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Particulate Removal from Water In a number of industrial and other applications, waters become infused with suspended solids such as clays (kaolins, montmorillinites, etc.) and precipitates (metal hydroxides, metal sulfides, etc.) For example, phosphate rock production in Florida requires colossal volumes of water (flow rates as high as 100,000 gallons per minute for a single mine), and that entire colossal volume of water becomes entrained with micron sized particles of kaolin that literally do not settle out even when the water is sent to an evaporation pond and allowed to stand idle for protracted periods. These micron sized clay particles remain entrained in water indefinitely. They do not settle without addition of expensive and potentially carcinogenic, organic chemicals called flocculants. It is very difficult to get the clay particles out of the wastewater even with protracted settlement in ponds using flocculants. This relegates phosphate rock mining companies for example to purchase huge quantities of land to store wastewater during the five, ten, or fifteen years of settlement and evaporation that is required to eliminate the entrained kaolin to the point at which the water can be recycled, disposed, or directed into wastewater sources. Such long term storage is not only expensive, it is environmentally degrading. Waterfowl have been known to die in these large phosphate/kaolin settlement ponds (whether from the water itself or from an entirely unrelated cause), and the water is thought to percolate into the subsurface aquifers. By and large, clay-containing wastewaters are relegated to long-term storage in tailings impoundments to settle the bulk of the solids, and there is real and perceived liability from such storage. There is mounting pressure, both economic and legal, for those companies to eliminate or reduce such ponds. In the most extreme case, companies will perhaps be required to filter the clay from the water using ultra filtration techniques. Such filtration would probably never occur in practice because industries would probably choose to shut down and quit the business and move overseas. Reticle has found that the clay particles entrained in such waters themselves have an intrinsic surface charge (zeta potential) that can be exploited to adhere to cathodes/anodes of Reticle carbon. This intrinsic surface charge is caused by the unsatisfied bonds of the elements on the surface of the minerals, not dissimilar in philosophy to the "little skins" of microorganisms. Much like charged Reticle Carbon electrodes, the energy at the surface of the micro-particles causes electric double layers to form in the water adjacent to those particles—the magnitude of this charge is a function of the particle chemistry and the water chemistry. The surface energy that can be measured on the particles is called the zeta potential, and it is unique in magnitude for every substance based on the different atoms that are present on the exposed surface and the ions in the water that can adsorb in the single layer next to the solids. Zeta potential is sufficient to attract the clay particles to a charged Reticle Carbon electrode and remove it from the water. Zeta potential can be affected by simple alterations in pH, the mix of H+ and OH- ions in the fluid available to stick to the clay particle. Careful control of water chemistry can change the magnitude and polarity of the zeta potential on specific solids, rendering electrostatic separation possible. As an example, the overall charge of the clay particles may be made negative while small particles of phosphate can be made positive by simply varying the pH, in effect adhering more negatively charged OH- ions to the clay particles. Such adherence renders those particles removable by an anodic Reticle Carbon electrode. The phosphate minerals (still positively charged) will be attracted to a cathodic electrode. This is profound. This allows the small clay particles to be charged by the simple addition of acid or alkali and thereafter allow them to be attracted to Reticle Carbon electrodes through standard electrostatic attraction. This can dramatically accelerate cleanup of kaolin-containing water, and it may prove to recover much of the lost phosphate minerals that were too fine for collection in the standard process. (An estimated 20% of the phosphate mined is estimated to be lost to tailings as the micron size particles of final phosphate product are washed into the tailings pond because they cannot be economically recovered.) Use of Reticle Carbon CDI to thicken or separate suspended clays from wastewater solutions offers real potential to actually purify clay entrained water and save many millions of dollars in storage cost, human liability, and wildlife liability not to mention storage cost and aesthetic pollution. The Reticle Carbon concept is to allow real-time recovery of water from slurries using the intrinsically low operating and maintenance cost Reticle Carbon CDI process. Using Reticle Carbon CDI, the only waste generated is a dewatered slurry that will consume MUCH less volume in tailings impoundments. Furthermore, in the phosphate mining business, the kaolin entrained in the water is actually quite rich in phosphate—the primary product of the mining business in the first place. The kaolin taken from the Reticle Carbon electrodes has as much value as the phosphate rock that is produced by the mine, and it can simply be mixed into the product of the mine and sold. This is a huge benefit—the byproduct is marketable at the same price and in the same way as the product. The byproduct has intrinsic value. The concepts discussed here are not symptomatic to only phosphate producers. Potash mines, clay mines, salt mines have all reported similar "fines" problems. Worldwide there are over 10 major mining companies that recover phosphates, salts, clays, etc., which are (by nature) micron-scale minerals. In addition, the technique can be used to clarify any process water with solid suspensions. The size of this industry is very large in the sense that the volumes of water are colossal. |
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