Comparison of mesh grinding and chemical washing techniques f or the beneficiation of kaolins used in the white cement industry

Kaolins are hydro aluminum silicates which are eithermonoclinic ortriclinic in structure in accordance to their origin. Sorne common applications of kaolins are paper products, paints, catalyzers, white cement, etc. In the present research four different kaolins which were extracted from Villa de Reyes, S.L.P.; Mexico, were beneficiated using two different techniques: mesh grinding and chemical washing, in order to meet the requirements for the white cement industry, i.e. the mass content of Fe2Ü3 should be less than two percent of the total kaolin mass. XRD, Optical Microscopy, Colorimetry analysis were performed on the four studied kaolins. Mesh grinding and chemical washing techniques showed a similar degree ofbeneficiation. XRD analysis showed six different phases that make up the four different kaolins and are: saponite, kaolinite, wollastonite, stellerite, crystoballite and quartz. Most of Fe2Ü3 was located in the kaolinite, wollastonite and crystoballite phases, as could be corroborated by the combination of XRD, Optical Microscopy and Colorimetry analysis.

The importance of cement in our society nowadays is evident, we find it everywhere we go, from the smallest town to the biggest cities. Cement is one of the most basic building materials and it is also used in many different ways that go from construction to artistic creations. History tells us that the use of cementitious materials goes back to around 3000 B.C., when egiptians used gypsum and calcium to build the pyramids. Many centuries later, in 1824 Joseph Aspdin invented portland cement by buming a finely divided mixture of calcium and clay, the sinterized product was finely milled and was called portland cement because ofthe similar quality this mixture had compared to the building rocks found in Portland, England ASTM defines portland cement as a hydraulic cement obtained by clinker milling, which essentially consists of hydraulic calcium silicates, and the addition of one or more forms of calcium sulfate.
One of the most important components in white cement production is kaolin. Kaolins are hydroaluminum-silicate of monoclinic or triclinic structure. Kaolins come from mineral alteration such as feldspar, granite, etc. during the tertiary and cretaceous environments 50 to 135 millions years ago.
Sorne of the major uses ofkaolins are: paper products, ceramic tiles, paints, cement industry,etc. The lack of chromophores elements in kaolins is an essential factor in the white cement production. Sorne of the most common chromophores compounds are: Mg02, Fe2Ü3, TiO2, etc. Toe most significant goal of this research was to remove Fe2Ü3 from kaolins to meet the white cement standard levels ofthis compound (Fe2Ü3 level should be less than 2% of the the total kaolin mass ), the way Fe2Ü3 was removed from kaolins was possible by using a physical and chemical technique. Sorne other techniques used in other research projects are mentioned next: selective flocculation processes in which an aqueous mixture containing the minerals to be beneficiated is prepared and sorne acids are added to this mixture in order to get good flocculation, a very common acid utilized in this process is oleic acid [11,12,13,14). Selective flocculation is used to separate a mixture of fmely divided minerals into its constituents. When mineral particles are bigger than 45µm (US mesh 325), components in the mixture can also be separated by other physical means like air or magnetic separation. When mineral particles are smaller than 45µm, the mineral separation is performed by the formation of an aqueous mixture containing the minerals to be beneficiated and adding chemicals which are responsible for the mineral separation.
A widely used process to separate phosphate or oxidized minerals with a silica matrix is froth flotation, it is conventional to use a fat acid collector and a salt promotor in this process. Mineral particles covered with the collector are separated from the matrix in the form of froth. Still another mineral beneficiation procedure, in particular for kaolins with titanium impurities is known as ultra flotation [17]. This process has been widely practiced over decades to improve the quality ofkaolins, ultra flotation has even been used in the beneficiation of other comercial value minerals such as casiterite (tin oxide), fluorite, and other non sulphide minerals.

Experimental Set up
The tests and analysis were performed in the ceramic laboratories of the

Mesh Grinding
Each set of kaolins was milled the ceramic ball mili for one hour, after they were milled, kaolins went through mesh grinding using mesh number US 325 and US 400. Afterwards the material was classified as follows in accordance to their size from biggest to finest: To solubilize Fe2Ü3 present in kaolins, chemical treatments were performed using HCI. Each kaolin was irnmersed in 20ml ofHCI and heated to 80ºC for 3 hours. After treatment with HCI and heat, kaolins were washed in water and were passed through filter paper, thus getting a clearer in color material. The chemical reaction between FezÜ3 and HCI is showed below: Toe remaining material in the filter paper are the washed clearer in color material.

