Smelters are facilities that extract more refined metals from ores. Copper, lead, nickel, silver, zinc, gold, cobalt and cadmium are extracted using this procedure from earth samples and residues. Smelting involves heating the ore with a reducing agent such as charcoal, coke or other purifying agents. Although it is beneficial to the metal production industry, it also has many hazards that if left uncontrolled can impact the environment. Smelting is perceived to be a primary cause of water pollution in its vicinity. It produces slag, a solid waste with metal or a high concentration of the metallic mixture. Smelters are also responsible for emitting high amounts of air pollutants such as sulfur dioxide, hydrogen fluoride, oxides of nitrogen, offensive and noxious smoke fumes, gases, vapors, and other toxins. Discussed below are the main sources of water pollution induced by smelting [1,2,3].
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Potential sources of water pollution in smelting that require remediation
Air emission to acid rain
Smelting processes release air emissions that are major factors for both air and water pollution. Acid rain may be produced as a result of sulfuric acid mist being formed from these smelting plants that permeates the atmosphere. Due to gravity and movements in the atmosphere, the acid may spread a few miles away, spreading its reach. Acid rain leads to a rise in acidity in surface water and soil wherein numerous vegetation, wildlife and fish population reside. Acid rain also expedites land erosion and physically endangers plants and animals in the long run .
Waste products from smelting include liquid waste infiltrating into water supplies. The water utilized to cool ore is usually disposed of. Accidental drainage happens, enabling this toxic water to infiltrate its surrounding environment. In addition to that, liquid effluent treatment wastes and residues derived from the treatment of wastewater streams also contribute to water pollution. Water from the smelting process usually requires cleaning in a wastewater treatment plant by neutralization and precipitation of specific ions. Gypsum (CaSO4), metal hydroxides and sulfides are the main residues from these systems. These wastewater and liquid effluent treatment water contain various hazardous chemicals such as chromium and lead, which are extremely detrimental to plant and animal life [4,5].
Slag is a solid waste product that can be distinguished in two main kinds: slag from steelmaking and slag from smelting. The slag produced when manufacturing steel has low levels of toxic elements. While slag produced by smelters contains higher levels of potentially hazardous elements. When smelting metals, a particular slag is generated which closely resembles a volcanic rock and comprises significant concentrations of a number of potentially toxic elements, including copper, zinc, lead, arsenic, cadmium, and barium. Studies have shown that when slag releases these elements into the environment under natural weathering conditions, pollution of soils, surface water and groundwater may occur .
Returning to the cycle
In modern smelting plants, where the economic value of the metal concentrate is reasonably high, most of the wastewater and slag are being returned to the process, thereby lowering the amount of waste required for permanent disposal .
Modern technologies are significant in lowering or even preventing widespread smelter pollution. Unlike early smelting, most of the sulfur dioxide generated nowadays is being initially captured before emission through the application of new equipment such as electrostatic precipitators. This machine is commonly used to capture dust particles and return them to the process. Moreover, recycling lead batteries and aluminum cans, and utilization of alternative raw materials decrease the demand to process ore, which also contributes to the reduction of water pollution .