Upgrading Egyptian Kaolin by Conventional Flotation
Collected and prepared from different sources by Eng. Atef Helal
Kaolin deposits in Egypt occur in three main areas namely, Sinai, Red Sea and Aswan. In Sinai, kaolins occur in the Nubia succession at Mussaba Salama, El-Tih and Farsh El-Ghozlan with total reserves of about 100 million metric tones. In the Red Sea area kaolin deposits located at the Hafayir and Abou-Darag localities, near Suez. The Hafayir kaolin is 12.5 m thick and confined to the sandy clayey calcareous Miocene sediments. The Abou-Darag kaolin bed is 17.8 m thick in the Nubia sandstone. The Wadi Kalabsha kaolin, holding around 17 million metric tones, is situated at about 150 km of west of Aswan.
Mineralogical and geochemical investigations indicated that these deposits are composed of kaolinite with traces of quartz and of detrital origin. Iron and titanium occur as ilmenite and anatase in very fine grains as rhombs and net structure. No iron and titanium were found in the structure of the kaolinite. The study suggests the authigenic origin of ilmenite and rutile that are probably formed during the diagenesis of these deposits.
Examples of Kaolin Production from Sinai Deposits:
Sinai International for Mining Co.
Kaolin is produced from company's mines in Eltieh area through underground mines and open cast area, the production capacity is about 72000 tons/ year. Used for several industries like ceramics ,white ware refractory industry , paper productions, cement.
Particle Size :
From 0-15cm or as customer request.
Kaolin & White Clay:
The production of Kaolin from Sinai is up to 100,000 MTPA. It is considered as one of the best Kaolin deposits in the Middle East. The most important markets for Kaolin are refractory, cement, ceramic and paper industries. In addition to the considerable portion needed for local demands of Kaolin, that ore is export to Spain, Turkey and some Arabian countries, mainly for the white cement industry.
The Froth "Flotation System" as shown in above Figure includes many inter-related components as follows:
· Chemistry components as collectors, frothers, activators, depressants and pH.
· Equipment components as flotation cell design, agitation, air flow, cell bank configuration and cell bank control.
· Operation components as feed rate, mineralogy, particle size, pulp density and temperature.
Froth flotation is a good example of an engineering “system”, in that the various important parameters are highly inter-related, as shown in above Figure. It is therefore important to take all of these factors into account in froth flotation operations. Changes in the settings of one factor (such as feed rate) will automatically cause or demand changes in other parts of the system (such as flotation rate, particle size recovery, air flow, pulp density, etc.) As a result, it is difficult to study the effects of any single factor in isolation, and compensation effects within the system can keep process changes from producing the expected effects (Klimpel, 1995). This makes it difficult to develop predictive models for froth flotation, although work is being done to develop simple models that can predict the performance of the circuit from easily- measurable parameters such as solids recovery and tailings solid content (Rao et al., 1995).
Flotation Cell Technolgy
Conventional flotation cells consist of a tank with an agitator designed to disperse air into the slurry, as shown schematically in Figure below. These are relatively simple machines, with ample opportunity for particles to be carried into the froth along with the water making up the bubble films (entrainment), or for hydrophobic particles to break free from the froth and be removed along with the hydrophilic particles. It is therefore common for conventional flotation cells to be assembled in a multi-stage circuit, with “rougher”, “cleaner”, and “scavenger” cells, which can be arranged in configurations such as the one shown in Figure below.
Flotation columns provide a means for improving the effectiveness of froth flotation (Eberts, 1986). A column essentially performs as if it were a multistage flotation circuit arranged vertically (Degner and Sabey, 1985), with slurry flowing downward while the air bubbles travel upward, producing a countercurrent flow. The first flotation machine design to use a countercurrent flow of slurry and air was developed by Town and Flynn in 1919. It was not until the work of Boutin and Tremblay in the early 1960’s that a new generation of countercurrent columns was developed that ultimately became industrially successful (Rubinstein, 1995).
A typical flotation column is shown in Figure below. The basic principle of column flotation is the use of countercurrent flow of air bubbles and solid particles. This is achieved by injecting air at the base of the column, and feed near the midpoint. The particles then sink through a rising swarm of air bubbles.
