Lead-Zinc Mining in Nigeria
Lead (Pb) is a bluish-white lustrous metal. It is a very soft metal with high malleability and ductility. It has a shiny chrome-silver luster when melted. It is one of the most important and widely used metals in the world. It is usually mined in the form of its sulphide (PbS) known as galena, carbonate (PbCO3) known as cerussite and sulphate ores (PbSO4) known as anglesite. Galena is however the most abundant and commonest ore. Over 60% of world lead production is used in lead-acid batteries for the storage of energy. Other uses include the sheet lead and pipes, cable coverings, pigments, solder and bearing metals. Zinc on the other hand is a brittle, crystalline, bluish-white metal. It uses include galvanization of steel plate, manufacturing of brass and other alloys, vulcanizing of rubber, production of pigments, certain medicines and chemicals. It is as well usually mined in the form of its sulphide (ZnS) known as sphalerite.
Lead and zinc mineralization in Nigeria are found in the rocks of the lower cretaceous age in the Benue trough. They are mesothermal type of hydrothermal deposits. That is, they are formed at moderate temperature and pressure in and along fissures or other openings in rocks by a deposition at intermediate depths from hydrothermal fluids. Lead-Zinc ores usually occur together and are often associated with copper, silver, siderites (FeCO3) and barites. Commercial deposits of lead and zinc ores have been discovered in Nigeria. Mineralization has been observed along a belt of some 30 -50km wide, with a stretch of approximately 560km from Ishiagu in Ebonyi state through Benue, Bauchi, Adamawa, Taraba, Nassarawa and Plateau states. Major occurrences have been discovered at Ameka, Ameri, Enyigba and Nyeba to Abakaliki in Ebonyi state, Zurak in Plateau state, and Gwana in Bauchi state.
Lead and zinc ore deposits have been mined on a very a large scale in Nigeria. Over 30 lodes of lead-zinc measuring an aggregate length of about 7,000m have been reported in the Nigerian lead-zinc field. The mineralization was shown by drilling to be up to 100m depth. Abakaliki field in Ebonyi State hosts the largest volume of Nigeria’s lead-zinc deposits. The Nigerian Lead-Zinc Mining Company (1956) gave a conservative indicated ore reserve of 693,000 tons with 9% lead and 7% zinc for the Ameri and Nyeba lead-zinc reserves. Lead-zinc mineralization has been found to be associated with calcareous shales and shaly limestone in the Abakaliki field.
Lead and zinc are primarily mined underground from their respective ores. As for zinc, once mined, the ore concentrates is heated to a temperature of about 9500 Celsius, causing the oxidation of zinc, sulphur and iron. Diluted sulphuric acid (H2SO4) is then used to reduce the zinc and iron oxides to their powdered forms. Leaching is followed with dilute sulphuric acid. The resulting solution is neutralized and followed by decontamination through filtration process. The zinc later on assumes its final form in the foundry.
Many mineral ores contain lead, but only the trio of galena, cerrusite and anglesite are economically viable. Therefore, lead is mostly obtained as a by-product of those ores. The lead ores are dug from underground mines. The ores are first grounded into powder. The grounded ores are then properly mixed with water in a process called floatation. Pine oil and air bubbles are then introduced into the mixture to cause agitation and formation of oil froth on the surface. The grounded lead particles are then collected in the froth which is scooped off and filtered to remove water. The powdered lead is then heated at a temperature of about 13500 to 19000 Celsius in a process known as sintering to oxidize impurities like sulphur. The molten lead produced is then allowed to flow into the lead molds where it solidifies before re-melting and skimming to increase its purity.
While zinc has nutritional values and is recommended for dietary intakes at certain proportions, lead on the other hand is poisonous, and lead exposure is generally considered harmful. There have been reported cases of lead poisoning in Nigeria through the intake of food or contaminated water. Series of deaths that have been linked to lead poisoning have been mostly attributed to underdeveloped domestic mining by villagers
Lead and zinc mining in Nigeria just like many other heavy metals have a wide range of detrimental impacts on the environment. The mines generally produce large volumes of waste because the ore bodies are always small fractions to the total volumes of mined or excavated earth materials. Lead mining causes the greatest degradations to the environments and the disruption of ecological balances – air, soil and water pollutions or contaminations, destructions of crops and vegetations and the deaths of floras and faunas. Smelting and refining processes release heavy metals such as cadmium (Cd) and lead (Pb) and enormous quantities of gaseous pollutants such as carbon dioxide, (CO2), sulphur dioxide (SO2) and oxides of nitrogen and as well as particulates matters, sewage waters and solid wastes. The soil is the major sink for heavy metals released into the environment by mining and smelting processes. Heavy metals contamination of soils poses serious threats to the balance of the ecosystem and human health through direct ingestions, the food chain relationship, contaminations of groundwater and reduction in lands’ value for agricultural purposes which can lead to food insecurity. Zinc is an essential nutrient for the body, but can be harmful at concentrations beyond the threshold. Water containing zinc at high concentrations for example, can lead to stomach cramps, nausea and vomiting.
Lead and zinc are important industrial metals and will continue to play a significant role in the future of the Nigeria’s mining sector. Environmental friendly mining can be achieved by putting measures in place to mitigate impacts on the environment. These set of measures include the reduction of input of the mine, reduction of outputs such as solid wastes, mine water and air particles. These can be achieved by using sustainable equipments and machineries. Other measures include proper waste disposals, mine reclamations and environmental restorations after mining.
