(571b) Spent Acid Recovery By Nanofiltration Membrane in Mining/Plating Industries – a Pilot Study

Mining process often involves a substantial usage of acid. For example, in titanium dioxide manufacturing process, the feedstock, treated by sulfurate process, required a large amount of sulfuric acid. This will generate a significant amount of spent acid (called AMD in mining industry). It was noted that when 1 metric ton of titanium white is manufactured, 200 to 400 ton of such waste streams will be generated.

Another industry that involves using acids and generate large amount of acid waste is steel industry. For example, in plating process, acid pickling is applied to remove oxide layer, a rust formed on metal surface, before plating. Acids, such as hydrochloric acid and sulfuric acid are often used at a relatively high concentration. Spent pickling acids need to be disposed and replaced with fresh acids, because the efficiency of pickling decreases with increasing content of dissolved metal, including zinc, iron, lead, and chromium, in the bath.

Spent high concentration acids cause environmental issues when disposing. It is toxic to aquatic organisms, destroys ecosystems, corrodes infrastructure and contaminates water in regions, because it is extremely low pH and a huge amount of Cl and SO₄ is added to environment. Thus, the spent acids must be treated according to a variety of environment regulations.

Several conventional methods have been applied for acid treatment before disposing, while the most widely used one is chemical neutralization. In this method, alkaline agent, such as limestone or caustic soda, is added to neutralize the pH and precipitate heavy metals in the stream at the same time. However, the method increases the overall operation cost and still generates a salty waste stream. Driven by more strict environment protection requirement and attempt to reduce operation cost, various approaches have been invested to explore the potential to recovery the acid and reuse it in the process. Among them, Nanofiltration membrane technology, a pressure driven membrane separation process, shows a promising capability comparing to Diffusion dialysis and Electrodialysis, which are all slow transmission processes and hard to scale up.

Nanofiltration membrane is a charged membrane with pores, which permeates water, highly rejects di, tri valent ions but partially allows monovalent ions to pass through. In spent acid recovery application, H⁺, Cl^- and HSO₄^- are only with one charge. At high concentration they can transport through membrane easily with water, results in an acid on the permeate side. Di, tri valent ions, such as Fe²⁺ and SO₄^2-, are retained on the retentate side. As a result, the purified acid as the permeate product can be reused.

Despite the success on heavy metal rejection or acid passage in these experiments, however, the long-term membrane performance at extremely low pH environment is a concern when scaling up the process for industrial applications, and the study on this is rather limited.

The challenges of using membrane at extremely high acid concentration, which may destroy membrane polymer structure, has led to the development of an acid robust NF membrane, KH membrane or so called Duracid. Based on the invention, the membrane polymer structure has been improved by incorporating of sulfonamide polymer matrices that is acid stable polymers into membrane. As a result, the membrane is able to withstand harsh acidic pH with an elevated acid transmission thru the membrane.

To apply this membrane in practical work, the pilot scale testing is critical, because not only the membrane itself, but also the element construction components may have impact on membrane performance in high concentration acid solutions. Additionally, the operation parameters at a pilot scale can be used in future for guiding process commercialization, and provide information for economic estimation on plant investment. Unfortunately, there is no previous literature on this can be found, which considerably constrains the application development of the membrane that will substantially benefit environment protection and nature resource reuse.

In this study, pilot plant trials were conducted using commercial available KH membrane elements at both 5% H₂SO₄ and 20% HCl solutions, given that the two acids are most widely used acids in mining and metal plating processes. Fe²⁺ rejection at various conditions was investigated. Long term membrane performance stability was also investigated and discussed. The solution flux was found that increased with increasing of operation pressure, and decreased with increasing of feed mineral concentration. This suggests that production flux will decrease as process recovery becomes higher due to the concentrating of minerals. The rejections of Fe²⁺ are greater than 90% for all the cases, suggesting a clean permeate acid product and a potential of reuse of it.

Overall, the membrane performance was stable during the trails and long term testing, which implies that the membrane structure is robust when contacting with high concentration acids.