Gujarat Cleaner Production Centre - ENVIS Centre

CLEANER PRODUCTION OPPURTUNITIES IN SMELTING INDUSTRIES

1

Gujarat Cleaner Production Centre - ENVIS Centre

Aluminium Smelting

History of Aluminium:

Aluminium is the most abundant metal in the Earth’s crust. Since the discovery of in 1821 by P. Bertheir, the metal has gradually evolved as a cheap, durable metal which finds applications in several industries.

The metal originally obtained its name from the Latin word for , alumen. The name alumina was proposed by L. B. G. de Moreveau, in 1761 for the base in alum and it was successfully shown in 1787 that the base was the oxide of a yet to be discovered metal. The existence of aluminium was established by Sir in 1808.

Initially, alumin ium was produced by reduction with alkali metals and was very expensive. In 1886 C. M. Hall and Paul Héroult, working independently, came up with a cost -effective solution called the Hall-Héroult electrolytic process that refined aluminium production. In this process, the is dissolved in molten and then reduced to the pure metal. The was deployed to purify the raw material alumina, recover it from hydroxide and used in the Hall -Héroult process.

The Hall-Héroul t process consumes a lot of energy but the development of alternative processes was scrapped due to technical problems and also because these were economically unviable. Therefore, the Hall -Héroult process has remained the relatively inexpensive mode of pr oducing aluminium.

Refining:

Aluminium refining is conducted primarily through the Bayer Process, which involves the separation of aluminium oxide from the oxide in bauxite. Under the Bayer Process, which was developed by Karl Joseph Bayer in 1888, the aluminium-bearing minerals in bauxite - Gibbsite, Böhmite and Diaspore - are selectively extracted from the insoluble components (mostly oxides) by dissolving them in a solution of sodium hydroxide.

2

Gujarat Cleaner Production Centre - ENVIS Centre

Gibbsite:

Böhmite and Diaspore:

Aluminium oxide precipitates when aluminium hydroxide is introduced to the liquid sodium aluminate. The crystals of aluminium oxide are washed and heated to get rid of the water. This results in a fine white powder (aluminium oxide) also known as alumina.

Smelting:

Aluminium smelting is conducted chiefly through the Hall -Héroult process. This process involves passing a direct current through a solution of alumina dissolved in molten cryolite; adding cryolite allows the electrolysis to occur at a lower temperature. The electrolyte is placed in an iron vat, which serves as the cathode, lined with graphite. In addition to this, carbon are immersed in the electrolyte. Upon passage of electrical current through the electrolyte, the molten aluminium m etal is deposited at the bottom of the cathode while carbon is oxidized to form carbon dioxide at the .

The reaction at the cathode is:

Oxygen is formed at the anode, which gets oxidized to form carbon dioxide:

There are two main types of smelt ing technologies; Soderberg and Pre-Bake. The principal difference between the two technologies is the type of anode used.

• Soderberg technology Söderberg technology uses a continuous anode which is delivered to the cell in the form of a paste and which bak es in the cell itself. • Pre-Bake Pre-bake technology uses multiple anodes in each cell, which are pre -baked in a separate facility and attached to rods that suspend the anodes in the cell. The newest primary aluminium production facilities use a variant of pre-bake technology called Centre Worked Pre -bake Technology (CWPB). • Plants using CPWB technology demonstrate very low fugitive emissions, which amounts to less than 2% of total emissions. The balance of the emissions is collected inside the cell and move d to scrubbing systems, which remove particulate matter and gases.

3

Gujarat Cleaner Production Centre - ENVIS Centre

Casting:

On completion of the smelting process, aluminium is casted into the various products such as ingots and wire rods. Casting is carried out in a casthouse, which has electrically heated furnaces and ingots casting machines. Aluminium casting processes can be subdivided into ingot casting and mould casting.

Under the ingot casting technique, molten aluminium metal is poured into separate moulds until the metal solidifies into ingot s. This technique consists of uphill and downhill casting. Uphill casting leads to better surface quality due to the usage of casting powder. The quality of products in this process is higher with simultaneous casting of many ingots. Downhill casting, on t he other hand, is cheaper but results in lower quality of products and casting rate.

Mould casting can be chiefly subdivided into die and sand casting. Under die casting process, molten aluminium metal is poured into mould cavities under high pressure; th e cavities are subsequently machined into dies. The die casting method is especially suited for applications where many small to medium sized parts are needed with a fine surface quality and dimensional consistency.

Sand casting allows smaller batches of p roducts to be made at a reasonable cost. Through this technique, manufacturers benefit from the small size of operations and can create products at a low cost. Sand casting also allows most metals to be cast depending on the type of sand used for the mould s. In addition to this, sand can be recycled many times and requires little maintenance

Industry Issues Aluminium • Disposal of red mud, bauxite tailings and other hazardous Smelting waste, dust emissions and high energy consumptions OR • Aluminium smelters produce a quantity of fluoride waste that unless carefully controlled, can be very toxic to vegetation around the plants. OR • The main environmental issues for primary aluminium are the production of poly fluorinated hydrocarbons and fluorides during electr olysis, the production of solid waste from the cells and the production of solid waste during the production of alumina. Similarly for the production of secondary aluminium there are potential emissions of dust and dioxins from poorly operated furnaces and poor and the production of solid wastes (salt , spent furnace linings, dross and filter dust).

4

Gujarat Cleaner Production Centre - ENVIS Centre

Process materials inputs and pollution outputs for aluminium smelting and refining Process Material input Air emissions Process wastes Other wastes Bauxite refining Bauxite, sodium Particulates, Residue hydroxide caustic/water containing vapour silicon, iron, titanium, calcium oxides and caustic Alumina Alumina slurry, Wastewater clarification and starch, water containing precipitation starch, sand and caustic Alumina Aluminium Particulates and calcination hydrate water vapour Primary Alumina, Fluoride—both Spent potliners electrolytic carbon anodes, gaseous and aluminium electrolytic particulates, smelting cells, cryolite carbon dioxide, sulphur dioxide, carbon monoxide, C2F6 ,CF4and perfluorinated carbons (PFC) Conclusion:

• Preference should be given to Prebake Technology rather than Soderberg Technology. • Use of computer control and point feeding of aluminium oxide to the centre line of the cell are necessary to control the bath composition and limit the anode effects. • Baking furnace gases may be used for energy recovery. • Use of dry scrubbing system with aluminium oxide as the adsorbent for control of gases from the cell and from anode bake oven is necessary. • Fabric filters or ESP for controlling particulate matters may be used. • Use of low sulfur tar for baking anodes will help control suphur dioxide emi ssions. • Use of inert anode in place of carbon anode will reduce the emission • The latest technique accepted for the control of fluorides is by using Dry Scrubbing system for the smelter gases, controlling pollution and also recycling the fluorides thereby affecting economy in the consumption of cryolite and Aluminium fluoride.

5