What Are Light Metals?

Metals with a relative density of less than 5. Divided into non-ferrous light metals and rare light metals. Non-ferrous light metals include aluminum, magnesium, calcium, titanium, potassium, strontium, and barium. The first four are mostly used as reducing agents in industry. Aluminum, magnesium, titanium, and their alloys have relatively low density, high strength, and high corrosion resistance. Strong, widely used in industrial sectors such as aircraft manufacturing and aerospace. Rare light metals include lithium, beryllium, rubidium, cesium, etc. Beryllium is mainly used to prepare beryllium bronze. Due to the small thermal neutron capture cross section of beryllium, it can also be used as a structural material for nuclear reactors. Lithium is used as a deoxidizer and deaerator in metal smelting, and as a thermonuclear reaction material.

Non-ferrous metals with a density of less than 5000 kg / m ³ are also called light non-ferrous metals. Including aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), potassium (K) and sodium (Na) a total of 7 metals. Although the density of the rare metals beryllium, lithium, rubidium, and cesium is also relatively small, they are generally classified as rare light metals. Calcium, strontium, magnesium, and barium in light metals are collectively called alkaline earth metals, and potassium and sodium are alkali metals. Alkaline earth metal refers to the heavier element of group IIA in the periodic table. Alkali metal refers to all elements in Group IA of the elemental cycle, that is, in addition to potassium and sodium, it also includes lithium, rubidium, cesium and rubidium.

Aluminum, magnesium and its alloys have many excellent physical and chemical properties and are important and commonly used non-ferrous metals. Alkaline earth metals calcium, strontium, barium and alkali metals sodium and potassium are usually used in the chemical industry as chemical compounds. In addition, light metals are chemically active and are strong reducing agents, which have important applications in the metallurgical industry. ^[1]

Light metal resources are abundant, and the abundance of crust of aluminum and magnesium is at the forefront, far exceeding the commonly used heavy non-ferrous metals. And unlike the case where heavy non-ferrous metals have only a single type of mineral resources, except for aluminum, most of the other light metals have two types of resources: minerals and brines (seawater and salt lake brines, underground brines, etc.).

Light metal industrial minerals are also different from heavy non-ferrous metal industrial minerals, which are mainly sulfide ores. The former are basically oxidized ores, mainly silicates (bauxite, nepheline), carbonates (magnesite, dolomite, Limestone), sulfate (alum stone, mirabilite, gypsum, barite, barite), and phosphate (apatite), etc., and some minerals are chloride (table salt, potassium salt, carnal) and fluoride ( fluorite).

The symbiotic valuable metals contained in light metal minerals have been comprehensively utilized in the production of aluminum industry. For example, gallium and vanadium in bauxite, and sodium and potassium in nepheline have begun to be recovered as by-products. In brine water, sodium, potassium, calcium, magnesium, boron, lithium, rubidium, cesium and other elements are generally contained, and comprehensive recovery of these elements is particularly important. In industrial production using brine as a resource, many light metals are actually the products of comprehensive utilization.

High-grade light metal minerals can be directly used for smelting after crushing treatment. Low-grade light metal minerals can only be directly smelted by specific methods, or need to be enriched by beneficiation first.

When brine is used as a raw material, it is generally necessary to perform evaporation and concentration first, and then to extract the enrichment of each metal salt in stages. For example, after salt lake brine evaporates in the sun, sodium chloride, potassium stone, carnallite, and hydrochlorite are crystallized out in sequence, which can be used as raw materials for extracting sodium, potassium, and magnesium salts, respectively.

The light metal smelting process can generally be divided into two stages. The first stage is to prepare intermediate compounds, such as alumina, magnesium chloride, sodium chloride, potassium chloride, calcium oxide, strontium carbonate or barium oxide, from salts precipitated from mineral or brine crystallization. In the second stage, the intermediate compounds are prepared by molten salt electrolysis or thermal reduction (see metal thermal reduction method). High-purity metals are usually further refined from crude metals.

Wet metallurgy is the main industrial method for preparing light metal intermediate compounds. In the production of some light metals such as sodium, potassium and strontium intermediate compounds and the extraction of magnesium chloride from brine concentrates, the all-wet process can be basically used. In the production of other light metals such as aluminum and barium intermediate compounds, the raw materials must be roasted or sintered to make the metal to be extracted into soluble compounds, then leaching and purification by wet method, and obtaining qualified products by precipitation, drying and calcination Intermediate products.

Chlorinated metallurgy (see halogenated metallurgy) is used for the chlorination of magnesium minerals, which can obtain the more pure intermediate magnesium chloride. In the production of calcium oxide, only a single calcination of the limestone raw material is required.

Most of the symbiotic valuable metals contained in the raw materials are recovered from the slag or solution at the stage of preparing intermediate compounds.

Molten salt electrolysis is the main method for industrial production of metal aluminum, magnesium and sodium. Now all of aluminum and sodium and 80% of magnesium are produced by this method. In addition, the molten salt electrolysis method is also used to prepare metallic calcium, strontium, and barium. Except for the use of fluoride molten salt electrolysis for aluminum production, the rest use chloride molten salt electrolysis. The fluorine or chlorine and its compounds emitted during the electrolysis process have become the most important sources of environmental pollution in the production of these metals. Purification and recycling of these harmful by-products have become an important part of the aforementioned light metal production.

The thermal reduction method is the main industrial method for preparing metallic potassium, calcium, strontium and barium. The products produced by this method in the production of magnesium metal account for about 20% of the total output. Except for the production of potassium metal and magnesium, which use sodium and silicon as reducing agents, the other three metals are prepared by aluminothermic reduction. Most of these metals are produced in small quantities and reduction facilities are limited to smaller scales.

Vacuum distillation is widely used to prepare all high-purity light metals except lead. These light metals have relatively low boiling points, generally only around 1273K (from the boiling point of potassium of 1047K to the boiling point of 1910K of barium). They can be easily purified by distillation and cold separation from other high and low boiling impurities. High-purity aluminum is generally made by physical metallurgy (area melting or directional solidification) or electrochemical metallurgy (organic solution electrolysis). ^[2]

Aluminum and magnesium are different from other light metals. Aluminum is an important engineering metal. Its output ranks first in non-ferrous metals. Pure aluminum or aluminum alloys are widely used in various industries and civil applications. Strictly restricted industries such as aerospace, aviation, and automotive have strong competitiveness. Magnesium is an important component of aluminum alloys. In these industries, magnesium-based alloys can replace some aluminum materials.

Sodium, potassium, calcium, strontium, barium and other metals are chemically active, can react strongly with oxygen and water and even cause combustion or explosion. They can only be enclosed in paraffin, kerosene or inert gas filled containers, which is difficult to apply directly in industry . Most of their main products are salts. Industrially produced metals, except for a few uses such as reducing agents, are mostly still used to make various compounds or alloys.

Light metals have the advantages of rich resources and low density, and their applications in the field of engineering materials will be further developed. Among the light metal resources, brine resources will occupy an increasingly important position due to the comprehensive utilization of multiple metals and low-cost production.