Countercurrent flow is accentuated in most columns by the addition of washwater at the top of the column, which forces all of the water which entered with the feed downward, to the tailings outlet. This flow pattern is in direct contrast to that found in conventional cells, where both the air and the solid particles are driven in the same direction. The result is that columns provide improved hydrodynamic conditions for flotation, and thus produce a cleaner product while maintaining high recovery and low power consumption. The performance differences between columns and conventional cells may best be described in terms of the following factors: collection zone size, particle/bubble contact efficiency, and fines entrainment (Kawatra and Eisele, 1987).
Columns exhibit higher particle/bubble contact efficiency than conventional machines, due to the particles colliding with the bubbles head-on. As a result, the energy intensity needed to promote contact is less, and so power consumption is reduced.
A second beneficial effect in certain types of flotation columns is the reduction of bubble diameter (Yoon and Luttrell, 1986). As bubble diameter is reduced, the flotation rate of both the coarser and finer particles is improved. Coarse particles can attach to more than one bubble if the bubbles are small, and therefore the chances of the particle being torn loose and sinking again is reduced. For fine particles, the probability of collision with the bubble is improved if the bubble is small, as then the hydrodynamic forces tending to sweep the particle away from a collision are reduced. The reduction of bubble diameter has the added benefit of increasing the available bubble surface area for the same amount of injected air. It is therefore desirable to produce bubbles as fine as possible.
Carrier flotation is considered as unconventional flocculation and flotation techniques compared by the conventional flocculation and flotation techniques. Carrier flotation is known as ultra-fine flotation. In flotation, extremely fine paerticles that are difficult to attach to air bubbles adhere to coarse carriers, and float with them . This technique was originally used to remove anatase (TiO2) from kaolin using coarse calcite as a carrier, and has been developed into autogenous carrier flotation, where fine particles are carried on coarse particles of the same ore. This is a great advance in flotation tehnology. Since it has advantage over conventional flotation techniques in speration result and reagent consumption, it has broad prospects for industrial use.
The concept of using particles of high floatability (usually coarser size) as a carrier to carry particles of less floatability (usually ultrafine in size) has been tested on a variety of ores and coals. In this technique, the particles to be floated coat the carrier material and the coated particles are then floated. The first commercial use of this process was in the purification of kaolin at Minerals and Chemicals Philipp's plant-Georgia USA (Seeton, 1961). This process was originally developed to remove titaniferrous impurities from kaolin clay using 60 μm limestone particles as the carrier mineral.
Conventional Froth Flotation of Ultrafine Egyptian Kaolin Ore
CMRDI,Cairo; Egypt – Abdel Khalek & Arafa
This paper aims to upgrade an Egyptian kaolin preconcentrate sample by reducing its anatase impurities (TiO2) through the conventional froth flotation technique. The flotation tests were carried out on a kaolin sample of about 80 wt.% below 1.59μm. Sodium silicate and oleic acid - (An oily liquid occurring in animal and vegetable oils and used in making soap; chemical formula: C18H34O2) – were used as a depressant and a collector respectively. The different operating (chemical and mechanical) parameters were studied.
The results showed that the mechanical factors just as the speed and time of conditioning played the vital role in determining the efficiency of the flotation process for such ultrafine particles. For example, changing the dosage of collector or the pH have no influence on the grade of the produced concentrates if the flotation process was performed at low conditioning speed or at small conditioning time. However, the reverse trend was noticed when the flotation tests were carried out at high conditioning speed and after long conditioning time.
At the optimum operating conditions a concentrate of about 0.63% TiO2 with a%age removal of 77.5 was obtained from a feed containing 1.52% TiO2.
Kaolin is used in a multiplicity of industries because of its unique physical and chemical properties. Shape, particle size, color, softness,and nonabrasiveness are physical properties that are especially important. Anatase (TiO2) represents one of the major discoloring impurities in kaolin that reduces its brightness. Kaolin with brightness greater than 90 is produced by incorporating either magnetic separation, froth flotation, and/or selective flocculation. Each of the individual processes has its own merit and limitations and therefore, a combination of two or three of these techniques is common in the industry to make the best use of mined kaolin. At least one plant is making use of a froth flotation process for kaolin that does not involve the use of carrier mineral. Although carrier flotation can work well with a given crude kaolin, it may not respond with another deposit.
