Rating Manual section 6 part 3: valuation of all property classes

Section 60: aluminium smelting works

This publication is intended for Valuation Officers. It may contain links to internal resources that are not available through this version.

1. Co-ordination

Aluminium Smelting Works as a class are subject to co-ordination as outlined in the relevant Practice Note to this Section.

2. Description

2.1 History of the industry

Aluminium metal was first obtained in pure form in 1825 by heating aluminium chloride with potassium-mercury amalgam. In 1854 Henri Sainte-Claire Deville produced aluminium from sodium aluminium chloride by heating with metallic sodium; this was an expensive process but continued on a commercial scale for the following 35 years.

Charles Hall first produced aluminium in 1886 by the present-day large scale method of electrolysis of alumina (aluminium oxide) dissolved in a bath of fused cryolite; in the same year Paul Heroult was granted a French patent for a process similar to that of Hall.

Electrolysis on a large scale requires a substantial amount of cheap electricity, therefore the first major plants were built near to hydro-electric power sources in Canada, Scandinavia etc. With an ever increasing world demand linked with improved plant efficiency and techniques other considerations of siting such as easy access to markets, deep-water ports etc, became of paramount importance and in 1965 permission was given for the construction of three Aluminium Smelters in the UK at Anglesey, Northumberland and in Scotland.

2.2 Glossary of technical terms

Alloy: A compound of metal with other metals or chemical elements mixed by heating to produce a metal of special structural properties.
Alumina: A product of the refining of bauxite, a white powder (aluminium oxide).
Aluminium: A silver white metal produced by electrolytic reduction of pure alumina in a bath of molten cryolite.
Anode: The positive pole in an electrical field.
Bauxite: A residual clay found in tropical regions.
Carbon: A non metallic element found in the form of diamonds, graphite and charcoal.
Carbon Dioxide: CO2. A colourless gas with a slight smell of soda water.
Cathode: The negative pole in an electrical field.
Collector Bars: A metal bar embedded in the cathode part of the smelting pot and connected to the negative side of the bus bar system.
Cryolite: (Greenland Spar). A fluoride of aluminium and sodium, a constituent of the electrolyte.
Electrolosis: A chemical change based on ionisation effected by the passing of an electric current through a molten substance.
Electrolyte: A solution which conducts a current by ionisation eg cryolite and fluorspar.
Fluorspar: A mineral containing fluoride.
Holding Furnace: A gas or oil fired furnace lined with refractory brick to hold the molten aluminium at a constant temperature prior to moulding. Either fixed or tilting depending on the method of emptying.
Homogenising furnace: A heat treatment furnace for the gentle heating and cooling of aluminium cast ingots to release any internal stresses caused in the casting process.
Metallic aluminium: The result of the electrolysis of alumina in a bath of molten cryolite.
Metallic sodium: The electrolysis of fused sodium chloride.
Petroleum coke: Coke produced from the heavy ends of the crude distillery of a petroleum refinery which is a more volatile coke than ordinary coke oven coke.
Rectiformer: An electrical device which both transforms the voltage of an alternating current and converts that current to direct current.
Ring Furnace: A pit furnace of honeycombe construction gas or oil fired in which the green carbon anode blocks are baked or cured.
Wet Scrubber: A vessel for the treatment of waste gas. Cleaning is by contact with a spray of water before discharge to the environment.

2.3 Process

The stages involved in the production of metallic aluminium by the electrolytic reduction of pure alumina are explained in detail in the appropriate paragraphs of this section.

The diagrams at the end of this section (RM Appendices 5:60:1-3) should be studied in conjunction with this paragraph and, as an introduction, the steps of the process are set out briefly hereunder:-

The pot linings of carbon cathode blocks are installed or replaced.

Carbon anodes are manufactured for use in the pots and installed therein.

The cryolite is prepared and its composition controlled; it is then heated to melting point in the pots.

Alumina is dissolved in the molten cryolite bath.

The solution of alumina in molten cryolite is electrolysed to form metallic aluminium.

The molten aluminium is tapped from the pots alloyed (if desired) cast into ingots and cooled.