X Ray Diffraction (XRD)
After mesh grinding, XRD was performed on the four different kaolins and their classification in size from biggest to finest. The XRD analysis was performed using the following characteristics for analysis shown on Table  I: Colorimetry A reflectance analysis was performed on the kaolins that had previously gone through mesh grinding, as well as the ones that went through chemical washing.

Optical Microscopy
Optical Microscopy analysis was performed on each of the kaolins that went through and were retained in meshes NM, R325,R 400 andM 400. In orderto be able to analyze kaolins in optical microscopy very thin samples of kaolins were gotten using a pressure of 2 metric tons during three minutes. Afterwards, the samples of material were mounted on bases made of a resin mixture. After cleaning up the bases in which the kaolins were embedded, optical analysis went on. Each kaolin sample went under the optical microscope at a IOOX zoom and proceeded to take five micrographs of each sample for its later analysis using an image analyzer software with a discriminating color function. Given the fact that FezO3 is red in color, appropriate accounts of its presence were measured.

Statistical Analysis
A complete statistical analysis was performed on the measurements made by the image analyzer software about the size of the Fe2Ü3 particles found in the kaolin samples. This statistical analysis was made with the purpose of observing the variation, homogeneity, and size tendency of the Fe2Ü3 particles present in the different studied kaolins during the different stages of mesh grinding.

3.lXRD
XRD analysis for the four different kaolins are shown below, including the major phases present in each one of the kaolins as well as their evolution through meshes.  0n Table II are shown the probable locations of Fe2Ü3 in the different phases that compose the studied kaolins.

Colorimetry
The reflectance results for the different kaolins are shown below.
Erero Marzo, 201 ó Under both treatrnents (mesh grinding and chemical washing) the studied kaolins show a similar beneficiation degree, that is, kaolins' reflectance tends to a clearer color.

Optical Microscopy
Below are a few ofthe most representative images of the optical microscopy analysis that kaolins underwent.

Statistical Analysis
Toe following graphs show Fei03 particle size evolution in the different studied kaolins. All the sizes shown are the average of the sizes measurements gotten by the five micrographs taken to each kaolin sample.

Scanning Electron Microscopy (SEM)
SEM analysis was performed on kaolins as they were taken out the ceramic ball mill with the goal of studying their superficial features and rnicrostructure. Sorne of the most representative images are shown below.

Elementary Mapping Analysis and EDAX
Elementary Mapping Analysis was carried out in a sample ofkaolin L 28NM, the images are shown below as well as an EDAX analysis practiced on the same sample.  Toe Elementary Mapping Analysis indicates that silica is the most abundant element in the kaolin sample, followed by oxygen and lastly iron. These images also show that iron and oxygen combine, thus forming Fe2Ü3 there's also the combination of silica and oxygen as ~xpected and confinned by the different kinds of analysis practiced on the studied kaolins.

Fei03 location in kaolin's phases
Toe probable location FeiO3 in the indicated kaolin phases on table II is obtained under the combined analysis ofXRD pattems as they go through the different mesh numbers, as well as the optical microscopy technique and its statistical analysis. It could be seen on the XRD pattems that certain kaolin phases diminish in intensity such as quartz, kaolinite and wollastonite when they advance in mesh number, after that, those same phases increase their intensity, ali of this with respect to the original intensity shown on each of the kaolins that still hadn't gone through any mesh, i.e NM. When analyzing optical micrographs, it could be seen thatwhen the mesh number got higher, kaolin 's color changes from dark red on NM kaolins, up to much clearer colors on M 400 kaolins. It can also be observed in these micrographs that Fe2Ü3 particle size diminishes when the mesh number rises, and also the particle's general distribution becomes more uniform. These particle size observations are also confmned through the Fe2Ü3 statistical analysis. With ali of these observations the probable location of Fe2Ü3 can be discemed.
These analyses indicate that FeiO3 particles are being retained in the greater size meshes and their size is becoming smaller as expected, thus obtaining a clearer color on kaolins. 0n XRD analysis it can be observed that kaolinite, quartz and wollastonite diminish in intensity as previously stated, as well as the kaolin's coloration becomes clearer, it is also observed at this point that the average particle size increases. Later, a clearer coloration is observed and that those same phases (k.aolinite, quartz, and wollastonite) now increase in intensity, and the Fe2Ü3 particle size diminishes. Lastly, in the last mesh number ( 400 ), it is seen in ali of the cases an even clearer coloration and the mentioned phases increase even more in intensity (kaolinite and quartz), and in sorne cases the intensity increases on an exponential manner (kaolinite) and the Fe2Ü3 particle size becomes smaller and more uniform. All of these observations give us the FezÜ3 probable location in the different kaolin phases.

