Bauxite is a naturally occurring mineral raw material composed principally of a mixture of one or more of the hydrated aluminum oxide minerals -- gibbsite (Al2O3 • 3H2O), boehmite (Al2O3 • H2O), and diaspore (Al2O3 • H2O). Bauxites also contain impurities of silica, iron oxide, titanium, zinc, and several elements in trace amounts. Bauxite is an end or near-end product of chemical weathering.
Depending on the amount of iron impurity, the color of bauxite varies from dark red and brown to pink to white. Some bauxite is finely divided, free digging earthy material, while most others are dense and rocky requiring explosives in mining. Many gradational forms exist. It is one of the most variable mineral raw materials in chemical composition and physical appearance.
It is generally believed that bauxite deposits resulted from intense chemical weathering of aluminum-bearing rocks or formations under tropical or subtropical conditions with alternating wet and dry seasons. Topographic conditions were favorable for drainage and the in situ accumulation of aluminum, iron, and titanium oxides. There was low to moderate topographic relief with a minimum of erosion during long quiet periods in the earth's history.
The largest known reserves are located in tropical and subtropical areas. The most important use of bauxite is the raw material for the production of aluminum. It is also used in the production of abrasives, refractories, aluminous chemicals and cement, and materials for the purification of some petroleum products.
The Gramercy plant processes bauxite mined on the north coast of Jamaica. Generally, bauxite from this area occurs in limestone sinkholes, some of which have ore over 100 feet thick. The ore is soft and earthy. Overburden consists of one or two feet of soil. Ore bodies are extremely variable in size, ranging from several thousand to several hundreds of thousands of tons.
The bauxite is mined with draglines, power shovels, backhoes, or scrapers, the selection of which depends on size and configuration of the deposit and the operator's equipment preferences. At the North Coast operation, the bauxite is partly dried and stored under cover prior to shipment to Gramercy. Mined-out areas are rehabilitated by shaping up the limestone footwall and replacing previously stockpiled soil overburden.
Currently, the bauxite usage at Gramercy is about 2.35 dry tons of bauxite per ton of alumina produced. At an annual production rate of 1.2 million tons of alumina, 2.8 million tons of dry bauxite will be required; however, this tonnage will vary as bauxite quality varies.
In the nearly 170 years since aluminum was first isolated by Oersted, the metal has grown from a laboratory curiosity to a position of major importance in the world's commerce. In terms of annual production rates, aluminum is first among non-ferrous metals.
Oersted's discovery, the reduction of aluminum chloride with potassium amalgam, was improved on by St. Claire Deville. Deville's sodium amalgam reduction of aluminum chloride led to the commercial production of aluminum in France in 1855. In 1866, Charles M. Hall (USA) and Paul T. Heroult (France) each invented a process for the production of aluminum by the electrolysis of alumina dissolved in molten cryolite.
Based on current bauxite quality, the soda losses are about 0.095 T/T of 100% caustic. A breakdown of these losses and caustic specifications follows:
Lime – CAO
Lime (CaO) is required in the Digestion and Clarification Press sections of the process. Currently, the usage is 0.06 tons CaO per ton of alumina. Lime is purchased from outside suppliers. The major specifications for this material follow:
The flocculants are used to settle the fine mud particles generated as a result of the bauxite digestion process. They bring these particles together and allow them to settle more quickly because of their increased mass.
Basic Polymer Chemistry
The flocculants used to settle the red mud at Gramercy are made up of polymers. Polymers are reacted from individual monomers and strung together to form very high molecular weight chains. The products delivered to the bulk tanks consist of these polymer
chains, wrapped tightly in small beads, which are covered with water and dispersed in oil. These polymers, which have negative charges on their surfaces, are prepared by mixing them with weak caustic solutions (“Dilute Caustic”).
In order to get the chains to extend, the neat polymer is mixed with dilute caustic to produce a 1.5% solution. This mixture must be agitated vigorously if not, the polymers will “gel” into white, slimy balls that cannot be used to settle mud. This is the reason the polymer feeders are designed to give “shear” in the mixing block.
