![]() Due to fine particulate nature, these sorbents were preferably applied in continuous stirred-tank reactor type equipment offering high kinetic rates, and not in typical column arrangements. Therefore, further size reduction is not needed, before the application. On the other hand, the method of sorptive flotation involves the preliminary abstraction or scavenging of metal ions using appropriate sorbents, which already exist at the fine or ultrafine particle-size range. This means that the system is not a solution anymore, but rather a solid/liquid dispersion- i.e., precipitate flotation. The increase of initial surfactants concentrations, at least to stoichiometric ratios with the respective metal ions to be removed, may lead to the preliminary precipitation of ion-surfactant floatable product, even before air is passed. The surfactants are generally recoverable from the floated product and can be reused, whereas the resulting clarified water may be also recycled. This permits the separation of dissolved and dilute ionic species within a small concentrated volume, usually less than 5%, as compared with the original volume of solution, of collapsed foam. As a result of this procedure, on the surface of solution and within the froth structure appears an agglomerate of fine solid particles (insoluble precipitates), containing the surfactant as a chemical constituent. The surfactant is usually an organic compound of hydrophobic and hydrophilic character, carrying opposite charge to that of the metal ion to be removed. Ion flotation involves the removal of (usually) surface-inactive metal ions (cations or anions) from aqueous solutions by the addition of specific reagents and by the subsequent passage of gas bubbles through these solutions. Today, for example, applications of flotation exist in paper manufacturing for deinking and waste paper recycling, emulsified oil from various industrial wastewaters, and the separation of used plastics. These flotation applications include mainly the treatment of water and wastewater. As opposed to settling, flotation is a solid-liquid separation technique that is applied to particles whose density is lower or has been made lower than the liquid they are in. The attachment of bubbles to particles transfers the solids from the body of water to the surface. ![]() As, for many years, various particulate solids besides minerals have been extracted from water by using this effective gravity separation method that is based on the idea of applying rising gas bubbles as the transport medium. ![]() Thus, flotation initially originated from the field of mineral processing, usually termed froth flotation. Without this process, many familiar metals and inorganic raw materials would be exceedingly scarce and costly, because the high-grade ores that could be processed by simple physical and mechanical methods have long since been used up. The importance of the flotation process to the economy of the whole industrial world is considered to be enormous. Sustainability in this field and its significance for the chemical and process industry is commented. The second separation process, also applied effectively, was a new hybrid cell of microfiltration combined with flotation. The ability of microorganisms to remove metal ions from dilute aqueous solutions (as most wastewaters are) is a well-known property. ![]() Innovative processes have been studied in our Laboratory, particularly for metal ions removal, involving the initial abstraction of heavy metal ions onto a sorbent (including a biosorbent): in the first, the application of a flotation stage followed for the efficient downstream separation of metal-laden particles. Various types of biological materials were tested and floated efficiently, such as bacteria, fungi, yeasts, activated sludge, grape stalks, etc. Concerning the necessary bubble generation method, typically dispersed-air or dissolved-air flotation was mainly used. However, it has, nowadays, found several other applications, as for example in the wastewater treatment field. Flotation constitutes a gravity separation process, which originated from the minerals processing field.
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