Aluminum electrolysis production technology (5)
①CO 2 Effect of global warming due to the gas purification system for the electrolytic production of aluminum contemporary use of flue gas treatment escaping CO 2 without any treatment, with water vapor in the sky one pair of global warming had a greater influence. To completely solve this problem, only a combination of inert anode and hydroelectric power generation is used. Table 8 lists the superposition of aluminum produced by the power plant and the aluminum plant and the mass of CO 2 gas discharged separately. Table 8  Superposition and separate CO 2 emissions from power plants / aluminum plants using both Hall - Erfa and inert anode processes project Hall - erufa Inert anode Hydropower Thermal power ( natural gas ) Hydropower ( coal ) Hydropower Thermal power (natural gas) Hydropower (coal) Electrolyzed CO 2 / ( t/t  Al ) 1.74 1.74 1.74 0 0 0 CO 2 / ( t / t generated by the power plant  Al ) 0 6.16 15.4 0 4.8 12 Total CO 2 escape ( t/t  Al ) 1.74 7.9 17.14 0 4.8 12 The effects of 2CF 4 and C 2 F 6 on the global warming effect CF 4 and C 2 F 6 produced when the anode effect of the electrolytic cell occurs have no toxic side effects on raw plants and plants, and have no effect on the isothermal ozone layer, but they are all global. The greenhouse effect works. Due to its ultra-stable stability in air, the degree of influence is 6500-10000 times that of CO 2 . According to a recent measurement of the prebaked cell, 0.12 kg of CF 4 per ton of aluminum produced per minute of anode effect per day escapes, and the amount of escape from the baking bath is about 2/3 of that of the prebaked cell. The primary countermeasure to reduce CF 4 and C 2 F 6 is to reduce the occurrence of anode effects. At present, many countries have stipulated that the anode effect frequency of the electrolytic cell of the aluminum plant must be reduced to 0.2 times/(day·tank), and some aluminum plants even reach 0.05 times/(day·tank). Table 9  Fine aluminum quality standard element Aluminium content (≥) /% Fe/% Si/% Cu/% Fe , Si , Cu grade (sum) / %≤ High grade 99.996 0.0015 0.0015 0.001 0.004 High second grade 99.99 0.003 0.0025 0.005 0.01 High third grade 99.97 0.015 0.015 0.005 0.03 High grade 99.93 0.04 0.04 0.015 0.07 There are three layers of liquid in the refining tank. The lowermost layer is an anode alloy, a primary aluminum and copper to be refined weighting agent composition, due to its greater density (3.2-3.5g / cm 3), so that it is submerged in the bottom, connected to the groove bottom of the anode carbon block; intermediate One layer is electrolyte, generally composed of Na 3 AIF 6 , AIF 3 , BaC1 2 , density of 2.5-2.7g/cm 3 , lighter than anode alloy, but heavier than aluminum, in the middle layer; the top layer is refined The refined aluminum has a density of about 2.3 g/cm 3 and is connected to a graphite cathode (or a solid aluminum cathode). A hand-held cutter can be used. Hand-held cutting machines, also known as stone cutting machines and marble machines, are tools for cutting stone. Marble Tools,Tile Tools,Head Waterjet,Marble Cutting Tool HARBIN QIAN FAN XI FENG MACHINERY CO.LTD , https://www.waterjetwamit.com
Regional air pollution from 3SO 2 gas The escaping SO 2 reacts with the moisture in the air to form “acid rainâ€. The solution is to use a petroleum sulphur with low sulphur or a wet purification system. The prebaked anode block containing 2.4% sulfur was measured. It was found that about 1 kg of COS gas per ton of aluminum escaped, and was oxidized to SO 2 after entering the atmosphere, and the remaining 95% or more of sulfur directly escaped as SO 2 gas. The total amount is equivalent to 3% of the amount of CO 2 escaping. [next]
4 Fluoride and PAH impact on the surrounding environment Since the effects of fluoride-evolved fluoride on organisms and plants have been recognized earlier, the measures taken by the world aluminum industry in environmental management and flue gas purification in the past two decades Great progress has been made in reducing fluorine emissions. The fluorine emissions of the European primary aluminum industry have decreased from about 3.8 kg/t A1 in 1974 to 0.7-0.8 kg/t Al in 1994, and some countries have now reached 0.4-0.5 kg/t Al. In some ways, the harm is almost non-existent.
The production of PAH is mainly derived from the bitumen smoke produced during the production of the self-baking tank. In the aluminum plant of the modern prebaked anode electrolyzer, there is almost no PAH, but there is a small amount of escape when the furnace is hot.
(2) Electrolytic flue gas treatment process and horizontal electrolytic aluminum production flue gas treatment are divided into dry method and wet method. At present, there are mainly the following forms of international dry purification:
1 flue is directly added to the A1 2 O 3 adsorption method (Alcan, Canada);
2 Venturi Reactor Method (French Air Company);
3VRI method (US PEC company);
4 boiling bed method (Alcoa, USA);
5 special pipeline method (Flakt, Norway).
