A waste treatment unit has been designed to recover water and various components from an aluminum can manufacturing process. Due to the scarcity of water in the Middle East and its inherent cost, water recovery is both necessary and cost effective. The manufacturing process generates: 1) an 11,356 L/hr (50 gpm) aqueous stream contaminated with hydrofluoric acid, dissolved aluminum, and various oil/surfactant complexes at a pH of <2 and 2) a 2,271 L/hr (10 gpm) aqueous stream with 3-10% emulsified oil. The waste treatment unit incorporates five separate membrane systems--three reverse osmosis, one nanofiltration, and one ultrafiltration--followed by an evaporator. The total unit has allowed the facility to recover more than 88% of the process water from the waste streams.
Key Words: aluminum, emulsified oil, hydrofluoric acid, nanofiltration, reverse osmosis, ultrafiltration.
Like many other metal working operations, the aluminum can manufacturing process generates a number of liquid waste streams contaminated with emulsified oils, acids, and dissolved metals. When these waste streams are combined for conventional end-of-pipe treatment, the facility loses the opportunity to recover water, metals, and acids. Membrane systems used within the manufacturing process can treat waste streams as they are generated. This integrated approach has several benefits: it greatly reduces final waste volume thereby reducing chemical usage, it returns a significant portion of water to the process, and it provides for the recovery of waste stream constituents such as acids and metals.
Membrane technology has been used for the desalination of natural waters for many years. Advances over the last ten years in membrane chemistry as well as element construction have allowed this technology to be used successfully in many industrial processes.
All of the membrane elements used in this facility utilize advanced thin-film membranes. Thin-films are characterized by the use of two or three layers of plastic material cast onto a non-woven fabric. The highly porous first layer provides mechanical support and a smooth surface for the separating layer(s). The thin-film separating layer(s) provides the retention characteristics of the membrane.
Reverse Osmosis (RO) is a separation process that removes water from a solution by forcing it through a semipermeable membrane. A semipermeable membrane can be defined as a barrier which would let water pass through much more readily than it will other components of the solution. RO membrane systems are capable of removing dissolved ions as well as low molecular weight organic compounds from a water stream. This facility uses Desal-11 for incoming water treatment and Desal-3 for process water treatment. The proprietary Desal-3 has a unique three-layer membrane structure and a smooth surface for reduced fouling potential in process streams.
Ultrafiltration (UF) membranes are characterized by the retention of colloids, emulsified oils and surfactants, particulates, and relatively high molecular weight organic species (MW>1000). The G-80 ultrafiltration membrane used in this facility is a proprietary two-layer thin-film membrane with a molecular weight cutoff of 15,000 on PEG.