The following is added to the paper by Atef Helal
The first commercial use of carrier flotation process was originally developed at Georgia USA (Seeton 1961) to upgrade kaolin. Titaniferrous impurities were removed with 60-μm limestone particles as the carrier mineral. In addition, it is not always necessary to externally add particles because the coarser particles originally present may act as a carrier. When coarse particles that may act as carrier particles are inherently present, the addition of carrier mineral may or may not improve the process.
A comparison of conventional froth flotation, carrier flotation, and column flotation techniques at their optimum conditions for separation of titaniferrous impurities from Egyptian kaolin ores is presented in above Table. Flotation was performed using oleic acid as a collector for the colored impurities, and sodium silicate as a depressant for kaolin. These results showed that conventional froth flotation gave a kaolin concentrate (84.75 wt %) containing 0.68% TiO2 and 0.56% Fe2O3 with a degree of whiteness 78 from a feed assaying 1.52% TiO2, 0.37% Fe2O3, and whiteness 56. Carrier flotation provided a concentrate of similar grade but of a higher degree of whiteness (90). Column flotation produced a concentrate with a lower TiO2 content (0.38%), Fe2O3% decreased to 0.49% and the whiteness reached 91.5. its higher value.
End of the addition – Atef Helal
The Egyptian kaolin is hard and massive. Attrition scrubbing followed by screening and hydrocycloning separation processes can remove the majority of its associated free quartz and some iron oxides. However, these preconcentrates still have high TiO2 content and consequently are not used in paper coating or fine ceramics. This paper aims to upgrade such preconcentrates by reducing there anatase mineral (TiO2) through the conventional froth flotation technique. The different operating parameters were studied.
Experimental And Materials:
A representative preconcentrate kaolin sample of El-Tih locality, Sinai , was used as a flotation feed , its chemical analysis is : Al2O3 (36.40%) and SiO2 (47.58%). This means that the concentration of the kaolinite mineral is high ( abt. 94.64%) while that of free silica is low (abt.4.69%). The sample contains also a relatively higher content of TiO2 (1.52%) than that required of paper coating and fine ceramics. The loss on ignition was 13.58%. On the other hand , the size analysis showed the very fine grade size distribution of the sample , where it contained about 79.92 wt.% below 1.95 μm. The fraction above 44.2 μm represented only 5.23 wt.% of the sample.
Laboratory grade oleic acid and technical grade sodium silicate were used as a collector and a dispersing agent respectively. Analytical grade sulphuric acid and sodium carbonate were used as pH regulators.
Flotation tests were carried out in a "Denver D12" flotation cell with 1.5 lit. stainless steel container. In each test, about 300 gm of kaolin sample was conditioned at pH 10.5 (if otherwise not mentioned), adjusted by sodium carbonate, in presence of one kg/ton sodium silicate at impeller speed 2500 rpm (if otherwise not mentioned). The collector was added as ammonium salt of oleic acid by its agitating with ammonium hydroxide (4:1 wt/wt ratio) in 100 ml water volume for 15 min. This prepared solution was added to the pulp and conditioned
for a certain period at solid/liquid ratio of 50%, and then flotation was carried out at an impeller speed of 2000 rpm (if otherwise not mentioned). The floated an non-floated fractions were dried, weighed and amalyzed for TiO2. The whiteness degree was measured by DR LANGE whiteness tester.
Results and Discussions :
In the evaluation of the flotation results the following definitions are used : 1- Retention Ratio (R-R) for TiO2 . This value varies from 1.0 for no separation to zero for complete separation. 2- Coefficient of separation (C.S). 3- Amount of TiO2 removed (Wang and Somasundarm,1980).
Effect of conditioning speed of kaolin with reagents
Figure 1 depicts the effect of changing the speed of conditioning of kaolin with reagents. It is clear that the conditioning speed plays a vital role in determining the efficiency of the flotation process. The TiO2 content of the concentrates was gradually decreased from about 1.52% in the feed to 0.93% with a % removal of about 61 and a R.R. of 0.62 by merely increasing the conditioning speed to 2500 rpm. The C.S was significantly improved about 10 times , Fig 1.
Wang and Somasundarm (1980) discussed the reasons of improving the separation at high conditioning speed. They attributed such improvement to 3 reasons: improving aggregation , increasing the rate of oleate adsorption , and increasing the temperature of the pulp due to the high speed.
