1111 E. Airline Hwy, Gramercy, LA 70052

Alumina Industry Terminology


Definition: A white Alumina crystalline substance which contains approximately 98.5% Al2O3• 3H2O and 1.5% impurities.

Monohydrate Aluminas (Al2O3 • H2O)

  • Boehmite (Alpha Monohydrate) - Jamaican Bauxite
  • Diasporic (Beta Monohydrate) - Greek & Chinese Bauxites

Monohydrate Available Alumina (MHA)

This represents boehmite (Al2O3 • H2O). Currently, this form of alumina constitutes about 12% of the available alumina in the bauxite processed at Gramercy. It is extracted in the monohydrate digest at about 470oF.

Lattice Bound Alumina

The Al+3 ion contained in the molecular structure of the goethite (FeOOH-AlOOH) form of iron found in Jamaican bauxite. Some of the Al+3 can be extracted by digestion with lime.

Alpha Alumina

A "dead-burned" or highly calcined alumina. This high density material is a specialty alumina (tabular, corundum).

Transition Aluminas

Chi, delta, eta, gamma, kappa, pseudogamma, rho, theta and upsilon aluminas. These transition phasesoccur as Al2O3 • 3H2O are calcined to Al2O3.


Trihydrate Aluminas (Al2O3 • 3H2O)

Gibbsite (Alpha Trihydrate) - Jamaican Bauxite

Al(OH)3 - Aluminum Hydroxide

Trihydrate Available Alumina (THA)

A compound of aluminum oxide with chemically combined water - gibbsite (Al2O3. • 3H2O). Current bauxite has about 88% of the available alumina as gibbsite and is extracted in the trihydrate digest at about 300 degrees Fahrenheit.

Total Available Alumina (TAA)

This represents the fraction of all forms of alumina in the bauxite (approximately 95%) that can be extracted at the plant's digest conditions of caustic, temperature and holding time.

Total Chemical Alumina (TCA)

A measure of all the alumina in the sample, regardless of type, state of hydration, availability, chemical compound, or solubility.

Sodium Aluminate (NaAlO2)

A compound formed by the action of caustic soda (NaOH) on bauxite.


Sodium Carbonate

A calculated quantity = total soda less caustic soda (NaOH +NaOH combined as NaAlO2) expressed as g/l Na2CO3.

Total Soda

The sum of free NaOH plus NaOH combined as NaAlO2, plus Na2CO3, expressed as g/l Na2CO3.

Total Sodium

The total amount of sodium in the liquor, both inorganic and organic, expressed as g/l Na2CO3.

Non-Alkaline Soda

A calculated quantity equal to total sodium less total soda, expressed as g/l Na2CO3. Non-alkaline soda includes all sodium compounds other than sodium carbonate and caustic soda.

Inorganic Non-Alkaline Soda

SiO2, P2O5, V2O5, Cl, SO4 are commonly determined. SiO2, P2O5 and V2O5 are reported as such in g/l Na2CO3. Cl is reported as NaCl, g/l and SO4 as Na2SO4, g/l.

Non-Caustic Soda

A calculated quantity equal to total sodium less caustic soda, expressed as g/l Na2CO3. It includes sodium carbonate, sodium oxalate, and all other inorganic and organic compounds that are combined with sodium.

Sodium Oxide (Na2O)

Na2O (expressed as g/l) is the form used by many plants outside the U.S. Na2CO3 (g/l) x 0.585 = Na2O (g/l).

Desilication Soda

The soda that combines with reactive and quartz (to a much lesser degree_ silicas to form DSP (Desilication Product) is a direct soda loss to the process. The general or empirical desilication equation:

Dissolution: (fairly fast clay type silica such as Kaolin)

Precipitation: (Slow)

Bayer Sodalite

Precipitation rate flavor favored by seeding, hence the higher the initial clay content, the more complete the desilication.

Leachable Soda

Soda that is removed from the hydrate by copious washing with water.