Due to the secondary pollution caused by wet purification and the complicated process, the purification of fluorine-containing flue gas is rarely purified by wet method. Aluminum (aluminum, as some European) at sea, in particular, high sulfur petroleum coke production plants and Soderberg anode aluminum, in order to meet stringent environmental requirements local, washed with water and the like to escape SO 2 Harmful gases; aluminum plants in inland areas are generally not used.
The typical pollutant emission indicators of the world's advanced modern prebaked anode electrolytic cell aluminum plant after environmental treatment are as follows:
CO 2 1.6t/t Al (including anode manufacturing)
Fluoride 0.5kg/t Al
CF 4 0.05kg/t Al
5. Production of refined aluminum The aluminum obtained by electrolysis of Na 2 AlF 6 -A1 2 O 3 molten salt generally contains no more than 99.8% aluminum, which is called primary aluminum; 99.99%-99.996% aluminum is refined aluminum; aluminum 99.999 The above are high-purity aluminum; the aluminum containing more than 99.9999% is ultra-pure aluminum. The quality standards of refined aluminum currently in China are shown in Table 9. [next]
Refined aluminum has better electrical conductivity, thermal conductivity, plasticity, light reflectivity and corrosion resistance than primary aluminum. The most valuable of these is its corrosion resistance. The higher the purity of aluminum, the denser the surface oxide film and the stronger the combination with the internal aluminum atoms, which makes it exhibit good corrosion resistance to certain acids and alkalis, seawater, sewage and sulfur-containing air.
Aluminum is a substance with very low magnetic permeability. It has good electromagnetic properties in an alternating magnetic field. The higher the purity, the smaller the magnetic permeability and the lower the conductivity of low temperature. Therefore, refined aluminum and high-purity aluminum have special applications in low-temperature electrical technology, low-temperature electromagnetic components and electronics.
There are many refining methods for aluminum, the most important of which are the three-layer liquid electrolytic refining method and the regional melting method.
(1) Three-layer liquid electrolytic refining Three-layer liquid electrolytic refining method was first proposed by AGBetts in 1905, and later recommended by Hoopes, and first applied in industry in 1922. For a long time, the principle of electrolytic refining has not changed fundamentally, but the structure of the electrolytic cell and the composition of the electrolyte have changed.
Figure 6 is a schematic view of a three-layer liquid aluminum refining electrolysis cell. [next]
Aluminum molten salt electrolytic refining is an electro-metallurgical process using a soluble anode, which is similar to aqueous electrolytic refining of metals. The basic principle is that when electrolysis, the aluminum in the anode Al-Cu alloy loses electrons, and the electrochemical anode dissolves to become Al 3+ into the electrolyte, and then Al 3+ obtains electrons on the cathode to perform electrochemical reduction. which is:
On the anode Al (l) -→A1 3+ +3e
A1 3+ +3e→Al (l) on the cathode
Impurities that are more positive than aluminum, such as Si, Fe, Cu, etc., do not undergo anodic dissolution, but remain in the anode alloy; impurities that are more negative than aluminum, such as Na, Ca, Mg, etc., although anodic dissolution occurs, with corresponding ions Entering the electrolyte, but because their precipitation potential is higher than aluminum, at a certain A1 3+ concentration, electrolysis temperature and current density, they will not precipitate on the cathode, but remain in the electrolyte, thus achieving the purpose of primary aluminum refining.
(2) The regional baking zone is a smelting method for preparing high-purity aluminum. For example, in combination with an organic solution electrolytic refining method, ultrapure aluminum can be obtained.
The regional baking method is to refine the refined aluminum (99.99%-99.996%) into a thin ingot, remove the surface oxide film with high-purity hydrochloric acid and nitric acid, put it into a spectral pure graphite boat, and then install it. A graphite boat with an aluminum ingot is placed in a quartz tube (vacuum is vacuumed inside the tube), and then heated slowly along the outside of the quartz tube to create a 25-30 mm narrow melting zone on the aluminum ingot. The melting zone temperature is 750 ° C. The zone melts 12-15 times, and the purity of the obtained product can reach 99.999% or more.
(III) Development direction of the primary aluminum industry 1. International Primary Aluminum Industry The US Government entrusted the US Department of Energy and the All-American Aluminum Association to prepare the "Technology Development Strategy for the US Aluminum Industry", which is the US government's plan to guide the long-term technological development of the primary aluminum industry. The document, which sets out the objectives of the US aluminum industry and the objectives of the primary aluminum industry, has certain reference value for the development direction of the global aluminum industry. The excerpts from the development of the primary aluminum industry are as follows: [next]
Increase aluminum production by 25% in 15-10 years;
40% increase in the use of aluminum in the automotive market within 25 years;
Increase the market for non-automobile transportation by 40% in 35 years;
50% increase in the use of infrastructure in 45 years;
In 53-5 years, the current efficiency is increased to 97%;
The power consumption drops to 13000 kW.h/t Al within 63-5 years;
Effectively reduce the anode effect coefficient within 73-5 years and significantly reduce the emission of fluorocarbons to reduce the serious impact on the atmospheric greenhouse effect.
2. The revolution of Hall-Elu prebaked anode electrolysis technology 1 The design of TiB 2 -G cathode element electrolyzer;
2 development of a venting type electrolytic cell;
3 development of inert anodes;
4 Research and development of new technologies and new technologies.
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