Colorimetry
Maxwell's equations predict the existence of electromagnetic waves whose speed in vacuum is determined by: where c is the speed of light or the speed of electromagnetic waves, µo : space permeability, eo : permittivity constant. From Maxwell equations it can be concluded that all of these electromagnetic waves have the same nature and differ only in their frequency and wavelength. When a colorimetry analysis is performed, it is only limited to avery narrow region of the electromagnetic spectrum called visible region. Results gotten using colorimetry show a graph of reflectance versus wavelength, that is, reflectance versus color. This means that the major reflectance percentage indicates the most predominant color in the analyzed sample. In the present research similar results were obtained for kaolins that went through chemical treatrnent as well as for kaolins that went through mesh grinding, in both cases showing a similar beneficiation degree, i.e. the same wavelength range was predominant for both beneficiation techniques.

Statistical Analysis
Statistical analysis performed on the micrographs obtained by optical microscopy was done with the purpose of observing FezO3 particle size variations. This was possible by using an image software analyzer with a color discrimination function. Because of the fact that optical microscopy analysis allows the obtention of measurements of FezO3 particle size, these measurements are done on the basis of the different optical phenomena that the light beam focusing on the kaolin sample produces. Toe measurement of FezO3 particle size is a function of an area whose mathematical algorithm is offractal dimension [18].
Five micrographs of each kaolin sample that went through the different mesh numbers were taken to measure the average area of FezO3 particles. Toe FezO3 particle size graphs shows that the average area ofFezO3 particles rises and then diminishes as the mesh number in creases in two of the studied kaolins (L 28, L 20), and Erero Marzo, 201 ó in the other two kaolins (L 64, L 66) the average Fe2Ü3 particle size simply decreases as the mesh number increases. These results indicate us that Fe2Ü3 particles are being retained in US 325 mesh in two ofthe studied kaolins (L 28, L 30), and on kaolins L 64 and L 66 FezÜ3 particles are being retained in more less uniform quantities as these kaolins advance in mesh numbers.
In mesh US 400 the Fe2Ü3 particle size becomes smaller and more homogeneous in its distribution in three ofthe studiedkaolins (L 28, L 30, andL 64) and in L 66 a lesser degree of homogeneity in FezO3 particle size distribution is observed.

SEM, Elementary Mapping and EDX
SEM analysis show similar morphologies in all the studied kaolins. It is observed that FezO3 particles are outside of the kaolin structure as was expected because of the beneficiation results gotten by physical means. Particles' morphologies went from polygonal, polyhedral upto irregular in shape. Particles' distribution went from uniform up to totally random. Particles' polygonal shapes correspond to clays as reported in literature. Particle irregular shapes correspond to different oxides and silica combinations, sorne polygonal shapes belong to Fe2Ü3 found on the kaolin samples. Fe2O3 was identified on the SEM micrographs due to the contrast shown on the images, being much brighter than the rest ofthe other structures on the samples, as well as because of its morphology and its corresponding elementary mapping.
Discreet element mapping shows that the most abundant element is silica, followed by oxygen and lastly iron as mentioned before. By studying these elementary mappings, the following can be concluded: a) Iron combines with oxygen, not with silica which corroborates the optical microscopy analysis.
b) Silica combines with oxygen, which <loes not differs from what was found in XRD and optical microscopy analysis and this is particularly relevant because it means that the studied kaolins come from a recent geological age meaning that their SiO2 content is particularly high and that these kaolins are from the feldspar type. This situation should be taken into count ifthese kaolins are to be used in the white cement industry.
EDX analysis shows five different basic elements that are: silica, oxygen, chrome, aluminium, and iron, as well as showing gold and carbon that come from the sample preparation and sample fixing respectively. The most abundant elements were silica and oxygen. Chrome and iron are natural pollutants found in sorne clays.

Conclusions
a) XRD analysis show the same six most abundant phases on the four studied kaolins, and these are: saponite, kaolinite, wollastonite, stellerite, crystoballite and quartz.
b) Jntensity of phases in XRD analysis varied as the number of mesh changed c) Mesh grinding and chemical washing techniques applied on the studied kaolins show a similar behavior or tendency, that is, the wavelength of the maximum reflectan ce of the studied kaolins tends to be that of the white color, this means that under both treatments (mesh grinding and chemical washing) kaolins show a similar beneficiation degree.