UrFU Scientists Have Developed a Technology for Recycling Potash Production Waste

14:21, April 20, 2021

It turns waste into a profitable commodity and prevents environmental disasters

A group of Ural Federal University scientists under the leadership of Aleksandr Cherepanov, associate professor of the Department of Experimental Physics, has developed a revolutionary, unparalleled technology for processing liquid wastes from potassium fertilizer production. Liquid waste (sludge) from potash production is a highly concentrated solution of sodium chloride with a residual content of unextracted potassium salts that also includes insoluble rock components (sand, clay, etc.). The high mineralization of such waste (almost 10 times saltier than seawater) makes it dangerous in the event of leaks and getting into rivers or underground sources. In Russia, the main deposits of potassium ore are located in Perm Territory and Volgograd Region.

Today, they are stored in huge open reservoirs - sludge depositories - as big as several soccer fields. The total mass of the accumulated waste is estimated at several hundreds of millions of tons and keeps growing. In case the reservoirs are overfilled, the basins of Kama and Volga, in other words, large areas of the European part of Russia, may face serious environmental consequences, and the producers will incur huge fines and losses.

"It would seem to be more convenient and safer to dry and store the sludge as powders. This requires evaporating all the water out of them. Such drying plants can theoretically be built. However, firstly, this method is too expensive and economically unprofitable. Secondly, dehydrated salt will still contain not only sodium, but also potassium and other impurities, including elements of rock. That is why this salt will not be suitable for any further industrial or household use," explains Aleksandr Cherepanov.

To solve the problem of sludge utilization, scientists from UrFU used a technology for pretreatment of solutions from insoluble components - rock elements. The technology, previously worked out by scientists in the oil production sector, is based on the use of ceramic membranes. At the same time the developers turned to the method of electrodialysis. Its idea is that an electric field applied to a purified solution "pushes" salts through membranes, thus reducing salt content in water. In "classical" electrodialysis, one type of membrane conducts positively charged ions, cations (in this case, sodium, potassium), the other - negatively charged, anions (chlorine). In this way, with the help of the membranes, all the components of the initial brine are distributed into two zones: with even more concentrated salt and with purified water.

"We went further and used a third type of membrane, a bipolar membrane. It is a bundle of the first two types of membranes, cationic and anionic, and provides decomposition of water present in the sludge into hydrogen H and hydroxyl group OH. Further, by attaching hydrogen to chlorine isolated on anionic membranes, hydrochloric acid HCl is formed, and by attaching the hydroxyl group to cations isolated on cationic membranes, alkali is formed. Thus, we get two solutions from the initial solution - an acid solution and an alkaline one. The purity of acid and alkali depends on the mode of electrical influence. We don't have to be as pure as in specialized chemical plants. First of all, a small residual salt content is not only acceptable, but in some cases even mandatory for practical use of the products. Secondly, this way we save on energy costs," describes Aleksandr Cherepanov.

The operating capability of the technology is highly dependent on the characteristics of bipolar membranes. Usually they are obtained by gluing cationic and anionic membranes together. However, glue creates high electrical resistance, so to obtain the required current values, one has to apply too high voltages, i.e. to spend excessive power. UrFU solved this problem by using membranes obtained by mutual implantation of cationic and anionic membrane materials. As a result of this approach, the energy efficiency of membrane electrolysis has increased, as compared with previously known solutions, by almost one order of magnitude.

"Acidic and alkaline solutions are chemical reagents that are used in industry. That means we not only recycle potassium production wastes but also process them into a marketable product using zero-cost raw materials. It means that the potassium industry enterprises won't have to buy chemical reagents, the potassium producers will get them from the waste," emphasizes Cherepanov.

Aleksandr Cherepanov predicts that the products of the established recycling technology will be in demand as the basis for production of household chemicals, as raw materials for glass manufacturing, and as detergents for cleaning equipment, for example, in food and oil industry. In addition, acid-alkaline solutions with residual salt content are ideal for eliminating contaminants inside oil wells and increasing the yield of oil-bearing formations.

"Considering the minimal expenses for waste processing the sales of the products we get can be no less profitable than the potassium salt production. Our development can also be used to desalinate seawater, because it is nothing but an aqueous solution of sodium chloride and some other compounds and elements, including potassium. For example, this could solve the problem of drinking water shortages in the Crimea," Cherepanov points out.

After completing the laboratory stage of development, the scientific team headed by Aleksandr Cherepanov begins to create an industrial modular unit. Its components (innovative ceramic and polymer membranes, sensors, etc.) are notable for their longevity: their projected lifetime of reliable operation is at least 10 years.

"If it is necessary to increase the capacity of the plant, the consumer only needs to dock the appropriate number of modules. We calculate the size of the equipment according to the dimensions of a 20-foot container, so it will be thoroughly protected from external influences and easy to transport," concludes Aleksandr Cherepanov.

The scientists already have two patents for this technology (Patent 1, Patent 2), and the researchers are preparing to apply for a third.