(432d) A Multiple-Effect Membrane Distillation Process for Concentration of Aqueous Hydrochloric Acid

It is well known that hydrochloric acid (HCl) is usually used as pickle liquor to remove surface oxide in a steel production industry. When pickling cannot be accomplished effectively and the quality of the treated metal surface deteriorates, the pickle liquor is discharged from the pickling tank, and the pickling tank is replenished with fresh acid solution. According to the data provided by the World Steel Association in 2011, the total world crude steel production was 1,490.1 million tonnes. And about 60 kg of spent pickle liquor was generated with the production of each ton of steel, and thus annual emission of spent pickle liquor was up to several million tons. As an example, 1000 ton of pickle liquor is being produced daily in a steel plant in north China, which contains 7% HCl and 15% FeCl₂. The spent pickle liquor usually contains 2-8 wt% hydrochloric acid and is considered a hazardous waste. The spent pickle liquor from steel processes is usually neutralized with lime and disposed in a landfill, which results into the following problem: high-salinity wastewater and sludge volume, the remaining salt residue processing difficulties, and most importantly, the acid unrecoverable. Therefore, there is an urgent need to recover and enrich hydrochloric acid to achieve economical and ecological benefits. Since the 1960s, hydrochloric spent pickling liquor is often treated in a hydrochloric acid regeneration system such as ion-retardation, diffusion dialysis and electro-dialysis, which recovers some of the hydrochloric acid and ferric oxide. Nevertheless, these regeneration processes produce lots of dilute hydrochloric acid solution. Thus, there still needs a novel and efficient technology to concentrate the recovered dilute HCl solution for further use. In the last few years, numerous studies have been performed to test the application of membrane distillation for concentrating dilute HCl solution. However, the high thermal energy consumption of the traditional MD process is one of the biggest barriers in its industrialization.

In the present study, multiple-effect membrane distillation (MEMD) based on AGMD module with function of internal heat recovery has been developed. This kind of MEMD process combines the advantages of MD process and conventional MSF process, avoids the disadvantages of MSF such as evacuation operation, and can provide a high PR value. The effects of feed-in concentration, cold feed-in temperature (T_c), hot feed-in temperature (T_h) and feed-in volumetric flow rate (F) on the performance of MEMD process were studied. The permeation flux (N) and energy efficiency, performance ratio (PR), and the average selectivity of water over HCl (β_avg) are the most important indicators for module performance evaluation. N indicates the productivity of this device; PR (performance ratio) is usually used to determine the thermal efficiency of evaporation-based process, which is defined as the amount of latent heat needed for evaporation of the produced water and the amount of heat provided to the system from an external energy source; β_avg is represents the measure of the preferential transport of water. The results showed that MEMD process could be used successfully for concentrating dilute HCl solution with the advantage of energy saving. The experimental data indicated that all N, PR and β_avgdecreased with the increase of feed concentration. When the feed concentration was below 12 wt%, PR could achieve 6.0~9.6, and β_avg was about 10~190. As the concentration of HCl achieved 18 wt%, the values of PR and β_avg were still about 4.4and 2.3, respectively. However, β_avgsharply decreased to a value around 1.0 when feed was further concentrated. It is also found that there exists trade-off phenomenon between N, PR and β_avg under experimental ranges, that is, the maximum N will be obtained with high temperature T_h, low temperature T_c and high flow rate F while the maximum PR is obtained with high temperatures T_h and T_c, as well as low flow rate F. the lowest β_avgwill be obtained with low temperature T_h, low temperature T_c and low flow rate F. During an operational stability test lasting for 30 days, the performance of MEMD modules was kept in good condition.