However, the level of the conditioning speed used in these flotation tests (1500-2500) are relatively high that adsorption can not be expected to be controlled by diffusion to any measurable extent. The beneficial effect of increased conditioning speed on flotation can not therefore be accounted for in the present case by a diffusion enhanced process.
Meanwhile, the measurements of temperature during the flotation tests indicated that temperature was slightly increased by about 4oC (from 20oC at the beginning of the conditioning process to 24oC at the end of the experiment). Such small increase in temperature as a result of increasing the conditioning speed suggests that it can only produce minor improvement in flotation.
From the above discussions , it could be suggested that the improvement of the kaolin grade at higher conditioning speed may be related to the aggregation between various particles of anatase,i.e. formation of anatase-anatase aggregates.
Effect of conditioning time of kaolin with oleic acid
Figure 2 depicts the effect of changing the conditioning time of kaolin with oleic acid. It is clear that such time plays a very important role in determining the grade of obtained concentrate. A significant drop in the TiO2 content from 1.35 to 0.86% with a % removal of about 61.5 was noticed by increasing the conditioning time from 5 to 20 min. At the latter conditions the R.R. and C.S. values reached to 0.56 and 0.20 respectively whereas the whiteness improved to about 69. Further increase in the conditioning time caused a slight improvement in the grade and whiteness degree. However the lowest TiO2 content (0.75%) with the highest % removal ( approx. 67.5%) and whiteness (79) was obtained at the longest conditioning time of 35 min.
At such conditions the R.R and C.S. reached their best values (0.49 and 0.32 respectively), Figure 2.
Trahar and Warren (1967) mentioned that ultrafine paticles float more slowly than those of intermediate sizes. Moreover, each of the ultrafine fractions may be further subdivided into slow and fast floating components. The decrease in the rate of flotation of fast component appears to be the main reason for the slow overall flotation rate of the ultrafines. In the mean time, Woodburn et al (1971) argued that the rate of flotation was equal to the product of three factors: the rate of collision between particles and bubbles; the probability of adhesion and the probability that the adhering particles would not be detached subsequently.
Thus the flotation rate will depend, among other things, on the particle size.The lower the particle size the slower the flotation rate. The effect of particle size will, then, be a predominant phenomenon in the flotation of kaolin since the flotation feed contains about 79.92 wt% below 1.95 μm. This means that the probability of collision between particles and bubbles will be minimum and consequently the flotation process may become a function of time.
Thus, at longer conditioning time (e.g. 35 min) the probability of formation of anatase –anatase aggregates may increase which in turn will improve both their collision rate with air bubbles and the probability of adhesion. This may explain the significant improvement in the efficiency of the flotation of such fine particles at longer conditioning time, For this reason, the longest conditioning time (35 min.) of pulp with oleic acid will be used in the next tests.
Figure 3 depicts the effect of changing flotation speed on the performance of the flotation process. The results indicate that the flotation speed plays, also, an important role in determining the efficiency of the flotation process. Increasing the flotation speed within the range 1200-2000 rpm caused a successive improvement in the grade of the obtained concentrates where the TiO2 content was decreased from about 1.38% at the lowest flotation speed (1200 rpm) to 0.79% only at 2000 rpm. At this range of speed the R.R. and C.S. values reached their best values (0.52 and 0.30 respectively) whereas the % TiO2 removed attained its maximum value (66.45%). The degree of whiteness showed a progressive improvement, from 62 to 72, with increasing the flotation speed from 1200 to 2000 rpm . On the contrary, the results indicated that when the flotation speed increased over 2000 rpm (e.g. 2250 rpm) a significant reduction in the flotation efficiency was noticed. At the latter case, the TiO2 content was increased to about 0.95% with a significant drop in its % removal to 53.75. The whiteness and C.S. grcreased to 68 and 0.18 respectively while the R.R. increased to about 0.63, Figure 3.
The flotation rate depends, as mentioned before, upon three factors : collision, adhesion and detachement. These factors can be affected by the flotation speed. An increase in the impeller speed, whatever the size of the bubble or the particle, results in a greater detachment force between a bubble and a particle caught in the turbulent field around the impeller. Elswhere in the stirred cell where the turbulence levels are less than in the impeller region , it is likely that an increase in the impeller speed will lead to an increase in the rate of capture of the particles by bubbles. The overall flotation rate is thus a balance between the competing mechanisms.
