Metals play important roles in nearly every aspect of modern life. Cellular phones, computers, and washing machines are only a few examples of the indispensable products that use metal. Metal mining, however, can be a complex process. This article explores how titration provides better control of the mining process and offers titration tips through two process examples; gold and aluminum determinations.
What are metals?
Metals are chemical elements that are malleable, ductile and conduct heat and electricity that are generally classified according to use. Base metals are typically used in industry as-is, without additional treatment. Examples include lead (Pb), zinc (Zn) and tin (Sn).A precious metal is a naturally-occurring metal that is rarer and has ahigher economic value, such as gold (Au) or platinum (Pt). Chemically, precious metals are less reactive than most elements. Finally, ferrous metals include iron (Fe), and include both pure iron or iron allows, and may also be characterized as magnetic.
Rocks and ores contain various compounds besides the target metal to be extracted. These naturally occurring compounds, such as other metals, carbonates and sulfide ions, can interfere with the titration reaction leading to erroneous results. To avoid interference, these compounds must be separated or chemically modified. Therefore, obtaining metals from ores requires a rather lengthy production process. The resulting pure metal undergoes additional modifications depending on its final use. The complete mining process from metallic ores to pure metal can be described as follows:
• First, a geologic survey localizes the mineral deposits and ore assays are used to evaluate yield expectations and extraction costs.
• Second, suitable ore preparation, such as concentration or extraction, is required to separate the metal compound from the rocks. Additional process steps lead to a raw material which can be refined.
• Finally, melting in an electric arc furnace, for example, in the presence of a suitable reducing agent extracts the elemental metal.
• Subsequent refining by electrolytic or other techniques, such as zone melting, produces the pure final metals.
Metal extraction can also be achieved by leaching. Leaching involves extracting metal from the crushed ore and dissolving it in aqueous solutions containing specific chemical reagents commonly used for the recovery of precious metals such as gold and silver.
Content determination by titration
Determination of metal content is necessary for different mining stages. Among potential analytical procedures, titration has proven to be a robust and rather simple analytical technique that can be used at nearly any point in the process and is now automated to a large extent from earliest sample preparation through data storage.
Only metal ions dissolved in a suitable solvent are available for titration. Therefore, adequate sample preparation is critical. Generally, metal release is achieved by heating ore powder in concentrated mineral acid solutions such as nitric or hydrochloric acid in a process known as digestion. Ashing and fusion represent additional sample preparation techniques which allow metals to be released from rocks, ores and alloys.
Nitric acid is effective in sample preparation due to its chemical compatibility, oxidizing ability, availability, purity and low cost. Hydrochloric, hydrofluoric and sulfuric acids are also used for acid digestions as well as mixtures of these acids.A common mixture is 1:3 nitric acid:hydrochloric acid, referred to as aqua regia, Latin for "royal water", due to its ability to dissolve gold. In some cases, inorganic strong bases can also be used.
Several sample preparation procedures and method programs have been tested in METTLER TOLEDO's Market Support Group Applications Laboratory in Switzerland. The results have been collated in Titration Application Brochure No. 42, a dedicated publication on titration methods for the metal mining industry, which can be located at http://us.mt.com/us/en/home/supportive_content/know_how/List_of_all_application_brochures.html?crel=ePR_PP_EN. An on demand Webinar with more chemistry, tips and tricks concerning metal titrations is also available at http://us.mt.com/us/en/home/supportive_content/specials/Titration_Webinars.html?crel=ePR_PP_EN
About Mettler Toledo:
METTLER TOLEDO specializes in the area of precision instruments for professional use. METTLER TOLEDO laboratory instruments are used in research, scientific, drug discovery, and quality control labs, amongst many others in the pharmaceutical, chemical, food and cosmetics industries. METTLER TOLEDO comprehensive industrial solutions cover the various steps in a host of manufacturing processes at many of the same customers that laboratory serves. Solutions range from receiving raw materials through various manufacturing processes, in-line process control and end-of-line packaging control, to logistics and shipping. Increasingly, these solutions are fully integrated into the customer's IT environment, helping automate their workflows. In food retail, our offering for fresh food management ranges from receiving and pre-packaging, to in-store solutions for self-service departments, deli counters, and checkout terminals.
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Titration of metals in the mining industry