NATIONAL TECHNICAL UNIVERSITY OF ATHENS
SCHOOL OF MINING AND METALLURGICAL ENGINEERING
Section of Metallurgy and Materials Technology
Laboratory of Metallurgy
 
HYDROMETALLURGY UNIT
 
e. Waste Management
 
 

Red Mud Addition in the Raw Meal for the Production  of Portland Cement Clinker
 

Red mud is produced during the digestion of bauxite with sodium hydroxide. It generally exits the process stream as a highly alkaline slurry (pH: 10–12.5) with 15–30% solids and it is pumped away for appropriate disposal. It is a complex material whose chemical and mineralogical composition varies widely, depending upon the source of bauxite and the technological process parameters. It contains six major constituents, namely, Fe2O3, A12O3, SiO2, TiO2, Na2O and CaO and small quantities of numerous minor/trace elements (as oxides) such as V, Ga, Cr, P, Mn, Cu, Cd, Ni, Zn, Pb, Mg, Zr, Hf, Nb, U, Th, K, Ba, Sr, rare earths, etc. Every red mud is composed of as many as 14-21 mineral phases. Its brick red color is due to the iron oxides.
 

The addition of red mud residue by 3.5% in the raw meal did not affect either the sintering or the hydration process during Portland cement production. More specifically, the cement produced presented the following characteristics:

 

Ø The alite phase occurred as small well-formed crystals.

Ø The belite crystals were few and evenly distributed in relation to alite, indicating that the clinkering reaction had proceeded extensively in the direction of alite and that the raw mix was homogenous.

Ø The liquid phase occurred as fine crystals, uniformly distributed.

Ø The values for setting times, water content for standard consistency and expansion were similar to those obtained with the reference ordinary Portland cement sample.

Ø The compressive strengths were at least as high as those of the reference sample during hydration.

 

Jarosite-Alunite Addition in the Raw Meal for the Production  of Portland Cement Clinker
 

The jarosite–alunite group of isostructural minerals is described by the general formula M(FexAlyCrz)(SO4)2(OH)6, in which “M” may be Na+, K+, NH4+ or H3O+ and x+y+z=3. For x=3, the formula represents jarosite, whereas for y=3 it represents alunite.
 

The precipitate is produced at some stage of a new hydrometallurgical process [1-3], which was developed at the laboratory of Metallurgy of the National Technical University of Athens in order to extract, economically and efficiently, nickel and cobalt from low-grade nickel oxide ores. According to the above process, the ores are subjected to heap leaching with dilute sulfuric acid and the resulting leach liquors, containing nickel, cobalt, iron, aluminum and chromium, are then treated, at atmospheric pressure and at temperatures not exceeding 100oC, to remove iron, aluminum and chromium before nickel and cobalt recovery. The jarosite-alunite precipitation is a hydrolysis reaction and can be represented by the equation:


 

2[Fe2(SO4)3](aq) + [Al2(SO4)3](aq) +M2SO4 +12H2O ®

 2M[Fe2Al(SO4)2(OH)6](s) +6H2SO4

 

The addition of jarosite-alunite precipitate by 1% in the raw meal did not affect either the sintering or the hydration process during Portland cement production. More specifically, the sample with jarosite-alunite presented the following characteristics:

  • The alite phase occurred as small well-formed crystals.

  • The belite crystals were few and evenly distributed in relation to alite, indicating that the clinkering reaction had proceeded extensively in the direction of alite and that the raw mix was homogenous.

  • The liquid phase occurred as fine crystals, uniformly distributed.

  • The values for setting times, water content for standard consistency and expansion were similar to those obtained with the reference ordinary Portland cement sample and within the standards limits.

  • The compressive strengths were at least as high as those of the reference sample during hydration.

 
 
 

Use of Gypsum/Brucite Mixed Precipitate Instead of Gypsum in Portland Cement

 

The Laboratory of Metallurgy of the National Technical University of Athens has developed a new integrated hydrometallurgical method, suitable to treat, efficiently and economically, low-grade nickel oxide (laterite) ores. It involves heap leaching of the ore by dilute sulphuric acid at ambient temperature, purification of the leach liquor and recovery of nickel and cobalt. A typical composition of the pregnant solution produced from heap leaching of Greek low-grade nickeliferous laterites with sulphuric acid is: Ni2+ = 5.0 mg/l, Co2+ =0.6 g/dm3, Fe3+=22.0 mg/l, Al3+=6.0 mg/l, Cr3+=1.0 mg/l, and Mg2+=8 mg/l. Iron, aluminium and chromium are removed from the leach liquor by hydrolytic precipitation after which nickel and cobalt are recovered from the purified liquor by solvent extraction. The resulting final solution contains only magnesium, which must also be removed in order to:

  • allow recycling of the purified barren solution to the heap leaching stage

  • obtain a magnesium–containing precipitate with commercial value and, thus, improve the cost – effectiveness of the whole process
     

The precipitation of gypsum/brucite was carried out in 5 dm-3, five-necked, round bottomed split reactors at 25 oC, using 1.1 times the stoichiometric quantity of Ca(OH)2 required to precipitate all of the magnesium. Calcium hydroxide was added in solid form. The reaction time was approximately two hours. Equilibrium pH was around 10.

 

The precipitate that was obtained composed of well-crystallized gypsum (CaSO4.2H2O) and aggregates of fine crystallites of brucite (Mg(OH)2). The mixture exhibited excellent filtering properties.

The addition of the above precipitate, instead of natural gypsum, by 4.1%, 5.2% and 6.3% in clinker did not adversely affect the properties of the cement mixtures produced.

More specifically, all the cement mixtures tested presented similar characteristics, such as grindability, compressive strength and expansion, with the reference sample.

Regarding the setting times of the mixtures, the replacement of natural gypsum with the gypsum/brucite precipitate accelerated the cement setting, within acceptable limits, a fact which was attributed to the lower solubility of SO3 contained in the precipitate than that in natural gypsum.: