Treating the surfaces of plastics
2 APPLIED PROCESSES AND TECHNIQUES
2.7 Common techniques for water and waste water treatment, process solution maintenance and materials recovery
There are a number of techniques that are used for treating water and water-based solutions, and are widely used in the surface treatment of metals for:
• treating incoming water supplies
• treating waste waters prior to discharge or for re-use
• process solution maintenance
• in-process materials recovery. This may also serve as process solution maintenance The basic techniques only vary slightly according (e.g. choice of resins for ion exchange) to the application, they are briefly described here and more fully in Chapter 4. They are well described elsewhere, and some are especially useful when considering closing the loop for process materials, see Section 4.7 [162, USEPA, 2000].
2.7.1 Filtration
There are many types of filtration. On the smaller scale they start with cellulose (paper) sheets usually clamped in filters (sometimes layered with activated carbon or other absorbents), and cartridges. On a larger scale, sand filters are used for cleaning raw water or polishing effluents, and belt filters or filters presses are used with higher solids applications such as waste water sludges, often in conjunction with coagulants.
Environmental considerations
Pumps (with an associated energy demand) are usually require for filtering, although in some cases (such as some sand filters) gravity is sufficient.
The filter medium, with the filtrate is usually disposed of as a waste.
2.7.2 Absorption techniques
Activated carbon is used to adsorb unwanted organic substances formed from breakdown products in a solution. It can be added to the solution on batch basis and then filtered out, or it can be used in a layered filter system.
Granulated polypropylene felt or other lipophilic filter media can be placed in filter housings and used for removal of oils by adsorption.
Polymer filtration uses chelating, water soluble polymers to selectively bind target metals in aqueous streams.
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Activated carbon will also remove a portion of the useful organic chemical additives, such as brighteners and these will need replenishing.
The absorbent material along, with the retentate and filter medium, is usually disposed of as a waste, although precious metals may be recovered.
2.7.3 Crystallisation
Various evaporation and cooling systems are used to bring solutions to a super-saturation point where solid crystals form and can be separated from solution.
Environmental considerations
Energy requirements for heating or cooling
2.7.4 Atmospheric evaporation
Atmospheric evaporation occurs when solutions are heated. As a basic technique, it reduces the volume of process solutions and allows drag-out to be returned or fresh chemicals to be added to the process solution. It can be increased by using air agitation and/or using an evaporator.
The evaporator may be filled with a packing material to increase the air-to-water evaporation surface. Evaporators are often used with a condenser to recover distilled water.
Environmental considerations
Evaporators can often beneficially use the heat from the solution when the process is self-heating, e.g. from the electrical current passed. The heat lost then assists with cooling the solution.
2.7.5 Vacuum evaporation
Reduced pressure and elevated temperature combine to separate constituents with relatively high volatility from constituents with lower volatility, e.g. removal of water from an acid plating solution. Evaporating the process solution (e.g., acid distillation) from contaminant phases with higher volatility can also purify process solutions. Multiple stages may be used to increase separation purity, to reduce energy requirements, or to accomplish multiple phase separations.
Environmental considerations
Energy requirements for heating and evacuation to vacuum conditions.
2.7.6 Electrolysis – plating out
Transition metals can be removed from waste water streams by plating out on high surface area electrodes in metal recovery cells. It can also be used in combination with ion exchange as a means to concentrate metal ions, see Section 2.7.8.
Environmental considerations
At concentrations below about 10 mg/l the metal deposition becomes very inefficient with 10 to 100 times the theoretical energy requirement actually needed for metal deposition.
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2.7.7 Electrolysis – oxidation
It is possible to oxidise both unwanted organic by-products and metals in solutions, such as Cr(III) to Cr(VI). This can be done with or without a ceramic membrane, depending on current density conditions.
2.7.8 Ion exchange – resin
Ions in solution are selectively removed by exchanging positions with resin-functional groups.
The direct ion exchange treatment of waste water provides a means of concentrating multivalent cations for subsequent treatment on column regeneration or by plating out (see Section 2.7.6) [3, CETS, 2002], [159, TWG, 2004].
Environmental considerations
Ion exchange requires energy for pumping during use and regeneration, and also requires extensive chemical dosing; the chemical requirement for metal ion capture is generally three to four times the theoretical requirement.
2.7.9 Electrodeionisation
Ions are removed using conventional ion exchange resins. An electric current is used to continuously regenerate the resin, instead of regeneration chemicals.
2.7.10 Acid (resin) sorption
Acid (resin) sorption is configured similarly to ion exchange. Resins are designed to selectively adsorb mineral acids while excluding metal salts (adsorption phase). Purified acid is recovered for re-use when the resin is regenerated with water (desorption phase).
2.7.11 Ion exchange – liquid/liquid
Ionic contaminants are removed from process solutions into immiscible primary liquid extraction solutions. Secondary liquid extraction solutions are used to remove the contaminants and to regenerate the primary extraction solution.
So far, the only application is ammoniacal etch solutions that are regenerated by removal and recovery of copper, with a closed loop extraction solution system.
2.7.12 Membrane filtration
There are various types of membrane filtration and their use depends on their pore size. These systems are pressurised because of the small pore size.
• microfiltration (MF) is a membrane filtration technology that uses low applied pressures with pore sizes in the range of 0.02 to 10 microns to separate relatively large particles in the macromolecular to micro particle size range (approximate molecular weights
>100000)
• ultrafiltration (UF) passes ions and rejects macromolecules (0.005 to 0.1 micron) and removes organics from process solutions
• nanofiltration (NF) is used for larger size rejection than reverse osmosis (rejects molecules larger than 0.001 to 0.008 microns).
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Energy is used in pressurising the systems.
2.7.13 Reverse osmosis
Reverse osmosis, effectively filtration of ions through a semi-permeable membrane at high pressure, provides an alternative means of concentrating metal impurities for subsequent removal. This approach can be capital intensive and any solids, together with organics, have to be removed prior to treatment.[3, CETS, 2002].
Environmental considerations
The technique runs at high pressures with a subsequent energy demand.
2.7.14 Diffusion dialysis
Diffusion dialysis is a membrane separation process that typically uses an anionic exchange membrane to transport acid anions and protons from waste acid solutions into deionised water streams (for treatment in waste water treatment plant) and the acid is recovered.
2.7.15 Membrane electrolysis
Membrane electrolysis uses one or more ion-selective membranes to separate electrolyte solutions within an electrolysis cell. The membranes are ion-permeable and selective. Cation membranes pass cations such as Cu and Al, but reject anions. Anion membranes pass anions, such as sulphates and chlorides, but reject cations
Environmental considerations
The electrolytic reactions may generate hazardous gases, depending on the solutions.
2.7.16 Electrodialysis
Anions and cations are removed from solutions with an applied electric field in cells with alternating anion- and cation-permeable membranes