| Because of the high rejection of bivalent and multivalent ions,the nanofiltration membrane can be used to remove the hardness ions of seawater and brine,which improve the yield of reverse osmosis permeate and reduce the discharge of the concentrate.Therefore,the nanofiltration process can be used as a purifying method for seawater,brine and underground brine,getting the purified brine enriched in monovalent ions which can be used as water and a resource of a raw material in chloralkali and soda industry.Nanofiltration has become an important technology in the field of seawater desalination and comprehensive utilization of sea resources.When using the conventional nanofiltration membrane treat seawater or brine,the recovery rate is low due to the calcium sulfate scaling phenomenon,which also causes a lot of environmental problems.Therefore,this paper tries to solve the problem of calcium sulfate scaling in order to improve the recovery rate of permeate and reduce the discharge of concentrated seawater.This paper focuses on the industrial application,aiming to "split" the sulfate ions and calcium ions effectively in the concentrated seawater drained from desalination plants or underground brine,to avoid the calcium sulfate scaling problem occurring in the further processing of the concentrated seawater.One kind of China produced nanofiltration membranes with high sulfate rejection and low calcium ion rejection was used to separate seawater or concentrated seawater.The performance difference between this nanofiltration membrane and conventional nanofiltration membrane on the separation of sulfate and calcium ions under various operating conditions was studied.The influence of operating parameters including temperature,pressure,feed concentration and feed flow rate on the performance parameters,such as the flux,rejection rate and the recovery rate of the permeate were tested and analyzed.The optimal operating parameters were determined and the long-term operational stability of the membrane was investigated.The experimental results demonstrated that when the domestic nanofiltration membrane was used to treat seawater in the feed flow of 300 L/h,pressure at 3.0 MPa,temperature at 25 ℃,the rejection of calcium ions,magnesium ions and sulfate ions was 58.82%,86.90% and 97.03% respectively.Compared with the conventional nanofiltration membrane,the rejection rate of calcium ions for this nanofiltration membrane was reduced by 14.72%,while the rejection rate of magnesium ions and sulfate remains basically unchanged,and the membrane flux was increased by 23.94%.When used to treat the seawater,this nanofiltration membrane did not show scaling phenomenon when the recovery rate was up to 80%.When used to treat the concentrated seawater drained the by-product brine from desalination plant,this membrane still not show scaling phenomenon when the recovery rate is up to 80%,while the conventional nanofiltration membrane start scaling when the recovery rate was 73.9%,so the recovery rate of this nanofiltration membrane was at least 8.3% higher than the recovery rate of the conventional nanofiltration membrane.In addition,offered by this nanofiltration membrane had low magnesium ions and sulfate concentration,which can be used as the feed for reverse osmosis to recover freshwater as much as possible.The permeate or the permeate further enriched by reverse osmosis can be used for chlor-alkali and soda industry.In the pilot-scale experiment for the treatment of underground brine,the recovery rate of this nanofiltration membrane was 54.5%,and it was 7% higher than the recovery rate of conventional membrane,which makes this process more economical efficient.This study provides technical support for the size reduction of desalination plant by-product,the concentrated seawater,and further utilization of concentrated seawater or brine. |
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