2.3.1 Section A - raw material reception

Bauxite occurs in large deposits in Australia, the West Indies and South America where it is refined into a white powder (aluminium oxide) and shipped in this form. Importation in bulk together with petroleum coke breeze requires harbour facilities which may be part of the smelting plant hereditament or may be sufficiently removed to warrant the former being dealt with as a separate hereditament. Whichever is the case the reception area comprises a jetty with its mooring facilities, unloading facilities by mobile crane with conveyor system to storage silos or storage buildings for both the alumina and coke. The normal emptying of the silos or storage buildings is from the bottom by way of an underground conveyor system either direct to the appropriate section of the smelting plant or if there is some distance between the reception depot and the smelting works, to an elevator system which loads the raw materials into railway trucks or other means of transport.

The only other buildings on the reception site are control and switch buildings, junction houses in the conveyor system, and normal amenity buildings for the workers.

2.3.2 Section B - carbon plant

This part of the works is sub-divided into green (uncured) carbon mix and press plant, the ring furnaces, the carbon rodding department and the pot lining building, all of which are dealt with in more detail in the following sub-paragraphs:-

(i) The green (uncured) carbon mix and press plant where the anode and cathode blocks are formed consist generally of a sheeted multi-storey plant structure supporting various types of crushers, mixers and presses where the petroleum coke is crushed and screened, mixed with hard pitch, heated to 160°C and pressed into the shape of the requisite blocks by vibratory formers or hydraulic presses.

The main constituent of the carbon mix is petroleum coke which is supplied from an adjacent storage source by elevator and conveyors to the crushing and screening plant at the top of the building and fed generally by gravity in dust-proof or covered conveyor ducts, to succeeding plant at lower levels. The formed carbon blocks are moved around the building on pallets attached to a continuous chain conveyor system and taken to the ring furnace building for curing.

(ii) The ring furnaces are used to bake or cure carbon anode blocks. They consist of a series of brick-lined pits set in the floor and gas heated indirectly through hollow flues alongside the pits.

The pits normally accommodate up to 5 anode blocks, are then filled up with granula coke and raised to a temperature of 1200°C. The blocks remain in the pits for a period of 21 days. The building forms a cover to the furnaces and incorporates overhead cranes which load and unload the pits from the continuous chain pallet conveyor system.

(iii) The anode blocks are then conveyed to the carbon rodding department where aluminium anode rods are joined to them and the assembly stored for future use. The anodes in use normally last about 21 days before having to be replaced; the spent ones being returned to the rodding department where the aluminium rod is reclaimed.

Generally the building will contain overhead travelling cranes, further section of the continuous chain conveyors, and process machinery together with its support stages, platforms, foundations and settings.

(iv) The pot lining building may not exist in every aluminium works as some pots are lined and relined in situ in the pot rooms. However, where it is the practice to line or reline the pots completely before conveying them to the pot rooms, the building will be equipped with overhead travelling cranes capable of lifting a pot weighing some 70 tonnes.

The lining comprises a layer of firebrick on which the carbon cathode blocks, with collector bars inset, are laid. Jointing is done with a carbon paste and a final inner lining of paste is compacted to form a smooth surface. The collector-bars are longer than the cathode blocks and protrude from the sides of the pot and are connected to the negative side of the electrical system by metal tabs.

2.3.3 Section C - pot rooms

The pot rooms or smelter buildings are heavy steel framed clad buildings with overhead craneage as in the pot lining building, constructed on two levels in an excavated area so that the operating floor is at approximately ground level and the semi-basement which has partly open sides, can obtain adequate air for ventilation. The pots are in rows normally running the length of each side of the building with a working area between. The operating floor has a series of holes through which each pot is lowered to rest by its own weight on foundations set on the semi-basement floor.

The pot room/smelter building can also be of single storey construction with the pots laid out in rows set in a shallow pit in the floor which runs the length of the building.

Pot dimensions are usually about 9.5 metres long, 4.26 metres wide and 1.4 metres deep and they are fitted with a removable hood which connects to ducting leading to the gas cleaning plant. Electrical connections to both cathode bars and anode assembly can be carried either over or beneath the operating floor.