The conversion of sodium carbonate to sodium hydroxide by

use of lime: Na2CO3 + Ca (OH) 2 -> 2NaOH + CaCO3

Accountable Soda Loss

The loss of soda from the system which can be accounted for, e.g., soda loss to red mud disposal.

Unaccountable Soda Loss

A soda loss from the process which cannot be accounted for by monitoring waste streams and uncontrollable soda losses.

Controllable Soda Loss

Soda losses which can be reduced by tighter controls within the plant or process, e.g., increased mud solids concentrations in the


Caustic soda

Caustic Soda

The sum of the free NaOH and the NaOH combined as NaAlO2, expressed as g/l Na2CO3.

Free Caustic

A calculated quantity representing the uncombined NaOH in the Bayer liquor; thus, free caustic = total caustic soda minus combined caustic soda as g/l Na2CO3.

NaOH = Sodium Hydroxide


Calcium Carbonate (CaCO3)

Some forms encountered are:

• Limestone

• Product of chemical reactions in the process, e.g., conversion of Na2CO3 to NaOH with Ca(OH)2.

Na2CO3 + Ca(OH)2 -> 2NaOH + CaCO3

Calcium Oxide (CaO) – Lime

It is used in the digest of bauxite to control P2O5 and condition Jamaican mud for better settling (promotes formation of hematite from goethite). Also used to form filter aid.

(3CaO.Al2O3.6H2O) for the filter press operation. Note that Gramercy reacts lime with liquor before adding to the process.

Calcium Aluminate (3CaO•Al2O3•6H2O)

The compound formed when Ca(OH)2 is added to Bayer liquor. It is used as a filter aid on the Kelly presses.



The portion of the total silica that is the more difficult form to solubilize in the Bayer process. The amount in bauxite varies,

e.g., Jamaican (about zero) compared to Weipa (1 to 3%).

Reactive Silica

The most readily soluble or reactive silica is in the form of kaolinite

- Al2O3•2SiO2•2H2O.

Desilication Product (DSP)

The silica (kaolinite plus that amount of quartz solubilized) that reacts with caustic forms DSP. A general equation for DSP is

3(Na2O•Al2O3•2SiO3•nH2O)•Na2X. Other forms of DSP are:

Cancrinite - 3(Na2O•Al2O3•2SiO3•nH2O) •Na2CO3

Noselite - 3(Na2O•Al2O3•2SiO3•nH2O). • Na2SO4

Sodalite - 3(Na2O•Al2O3•2SiO3•nH2O). • 2NaCl

Zeolite - Na2Al2Si2O2•.5H2O•1/12 Na2 (Al2, 2OH, CO3, SO4, 2Cl)


organic carbon

Organic Carbon

The carbon content of organic matter contained in the bauxite is determined in the laboratory and expressed as percent C. Jamaican bauxite ranges from 0.15 to 0.25 percent. Other bauxites range from basically none (Surinam, Trombetas) to 0.35 percent (Guyana).

Organic Soda (ONa)

Bayer liquor contains series of organic compounds having 1 to 22 carbon atoms. The simplest and largest percentage of the total (about 65%) are sodium formate, oxalate, acetate and succinate, or compounds having 1 to 4 carbon atoms.

Sodium Oxalate (Na2C2O4)

This compound is an organic impurity in the process and is expressed as equivalent Na2CO3. It is formed as a result of the attack by NaOH on the organic carbon contained in the bauxite with subsequent degradation to Na2C2O4.

Total Oxidizable Organic Carbon (TOOC)

A measure of the liquor contamination by the carbon content of the bauxite. It is determined in the laboratory by oxidizing the organic carbonaceous matter to CO2.


Allowable Charging Ratio

The allowable charging ratio is alumina to caustic ratio aim in the liquor exiting digestion. The equilibrium A/C ratio is calculated as a function of the caustic soda concentration of the liquor, liquor impurities, and temperature in digestion. A safety margin of 15 – 25 points is subtracted from the equilibrium A/C. The resulting number is the aim or the allowable charging ratio. The safety margin is referred to as the “allowable margin”.

