Zero-Liquid-Discharge, Zero-Mineral-Discharge Process Recovers Water, Extracts Salts

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 225181, “Industry-First Zero-Liquid-Discharge and Zero-Mineral-Discharge Process for Produced Water,” by Pramod D. Patil, SPE, and Moin Muhammad, SPE, NESR, and Hamidreza Samouei, Texas A&M University. The paper has not been peer reviewed. _ The zero-liquid-discharge (ZLD) approach is a clean technology for valorizing low-salinity brines such as seawater using desalination reverse-osmosis (RO) plants. A key aspect is calcium removal and recovery to avoid scaling problems in successive advanced separation units for recovering other valuable salts. Most current technology relies on discarding some salts while recovering others. In this paper, a systematic process is proposed by which all the salts of sodium, calcium, magnesium, strontium, and lithium can be recovered in their highest purity from produced-water brines from oil wells with very high salinity. Problem Statement While current wastewater-reclamation and -minimization technologies are effective, they demand high energy consumption. Conversely, the advancement of membrane-based RO technology, integrated with ZLD systems, is cutting energy usage. Although incorporating RO technology can boost energy efficiency compared with thermally powered ZLD systems, its effectiveness is limited to feeding waters within a particular salinity range. Recently, alternative salt-concentrating methods have emerged. In the RO process, the feed-saline solution is pressurized and directed to the feed side of a perm-selective membrane, which rejects salt ions and allows water to permeate to the low-pressure side. The applied pressure gradient must overcome the osmotic pressure difference between the feed and permeate liquids. The brine concentrate rejected by the membrane passes through an energy-recovery device, which partially pressurizes the feed stream, thus reducing the energy needed. In the ZLD process, which primarily recovers desalinated water while disposing of resulting minerals as landfill waste, the treated water can be repurposed for various applications. The existing ZLD process falls short in producing individual mineral salts in their purest forms, however, instead yielding mixed salts through conventional thermal and membrane-based methods. If these salts were to be refined to their purest states, they would hold significant market value and could be used across diverse industries. In this study, a novel process for recovering multiple mineral salts in their purest form (greater than 99%) is proposed. Fig. 1 outlines the process flow, wherein produced water initially is reacted with a sequence of reactants to yield pure-form minerals. The filtered brine is subsequently fed into a multistage evaporator where the brine is evaporated and the condensate is collected as pure water. This evaporation step is designed for commercial use to maximize the precipitation and separation of monovalent salts, such as sodium chloride. The remaining concentrated brine is then processed in a direct lithium-extraction unit, where Li+ is isolated from the concentrated brine through methods such as adsorption, ion exchange, or solvent extraction.