Increased emphasis on industrial pollution control has intensified investigation of waste treatment technology for methods of reducing the amount of pollutants discharged by industry. The mining and mineral processing industry can point to a record of considerable accomplishment in the field of wastewater treatment and reclamation because these operations are so frequently an integral part of the entire operation of an ore dressing facility.
Neutralization of excessive acidity or alkalinity in wastewater is a fundamental pollution control measure that is required to preserve receiving stream biota. A wide variety of materials are available for pH adjustment but for large consumptive uses first consideration usually is given to a low-cost material such as lime or limestone for acid neutralization and sulfuric acid or carbon dioxide for alkali neutralization. Since neutralization may involve many factors in addition to pH adjustment, the chemical characteristics of the wastewater must be considered.
The use of limestone for neutralizing acidic wastewater is often attractive because this material is available in many areas at the lowest cost per unit of basicity. However, limestone neutralization has a reputation of limited reliability due to the high incidence of failures recorded in the past. It is important, first of all, to establish that this process is really applicable to the wastewater to be neutralized. A common problem with stationary beds of lump or granular limestone is fouling caused by the presence of certain metal ions, sulfates, phosphates, fluorides, oils, greases, etc. Such fouling can be the result of chemical action (i.e., formation of insoluble metal oxide coatings) or physical (i.e., coatings of oils and greases).
Gravity sedimentation is widely used in ore dressing operations for thickening and water clarification for reuse or discharge.
Settling tanks or thickeners and tailings ponds are commonly employed to separate water from finely ground ore concentrates and tailings. In addition to water recovery and pollution abatement some benefit is often gained from recovery of chemical reagents used in flotation circuits. The introduction of new flocculation aids (e.g. polyectrolytes) has greatly increased the efficiency of the sedimentation process in recent years and these materials are now extensively used in these operations.
Activated carbon may be used in either the granular form or the powdered form. Successful and economical plant scale application of granular activated carbon to the treatment of secondary effluents has been demonstrated at South Lake Tahoe. Spent carbon is withdrawn from the carbon beds in this plant for thermal regeneration and is returned to the beds for reuse. Wastewater treatment processes employing powdered activated carbon are currently under development and these show considerable promise for reducing the capital costs of treatment facilities. Both dissolved organics and suspended matter are removed simultaneously in the powdered carbon processes.
Many physical-chemical methods are available for the removal of undesirable dissolved solids from wastewater. As a rule, wastewater treatment will involve the selective removal of one or more substances rather than bulk separation of all of the dissolved matter from the wastewater. The latter is usually an expensive operation at high dissolved solids concentrations although product or water recovery may provide sufficient incentive in some instances.
Chemical treatment is quite commonly used for precipitating, oxidizing, or reducing undesirable constituents in wastewater. Chemical precipitation methods offer one of the most economical approaches to contaminant removal and clean-up of the wastewater in question. Inexpensive chemicals such as lime are often used to effect precipitation reactions. Furthermore, the equipment required for precipitation processes is generally relatively inexpensive due to the low cost of the sedimentation process.
Chemical oxidation of wastewater constituents may be accomplished through the addition of chlorine, hypochlorite salts, ozone, chlorine dioxide, permanganate salts and other oxidants.
