed-mediated enhanced precipitation method is7.5mmol·L^-1 and 8mmol·L^-1.Compared with the chemical precipitation method,6.25%calcium hydroxide and 20%sodium carbonate are reduced respectively.The order of the influence of different stirring factors on the removal effect of calcium ions is fast stirring intensity>fast stirring time>slow stirring time.Appropriate stirring intensity and stirring time are conducive to uniform mixing of the medicine and crystal growth.The optimal stirring is rapid stirring intensity of 400 r·min^-1 for 2 min,and 100 r·min^-1 for slow stirring for 10 min.Pilot experiments verify that the decalcification effect of seed crystal reflow is better than that of non-seeded crystal,and seed crystal reflow can reduce the pharmaceutical cost by 15%.Seed-mediated enhancement of calcium removal effect by reducing the critical supersaturation and apparent activation energy of calcium carbonate.The critical supersaturation of calcium carbonate is reduced from 928.14 to 446.42,and the apparent activation energy is reduced from 49.67 k J·mol^-1 to 40.35 k J·mol^-1.XRD,FTIR,TG,SEM and particle size analysis showed that the crystal form of the precipitated product with or without crystal mediation is calcite,and the particle size of the seed crystal-mediated product is larger than that of no seed crystal.4.Engineering application of seed-mediated enhanced chemical precipitation method for calcium removal.A paper-making company with a design treatment scale of approximately 32000m³·d^-1 is selected,and a seed-mediated decalcification tank is added between the final sedimentation tank and the reclaimed water reuse system,and the pharmaceutical dosage plan was 11.04 mmol·L^-1Ca（OH）²+3.40 mmol·L^-1Na²CO³+2.0 g·g^-1 reflux sludge（calcium carbonate seed crystal）,the average concentration of calcium ions in the effluent is 1.81mmol·L^-1,which meets the requirements of RO membrane inflow.