When the 1.5% solution is prepared properly, it has a white appearance and is viscous. It also has a stringy texture. If there are white balls or “fish eyes”, the chains have not been fully extended and there is a problem with the mixing system. This can be caused by a lack of shear, chemical contamination, or too low soda content in the make-up water. It can also appear pink or reddish if the dilute caustic contains process liquor carried over from evaporation.
For maximum performance in the washes and settlers, the dilute caustic should be between 10 and 40 g/l. Below 10 g/l, the flocculant becomes more viscous and difficult for it to flow out of the dilute floc tank and into the system. Above 40 g/l and the product begins to degrade, or break down, becoming less effective, even to a point where it will not settle mud.
Once the flocculant is prepared (inverted) it is post-diluted and sent to the settlers and washers. The dilution sources are #4 Washer Overflow, Dilute Caustic, or #7 or #8 Washer Overflow, in order of preference.
When flocculant contacts the mud, it gathers the particles together, giving them more weight and allowing them to settle at a higher rate. The most ideal way to add flocculants is as dilute as possible and with enough mixing to allow flocculant/mud contact without breaking apart the flocculant. The flocculant is injected into one or more feed points. The feed points used are either on the slurry feed line, into the center feedwell, into an overflow recirculation line, or into the mix pots outside the vessel.
The dosage response of flocculants is steep, meaning that it takes only small changes in dosage to quickly increase settling rates. Too high of a settling rate is undesirable because the solids will compact quickly and could cause high rake torques. High settling rates can also result in fines not being settled and overflowing into the overflow stream. It also means that product is being wasted.
Cytec “CYFLOC HX 200 is a liquid emulsion called settler flocculant and is applied to the settler and “0” washer. It is added into the feed well of these vessels and mixed with Mud slurry from Digestion for the settlers and from upstream washer for “0” washer. It is added a rate of 50 to 90 pm to the settlers and 4 to10 gpm to “0” washer. It is used for clarity control for 0 washer.
Front –end Floc
Nalco 85252RRA is a liquid emulsion called ‘front-end’ flocculant and is applied to washer 0 thru washer 4 and to the Pressure Decanters for mud settling. It is added into the feed box of the washer and mixed with Mud slurry from upstream washer and washer overflow from downstream washer. It is added at a rate of 5 to 30gpm at 0.31% in the wash train. It is pumped from the 1.0% floc solution and is diluted to 0.75% and added to the Pressure Decanters feed well to settle the mud. It is added at a rate of 8 to 27gpm at the PDs.
Nalco 85292RRA is a liquid emulsion called ‘back-end’ or Washer flocculant and applied to the last vessels of the Washing Train i.e. 5th to 8th stages. It is added into the feed box of the washer and mixed with Mud slurry from upstream washer and washer overflow of downstream washer. It adds a rate of 5 to 30 gpm.
Nalco 85549 Humate Removal Agent is added to the process as an organic remover (humates) to improve the color (whiteness) of the hydrate product produced at Gramercy. It is added to the suction of the Dilution Pumps, which pumps washer overflow to Digestion to control caustic concentration. It removes humates (long-chain organic compounds) and improves hydrate color.
Crystal Growth Modifier
Nalco 7837 is a crystal growth modifier added in the precipitation circuit to:
- Control or suppress nucleation (control the generation of fines)
- Enhance agglomeration (stick together small particles)
- Improve particle strength (strengthen agglomerates)
- Promote growth of large particles
- Control foaming in precipitation
- It is injected into the suction of the three separate fill pumps adding L to P liquor to Precipitation. It is added at a rate of20 ppm to the suctions of the North Continuous Train pump, South Continuous Train pump, and the Batch Fill Pump. The addition rate is changed at times in response to process conditions in precipitation.
Nalco 85734HC is added to the Secondary Overflow feeding the Tertiary Thickeners in Precipitation to control the quantity of fine hydrate that overflows the vessels in the liquor feed to Evaporation. It also improves the rheology of the underflow slurry and compaction of tray underflow solids. It is added at a rate from 2.7 to 3.5 ppm, about 30 gal/day for all 3 pumps.
Nalco 85488 is used to reduce the moisture content and leachable soda of the hydrate discharged from the Pan Filters in Calcination. It is added to the water used to wash the soda from the hydrate on the Pan filters. It is added at a rate of 85 gal/day. It controls the moisture content of the Kiln feed to about 8%.