Thus with increasing the flotation speed from 1200 to 2000 rpm the rate of collision between particles and bubbles, and their subsequent adhesion would increase. This may lead to each significant improvement in the grade of kaolin and its whiteness. However, at the highest impeller speed (2250 rpm) the rate of detachment would be dreater than that of collision and adhesion leading to poor flotation of anatase as shown in Figure 3.
Effect of Changing pH
The effect of changing pH medium was studied over a relatively wide range (2-10.5) at different speeds of conditioningand flotation, the results of which are shown in Figure 4. The results showed, again, the importance of optimizing the chemical factors after the mechanical ones. For example, changing the pH of the medium over the all pH region (2-10.5) at lower conditioning and flotation speeds (2000 amd 1500 rpm respectively) did not show any variation in the TiO2 content (1.36 %) with a very small removal percent (abt. 10 to 16%). This made the C.S. very poor (abt. 0.02 to 0.09) allover the pH region (results are not shown).
However, repeating the same flotation tests at the optimum speeds of conditioning (2500 rpm) and flotation (2000 rpm) showed the dependence of the efficiency of the process on the pH of the medium, Figure 4. The TiO2 content was not affected within pH 3-6 afterwhich the anatase impurities started to gradually decrease with increasing pH till 10.5 where the lowest TiO2 (0.69%) with a %age removal of about 64.11 was obtained. At the latter case , the R.R. reached its minimum value (0.45) whereas the C.S. was significantly improved to 0.34. Also, the whiteness improved to 79, Figure 4.
The variation of pH affects the sign and magnitude of the charge of kaolinite and anatase. It is known that the face surface charge on the kaolinite remains negative over a wide pH range (3-11) (Grim,1953).Abdel Rahman (1996) has found that pure kaolinite and anatase remain negative at pH's above their point of zero charge PZC (3.23 and 3.65 respectively) and both minerals are completely dispersed in the pH range 6-10.
It is clear that the effect of pH can be understood by considering the degree of dispersion, distribution of collector species and surface charge charahteristics of the anatase. When the flotation tests were performed at the optimum speeds of conditioning and flotation (Figure 4), the results showed a dependence upon the pH. In the latter case, the TiO2 content was not changed within the acidic pH region (2-6) probably due to the lack of good dispersion for the pulp. Meanwhile, the flotation process was significantly improved within the alkaline pH where successive reduction in the TiO2 content was noticed with raising the pH till 10.5 at which the lowest anatase content was obtained. Also, the R.R. reached its minimum value (0.45) while C.S improved to 0.35. Such improvement at pH 10.5 could be related to the complete dispersion of the kaolin pulp which, in turn, may lead to better distribution for oleate species on the anatase particles.
Effect of oleic acid dosage
The effect of changing the dosage of oleic acid was studied at two different levels of conditions. First at lower conditioning speed (2000 rpm) and time (15 min.) and the second series at the optimum conditioning speed (2500 rpm) and time (35 min), the results of the latter are shown in Figure 5.
The results indicate that the dosage of oleic acid had a major effect if conditioning was performed at lower speed (2000 rpm) and for small time (15 min.), regardless of the dosage added. The TiO2 content was not changed with increasing the dosage of oleic acid up to 2.0 kg/ton above which a minor change to 1.21 % was noticed at a dosage of 3.5 kg/ton. At such conditions, the R.R. was reduced to 0.80 while the C.S was only 0.11. It seems that the flotation process is not selective at a relatively weak conditioning level. This could be related to the weak dispersion of the kaolin pulp which may adversely affect the adsorption of oleic acid on the anatase particles.
