Analysis of Red Mud

• By: Mitra S.K ADMIN
• April 05 2022
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EXPLORATION OF REDMUD

Introduction:

Red mud, now more frequently termed bauxite residue, is an industrial waste generated during the processing of bauxite into alumina using the Bayer process. It is composed of various oxide compounds, including the iron oxides which give its red colour. Over 95% of the alumina produced globally is through the Bayer process;

Due to this high level of production and the material’s high alkalinity, it can pose a significant environmental hazard and storage problem. As a result, significant effort is being invested in finding better methods for dealing with it such as waste  to create useful materials or   cement and concrete .  Less commonly, this material is also known as bauxite tailings, red sludge, or alumina refinery residues.

The alumina content of the bauxite used is normally between 45% and 50%, but ores with a wide range of alumina contents can be used. The aluminium compound may be present as gibbsite (Al(OH)₃), boehmite (γ-AlO(OH)) or diaspore (α-AlO(OH)). The residue invariably has a high concentration of iron oxide which gives the product a characteristic red colour. A small residual amount of the sodium hydroxide used in the process remains with the residue, causing the material to have a high pH/alkalinity, normally >12.

Reaction involves during Alumina extraction by Bayer Processes:

Al₂O₃ (Available Aluminium) + 2 NaOH → 2 NaAlO₂ + H₂O

2 NaOH + SiO₂ → Na₂SiO₃ + H₂O

2 NaAlO₂ + 3 H₂O + CO₂ → 2 Al(OH)₃ + Na₂CO₃

2 H₂O + NaAlO₂ → Al(OH)₃ + NaOH

2 Al(OH)₃ → Al₂O₃ ( Recovered Alumina) + 3H2O

SELECTING AN ANALYTICAL METHOD:

In order to select an analytical method intelligently, it is essential to define clearly the nature of the analytical problem. In general, the following points have considered while choosing an instrument for measurement.

1.  Accuracy and precision required,
2.  Available sample amount,
3.  Concentration range of the analyte,
4.  Interference in sample,
5.  Number of samples to be analyzed,
6.  Speed, skill and cost of analysis.

Selecting Parameters :

• Phase Analysis : Other elements & structure of Alumina by using XRD Study of Redmud,
• Chemical analysis.

XRD Study of Red Mud:

Further study of  the other elements & structure of Alumina .We were doing various Phase analysis by Bruker D2 Phaser and findings as follows-

Glossary:

H = Hematite ,Go = Goethite, G = Gibbsite ,R = Rutile , B = Boehmite , M = Magnesium calcite

Most probable Phase Composition Cancrinite 0.52% , Sodalite 4.14 % , Goethite 33.33 %, Quartz 0.78% , Magnesium calcite 5.49% , Kaolinite 1.46 % , Gibbsite 1.93 % , Rutile 1.93 % , Boehmite 10.92 % , Hematite 39.51 %

Most probable Phase Composition Anatase 16.73% , Sodalite 8.19 % , Goethite 21.81 %, Quartz 4.78% , Calcite 0.56% , Kaolinite 1.46 % , Gibbsite 2.22 % , Rutile 1.93 % , Boehmite 9.97% , Hematite 28.32%,Fedorite 10.06%

The data obtained (Sample – 01 & Sample – 02) after phase analysis and subsequent quantification using Powder X Ray Diffraction of the red mud clearly indicates that Al was not completely extracted during the Bayer’s process. The residual Al is present mainly as Boehmite in the red mud and a small amount of Gibbsite  is also present. Altogether these Al bearing phases account for around 7-8% of Alumina which was not extracted. We also looked for the presence of Diaspore (α AlO(OH)) although it was not identified in the PXRD scan which may be due to the presence of the said phase at a very low concentration.

Furthermore, there is some difference between the silica content obtained in wet classical analysis and the PXRD data. Although the PXRD is a semi quantitative method the considerable difference between the results obtained in the two methods can be attributed to the presence of amorphous silica in the samples which is not detectable in the PXRD method.

Chemical analysis : Parameter wise method details with comparison of Results :

Iron content Analysis :

Potassium dichromate (VI) solution turns green as it reacts with the iron (II) ions, and there is no way you could possibly detect the color change when you have one drop of excess orange solution in a strongly colored green solution. With potassium dichromate (VI) solution we have to use a separate indicator, known as a redox indicator.

Cr₂O₂⁻⁷+14H⁺+6e⁻→  2Cr³⁺+7H₂O

Cr₂O₇⁻²+14H⁺ +6Fe²⁺→  2Cr³⁺+7H₂O+6Fe³⁺

You can see that the reacting proportions are 1 mole of dichromate (VI) ions to 6 moles of iron (II) ions. Once you have established that, the titration calculation is again going to be just like any other one.

Silica content Analysis :

Entire process done by gravimetric method. Final reactions are as follows.

4HF(aq)+SiO₂(s)→SiF₄(g)↑+2H₂O(l)

Hydrofluoric acid (HF) reacts with silicon dioxide to produce silicon tetrafluoride ( gas – Volatile) and water.

Alumina (Al₂O₃) content Analysis :

Methods of aluminum determination, and complex stability constant for Al³⁺ is so high, that EDTA titration is tempting. Of several possible solutions, back titration proves to be the best approach. EDTA and aluminum are allowed to react in the hot solution for several minutes, then excess EDTA can be fast and easily titrated with Zn²⁺.

If solution was prepared by dissolving sample containing Al in a strong acid, it may have very low pH. Buffer we use during the determination  pH 5.5 Sodium acetate – is relatively far from its maximum buffer capacity.

Sodium Oxide  (Na₂O) content Analysis :

The flame photometer A traditional and simple method for determining sodium and potassium in liquid involves the technique of emission flame photometry. This relies on the principle that an alkali metal salt drawn into a non-luminous flame will ionise, absorb energy from the flame and then emit light of a characteristic wavelength as the excited atoms decay to the unexcited ground state. In this practical (Experiment A) you will calibrate a flame photometer using standard sodium and potassium solutions then measure the Na⁺ and K⁺ concentrations.

Titanium Oxide  (TiO₂) content Analysis :

Reaction of yellow colour development of TiO2 determination.

TiO₂ + H₂SO₄ (Conc) → TiOSO₄

TiOSO₄ + 5H₂O ↔[Ti(OH)₃(H₂O)₃]⁺ + HSO₄^–

[Ti(OH)₃(H₂O)₃]⁺ + H₂O₂ ↔ [Ti(O₂)(OH)(H₂O)₃]⁺ + H₂O

Loss of Ignition  content :

Weight accurately 1 g of sample into a previously weigher1 platinum crucible., then at a gradually increasing temperature. Finally ignite at 1000°C for one hour. Cool in a desiccator containing preferably magnesium perchlorate and weigh. Repeat the heating till constant weight.

Loss on Ignition (LOI) – Determining the Total Volatile Content

Determine % loss on ignition by dividing “initial loss” by initial weight of sample and multiply by 100

Objectives:

1. Red mud is a solid waste residue of the digestion of bauxite ores with caustic soda for alumina production. Its disposal remains a worldwide issue in terms of environmental concerns.
2. One of the economic ways is using red mud in cement production, which is also an efficient method for large-scale recycling of red mud.
3. Find out the easiest, fastest and most reliable methods to find the composition of Red mud. As on date we are trying to establish Red mud Chemical composition by classical methods. In the future we will introduce XRF technics to established to find chemical composition of Red Mud..
4. Enrich our scope of work and open the door to some recycling based industries.