To ‘start’ a pot a thin layer of coke particles is spread over the cathode to act as a heating medium. The anodes are lowered to touch this layer and a positive electric current connected. The result is a resistance or short circuit creating intense heat which quickly brings the pot up to its working temperature of 970°C. A mixture of cryolite and fluorspar is added which becomes a molten electrolyte at this temperature.

The anodes are raised to their normal operating position and alumina added to the electrolyte.

The reduction process commences when the released oxygen combines with the carbon anode to escape as carbon dioxide and the metallic aluminium is deposited on the bottom of the pot.

The process is continuous; the molten aluminium is regularly syphoned off into mobile ladles and more ingredients are fed in automatically or manually from a mobile storage vehicle.

During the process a crust forms on top of the molten liquid and this is broken up at intervals, either automatically by machinery incorporated in the superstructure or manually by a mobile crust breaker.

The gas cleaning plant referred to earlier is situated outside the building and will be either a Bag Filter or Wet Scrubber type.

2.3.4 Section D - casting plant

Molten aluminium is received in the casting department by way of mobile crucibles and is normally emptied into gas fired holding furnaces operating at a temperature of 800°C.

The molten metal is transferred from these furnaces to the casting machine for casting in pans or moulds.

The casting shop comprises a steel framed and clad building with sub-basement pits for furnace supports and overhead craneage.

The number and type of gas fired holding furnaces varies according to the size of the particular works, and in addition there will be an homogenising furnace operating at approximately 600°C for heat treating the machine extruded ingots.

2.3.5 Section E - power services

The power services vary according to the particular works. Where the power source is the National Grid, the services will consist of the main transformers reducing the voltage from the grid supply to:

a. the level for use in the works b. with the aid of rectiformers, to a low voltage but high amperage DC current to be used in the electrolysis process in the pot rooms

Where electricity is generated on the site, the power services will include steam producing plant driving turbo alternators producing HT (High Tension) current. It is usual practice to split the supply, part being transformed for use in the works with the exception of the pot rooms and part transformed down by rectiformers to high amperage low voltage DC current for use in the pots.

It will invariably be found that where power is generated on the site, facilities are provided for an emergency intake of power from the grid and possibly facilities to transfer surplus generated power back into the grid. In these circumstances there will be a separate system of transformers and associated switchgear keying in with the main system. The rectiformers in all cases will be found in close proximity to the pot rooms.

2.3.6 Section F - central administration and maintenance

This collection of buildings consists of typical industrial type, framed and sheeted buildings. The office block is not elaborate, but due to the nature of the process works, amenities building are of good quality with facilities of a high order.

3. Survey requirements

A separate guide to referencing Aluminium Smelting Works has not been produced but, where appropriate, the Guide to Referencing Steel Works should be followed.

4. Valuation considerations

4.1 Plant and machinery

Aluminium smelting plants contain a considerable number of items of plant and machinery named in The Valuation for Rating (Plant and Machinery) Regulations. Rateability should be tested in accordance with the principles set out in Rating Manual section 4 part 3.

The pots located in the pot room/smelter building are identified as furnaces in Class 4 Table B of the 1989 Plant and Machinery Regulations and Class 4 Table 4 in the 1994 and 2000 Regulations. However these will only be rateable in the unlikely event that exceptions (e) 1989 Regulations or (d) – 1994 and 2000 Regulations found at the beginning of Class 4, are not satisfied.

4.2 Site

Apart from considerations of national industrial planning strategy, site requirements include a source of cheap electricity, deep water facilities for the importation of raw materials and good road and rail communications.

5. Basis of valuation

Aluminium Smelting Works are highly specialised hereditaments and the appropriate method of valuation is the Contractor’s Basis (see RM Section 4 Part 3).

Appendix 1: aluminium works

Production flow diagram

Appendix 2: the smelting process

Process diagram on raw materials and their use in the smelting process

Appendix 3: section through a pot

Diagram for this section