If the degree of dispersion plays such vital role in determining the selectivity of flotation , the results should be improved while repeating the experiments at the optimum conditioning speed (2500 rpm) and time (35 min). This has been confirmed in the results shown in Figure 5. A large drop in the TiO2 content (from 1.52% in the feed to 0.75%) was noticed while adding only 1.0 kg/ton of oleic acid. At the latter case, about 60% of TiO2 was removed while the R.R. was reduced to 0.49 with a significant improvement for the C.S. value (0.32). Such reduction in TiO2 content was not obtained at the weak level of conditioning even after adding 3.5 kg/ton of oleic acid. The results in Figure 5 also showed that further addition of oleic acid to 2.0 kg/ton decreased again the TiO2 % to 0.61 with a %age removal of about 79. At such conditions, the whiteness was improved to 81 while the R.R. and C.S. values were nearly constant. Thr results showed that a dosage of 1 kg/ton of oleic acid was enough to be used in the next experiments. It seems that performing the flotation process at the optimum conditioning speed and for enough time can greatly help in improving the selectivity. Such high conditioning speed can facilitate the aggregation process of anatase-anatase aggregates which at enough conditioning time enhance their collision and subsequent adhesion with the oleate species thereby improving their flotation as shown in Figure 5.
Effect of dosage of sodium silicate
Figure 6 shows the effect of changing the dosage of sodium silicate on the efficiency of the flotation process. The results indicated that the TiO2 content was decreased from 0.92% to 0.75% with increasing the dosage of sodium silicate from 0.5 to 1.0 kg/ton. At such dosage about 60% of TiO2 was removed which improved the whiteness to about 78. Also, the C.S. and R.R. were at their optimum values (0.32 and 0.49 respectively). The results also showed that the higher dosage can deteriorate the selectivity of the flotation process. It seems that 1.0 kg/ton of sodium silicate is the optimum dosage.
The importance of the flotation performance with increasing the dosage of sodium silicate up to 1.0 kg/ton could be related to its dual role in the kaolin pulp, i.e. as a depressant for the kaolinite mineral and at the same time as a good dispersing agent for the kaolin slurry. So, with increasing the amount of sodium silicate up to its optimum dosage the silicate ions can adsorb, oftenly physically at the positive edges of the kaolinite mineral leading to a corresponding reduction in the possibility edge-to-face flocculation process and consequently enhances the dispersion of kaolin. This makes the flotation process more selective by floating only the anatase impurities. Such adsorption is expected to increase at the higher dosage of sodium silicate leading to an excessive increase in the negative charge of both the kaolinite and the anatase minerals thereby hindering the adsorption of the collector (oleate ions) on the surface of the latter and consequently deteriorates the selectivity of the flotation process as shown in Figure 6.
Effect of changing the conditioning temperature
Figure 7 shows the effect of changing the conditioning temperature. These results showed a significant reduction in the TiO2 content from 0.75% to 0.63% with a considerable improvement in the whiteness of the obtained concentrates from 74 to 79, with raising the conditioning temperature from 20 to 40oC. This was, also, reflected in an improvement in the R.R and C.S. values as well as the %age TiO2 removed. It is clear that the increase in conditioning temperature is beneficial for improving the grade of kaolin.
The variation in temperature can affect the system through increasing the collector adsorption on anatase, solubility of collector, solubility of minerals and viscosity of the liquid medium. The adsorption ………
Meanwhile, flotation can be expected to improve to some extent with increase in conditioning temperature owing to the decrease in viscosity of water also . Based on the above discussions the increased collection adsorption on anatase mineral is proposed to be mainly responsible for such improvement in flotation with increasing the conditioning temperature.
1. In the froth flotation of Kaolin preconcentrate sample (97.92 wt.% below 1.95 μm) the mechanical parameters (such as conditioning speed and time and flotation speed) should be optimized before that of the chemical ones. Otherwise no change in the performance of flotation can be noticed. This because kaolin needs a high conditioning speed, a long conditioning time and a suitable flotation speed. Meanwhile, the degree of dispersion of the pulp plays a vital role in determining the selectivity of anatase flotation. Such degree of dispersion can greatly improved by performing the tests in an alkaline medium (pH 10.5) with the optimum dosage of sodium silicate. The selectivity of the process can be further improved at conditioning temperature of about 40oC.
2. At the optimum conditions for froth flotation of anatase impurities from the kaolin, a concentrate containing only 0.63% TiO2 with a whiteness of 79 could be obtained from a feed assaying 1.52% TiO2 with a whiteness of 56.
Note : The rest of
present paper discusses " Charachteristics of
tailings and tailing management", and not available electronically. l Atef Helal
l Atef Helal