Alumina to Caustic A/C

In Bayer liquor, this is the ratio of: Al2O3 g/l / Caustic Soda, g/l, as Na2CO3

Caustic Soda to Total Soda (C/S)

The ratio expressed as: Caustic Soda, g/l, as Na2CO3/Total Soda, g/l as Na2CO3

Hematite to Geothite (H/G)

The ratio of these iron minerals is an index to the settle ability of muds from digestion. Generally, the lower the H/G ratio, the more difficult it will be to settle the resultant digest muds, especially if the high temperature pretreater or other digester vessels are off line.

Hematite = Fe2O3 Geothite = FeOOH – AlOOH

Silica to Caustic (Si/C)

The ratio of the Si (g/l) in solution to caustic (g/l). This ratio should be controlled between 1.6 and 2.5 x 10-3. Experience has shown that this limit will result in a manageable heater cleaning program in Digestion.



It is a major constituent of bauxite and the principal component of red mud. It is present largely as an oxide and expressed as percent Fe2O3.


Manganese in bauxite and mud is determined by a colorimetric method in the laboratory. It is reported as percent MnO.


A laboratory determination that is generally expressed as g/l TiO2 in liquor or percentage weight in alumina or bauxite. It must be controlled by lime addition.


Zinc is reported as ZnO. It is a liquor contaminant for Jamaican bauxite plants and is controlled by addition of a sulfide (Na2S or NaHS) to the pregnant liquor stream.


An important constituent of bauxite, and is reported as percent P2O5. It must be controlled by lime addition.


The free water and any other substance volatile at 100oC as measured by loss in weight when heated to 110oC for four or more hours.

Other Elements

Elements such as Calcium, Chromium, Gallium, Vanadium, and other trace elements are generally determined by x-ray analysis and reported as the oxide.


The term is derived from the Greek word Pisos, which means peas. Jamaican bauxite pisolites are hard, pea-shaped aggregates having the typical composition above.



A measured portion of a liquid sample submitted for analysis.

Loss On Ignition (L.O.I.)

A measure of bound water and other volatiles (such as water of hydration in Al2O3•3H2O), and any other substances volatile at or up to 1000oC (measured from 300-1000o). It is expressed in percent.

Mud Samples

The washed and dried solids obtained from a mud slurry. The dried product is usually in the form of a cake which is ground in a mortar to pass a 100 mesh screen.

Mud Settling Test

A laboratory test used to determine the settling quality of plant muds using synthetic polymers. The flocculent used is mixedwith settler feed, and the time taken for the mud level to pass given points in a 1000 ml cylinder is recorded, and the rate determined, usually in ft/hr.

Settling Index

This index is a measure of the settle ability of a bauxite to be processed in the plant. It is based on a laboratory digest and settling rate determination for a standard bauxite, which is known to process satisfactorily, and is given an Index of 1. Other bauxites can be similarly processed in the laboratory to predict their behavior: (1) if their settling rates are faster than the standard, their indices will be greater than 1, and (2) if their rates are slower than the standard, their indices will be less than 1.

Bauxite Mud Factor

This is the amount of mud per unit of bauxite for a given process. Tie elements are used to determine the mud factor for a given bauxite and set of process conditions. Fe2O3 or TiO2 are essentially unaltered in the digestion process; therefore, make good tie elements. The mud factor at Gramercy is 0.8 to 0.9.

Attrition Index

Attrition index is the measure of the breakdown of alumina particles (inverse relationship to strength) and is measured by subjecting a sample of alumina to air agitation in a cylinder. The index is:

AI = (% -45 micron after attrition) - (%45 micron before attrition) X 100 / (%45 micron before attrition)

Agglomeration Index

Agglomeration index is the measure of the increase in size of particles during precipitation where small particles are stuck

together. The agglomeration index is: Agg. I = (% +325 mesh of product)- (% +325 mesh of seed) X100 / (% +325 mesh of seed)

Analytical Precision

This refers to how well analytical results can be reproduced. This is a measure of repeatability of an analytical method.

Analytical Accuracy

This refers to how close the analytical result is to the real value being determined.