3 replies to this topic

Hi everyone.

I'm currently undergoing design project on my final semester, on bayer process, to produce alumina from bauxite. but its really hard to find details of the process, for example the conversion of the crystallizer, the sollubility of alumina in soda solution, and the condition of the equipment in the process. I really do not have direction to find those information. If anyone have the information may you share with me? Many thanks and cheers!!

[kkala]

I'm currently undergoing design project on my final semester, on bayer process, to produce alumina from bauxite. but its really hard to find details of the process, for example the conversion of the crystallizer, the solubility of alumina in soda solution, and the condition of the equipment in the process. I really do not have direction to find those information. If anyone have the information may you share with me?

It seems that you start a task not having tried before by your instructor's group, and this may be an elementary "simulation" of the Bayer process.
I once worked in an Alumina Project, but this was canceled, so I changed work in 1996, after a long stagnation. After so long a time I can give only general info, as well as an opinion.

1. Risking to be trivial, I note that Bayer process is a cyclic process: Bauxite is ground with soda and resulting slurry is heated in tubes or autoclaves (digestion, 150-270 0C) to transfer Al2O3 into liquor. Slurry passes from thickeners(90 0C) and polishing filters to get rid of bauxite residue (red mad), while overhead liquid (pregnant liquor) is diluted and passes from decomposers / precipitators, where crystallization of Al2O3.2H2O (boemite) takes place (90-65 0C). Resulting alumina slurry undergoes classification (fines returning to Precipitation), while coarse particles are filtered and calcined to produce Al2O3 (1100 0C). Filtrate (spent liquor) is evaporated, make-up soda is added and mixed with new bauxite on the way to grinding.
2. Indicated temperatures are approximate, also depending on bauxite quality (boemitic or diasporic). Concentrations of Al2O3 also vary, say from 185 g/l in pregnant liquor to 140 g/l in spent liquor (I am not sure about these values). Caustic ratio (moles Na2O / moles Al2O3 in liquid, inverse indice is often used, or similar) is about 1.5 in pregnant liquor and 3.0 in spent liquor. Precipitation yield may be 180 g/l (solid AL2O3 created per l of pregnant liquor, value not sure).
3. As I can remember, Process is rather complex and difficult to be understood well in a six month Project of one person. Do not expect realistic results. If process parameters are documented and judgement is used for the "simulation", task would reach success. According to that logic:
4a. Find a good book on Alumina production and study it thoroughly to understand the process. If possible visit a local Alumina refinery to collect operating data. Do not step further, without complete understanding Bayer process.
4b. Define operating parameters of the Plant (do not consider above as reliable, they should be interrelated). Alumina passing to liquid can be 90-96% of "available" Al2O3 in bauxite, where available Al2O3 is Al2O3 content minus SiO2 content in bauxite (others give a slightly different definition of available Al2O3). These parameters (mentioned above) should be taken from the book or the local factory, ask instructor's help in assessing them. They are not independent of each other, also depend on bauxite quality. Assume a parameter that you cannot find (e.g. extent of evaporation of spent liquor), to proceed with your exercise / project.
4c. Start making the "simulation", that will have a lot of iterations if a spreadsheet is used. It is not an easy job. Corrections may be necessary when you arrive at the end. Better to consider a process as simplified as possible, e.g. without considering organics removal (downstream classification), desilication rates, caustic cleaning. Caustic ratio cannot be too low (probable limit 1.3?) to avoid scales in the lines.
4d. Equipment size is based on above, precipitation residence time 35-90 hr. Digestion time 0.2 - 4 hours approximately, depending on bauxite, temperature, concentrations.
4e. Solubility of boemite in Bayer liquor is hard to find, googling "alumina solubility in Bayer liquor" gave a book, etc. It also depends on caustic ratio. Nevertheless pregnant liquor is oversaturated.
4e. In the past floury (e.g. finer) alumina was produced, nowadays sandy (coarse) alumina is made to be used in the gas scrubbers above aluminium production electrolytic sells. Probably you can neglect this parameter for simplicity.
5. Wishing success, you have taken a challenging yet exciting task. It can be very useful.

Edited by kkala, 15 January 2010 - 06:32 PM.

Thousands thanks for your guidance and information. For your information, I'm doing mass and energy balances for the process recently, so it would need many information to perform and support the calculation. So for now, I should get more information from book as well as get it from industry, though there is no such industry in my country (or maybe I cannot find it), I should search the information through some website. However, if you do not mind, may I take your information given as a reference and assumption whenever I need to do so? Again, many thanks and wishing you have a great day.

[kkala]

... I should get more information from book as well as get it from industry, though there is no such industry in my country (or maybe I cannot find it), I should search the information through some website. However, if you do not mind, may I take your information given as a reference and assumption whenever I need to do so?

Certainly you can use the information, which try to give a general idea of the conditions of the alumina plant. Nevertheless it is pointed out that you have to give a specific e.g. Al2O3 / NaOH concentration for each flow, being consistent with the rest flows. In my opinion this is the challenging task, to make this consistent balance even by simplifying assumptions. It is a student task, most probably the purpose is to understand the process rather than to precisely illustrate it. You will apply a degree of simplification, depending on time and available data.
In reality bauxite to be used undergoes patient pilot plant tests, so that alumina plant main parameters can be specified. Oil stocks are evaluated in a similar manner, as I guess from Nelson (Petroleum refinery engineering). But a student has to make (ball park) assumptions on main plant parameters, then develop a flow sheet with consistent mass balances.