Gujarat Cleaner Production Centre (Established by Industries & Mines Department, GoG) ENVIS Centre on: Cleaner Production/Technology Supported by:

Transcription

1 2015 Cleaner Production Guidelines G in Aluminium Smelting Sector Gujarat Cleaner Production Centre (Established by Industries & Mines Department, GoG) ENVIS Centre on: Cleaner Production/Technology Supported by: by: Ministry of Environment, Forest & Climate Change, Government of India rd Block No: No 11-12, 3 Floor, Udhyog Bhavan, Gandhinagar Phone:: + 91 (079) Mail: gcpc11@yahoo.com ; info@gcpcgujarat.org.in; Website Website:

2 History Of Aluminum Smelting Initially, aluminum 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 aluminum production. In this process, the aluminum oxide is dissolved in molten cryolite and then reduced to the pure metal. The Bayer process was deployed to purify the raw material alumina, recover it from sodium hydroxide and used in the Hall-Héroult process. The Hall-Héroult 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 producing aluminum. Refining Fig- Aluminum Process flowsheet Aluminum refining is conducted primarily through the Bayer Process, which involves the separation of aluminum oxide from the iron oxide in bauxite. Under the Bayer Process, which was developed by Karl Joseph Bayer in 1888, the aluminum-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.

3 Gibbsit Al(OH) 3 + Na + + OH - Al(OH) 4 + Na + Böhmite and Diaspore Aluminum oxide precipitates when aluminum hydroxide is introduced to the liquid sodium aluminate. The crystals of aluminum oxide are washed and heated to get rid of the water. This results in a fine white powder (aluminum oxide) also known as alumina. Smelting Aluminum smelting is conducted chiefly through the Hall-Héroult electrolysis 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 anodes are immersed in the electrolyte. Upon passage of electrical current through the electrolyte, the molten aluminum metal is deposited at the bottom of the cathode while carbon is oxidized to form carbon dioxide at the anode. 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 smelting 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 bakes in the cell itself.

4 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 aluminum 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 moved to scrubbing systems, which remove particulate matter and gases. Casting On completion of the smelting process, aluminum is casted into the various products such as ingots and wire rods. Casting is carried out in a cast house, which has electrically heated furnaces and ingots casting machines. Aluminum casting processes can be subdivided into ingot casting and mould casting. Under the ingot casting technique, molten aluminum metal is poured into separate moulds until the metal solidifies into ingots. 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 the 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 aluminum metal is poured into mould cavities under high pressure; the 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 products 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 moulds. In addition to this, sand can be recycled many times and requires little maintenance Industry Aluminum Smelting Issues Disposal of red mud, bauxite tailings and other hazardous waste, dust emissions and high energy consumptions Or Aluminum smelters produce a quantity of fluoride waste that unless carefully controlled, can be very toxic to vegetation around

5 Or the plants. The main environmental issues for primary aluminum are the production of poly fluorinated hydrocarbons and fluorides during electrolysis, 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 aluminum there are potential emissions of dust and dioxins from poorly operated furnaces and poor combustion and the production of solid wastes (salt slag, spent furnace linings, dross and filter dust). Process materials inputs and pollution outputs for aluminum smelting and refining Process Material input Air emissions Process wastes Other wastes Bauxite, sodium hydroxide Bauxite refining Alumina clarification and precipitation Alumina calcination Primary electrolytic aluminum smelting Alumina slurry, starch, water Aluminum hydrate Alumina, carbon anodes, electrolytic cells, cryolite Particulates, caustic/water Vapor Particulates and water vapour Fluoride both gaseous and particulates, carbon dioxide, sulphur dioxide, carbon monoxide, C2F6,CF4and perfluorinated carbons (PFC) Wastewater containing starch, sand and caustic Residue containing silicon, iron, titanium, calcium oxides and caustic Spent potliners

6 Conclusion Preference should be given to Prebake Technology rather than Soderberg Technology. Use of computer control and point feeding of aluminum 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 aluminum 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 emissions. 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 Aluminum fluoride. ***