A floation process is presented and the work carried out to find optimum conditions to minimise the colouring material in ground calcium carbonate. (GCC) is ...
Flotation of Fine Particles of Egyptian Ground Calcium Carbonate As Added Value of Filler in Some Industries Fatma H . Abd EL-Rahiem Central Metallurgical Research and Development Institute, Helwan, Cairo, Egypt Mahmoud A. Arafa Central Metallurgical Research and Development Institute, Helwan, Cairo, Egypt Mohsen M. Farahat Central Metallurgical Research and Development Institute, Helwan, Cairo, Egypt
Keywords: Egypt, Bahriya Oasis, Ghorabi iron ore, attrition, classification, magnetic separation.
ABSTRACT A floation process is presented and the work carried out to find optimum conditions to minimise the colouring material in ground calcium carbonate (GCC) is described. The feed material is the over flow product from a hydrocyclone , which results from using a Dever Attritioning machine with semiautogenous grinding for one hour a 1:1 ore / water ratio. By using sodium silicate as deprassant and oleic acid as collector with pH 9, the iron content ( as colouring material impurities ) is decreased from ~ 0.9 in the original sample to ~ 0.45 in the rougher concentrate. However , by cleaning the rougher concentrate, the iron content is decreased to 0.26% .
1. INTRODUCTION GCC and several minerals like kaolin, talc and titanium oxide are used for coating and filling process ranging from automobile dashboard plastic to ultrapremium paper. But nowadays the world has a trend to use GCC instead of other minerals which were used in these processes. There are several reasons why consumption of GCC has increased greatly. There are rich resources of GCC : the first reason is the rich resources and wide distribution of limestone. The second reason is the high degree of limstone whiteness that reaches a value over 90% . The third reason is the relative low cost . Moreover high efficiency of grinding equipment have been developed to make the production of GCC with various kinds of > 90% of less few microns superfine powder which is a high quality for coating and filler grades . Moreover, the energy consumed in production per ton of such products has decreased from 250 Kwh/t to 120-170 kwh / t(1). In U.S.A., The cheaper grades of calcium carbonate , where whiteness is not too important and fineness is limited to a top size of 40µm , are supplied by Canadian sources (2). The better grade for pigment and plastic uses have, however, up till now, been
supplied by imports amounting to several millions dollars / year. Calcium carbonate specification vary widely for the same application due to individual company processes bring slightly different. Many trade names are used, which further complicates any evolution process. The properties that help to make calcium carbonate important as a pigment are whiteness, fine particle size (- 10µm ), close particle size distribution, strong affinity for the binders used, good adhesion in highly loaded systems, and thus the ability to reduce shrinkage, and low cost in relation to the properties it contributes to the finished product (2). Flotation process is one of the beneficiation process where the other process failed to yield a reasonable concentrate. The other process are gravity and their magnetic properties are also very near to each other, so flotation and flocculation may be the chief processing techniques . The main difference in properties could be changed by changing the particle's surface properties with modifiers (3) . The problem of foaming with finely ground ores may be seen as a result at of the finess of the particles , their concentration in the pulp and their hydrophobicity. Fine particles , instead of silding down the bubbles walls like large particles, remains distributed over the bubbles surface and hinder coalescence. The concentration and ultrafine must also be large enough to form nearly complete bubble coatings. It is also necessary that particles be hydrophobic. It was concluded that foaming did not occur during flotation garnet alone, but sufficient ultra fine hydrophobic calcite or sheelite in the ore lead to extensive foams . The enhanced ability of ultra fine hydrophobic calcite to cause foaming is probably a result of the shape of ground calcite particles (4) . A large proportion of these particles, even in the micrometer size range, have parallel float surfaces as a result of the perfect rhombohedral cleavage of calcite(4) . This investigation aims to beneficiate the ultrafine ground calcium carbonate by flotation process. Parameters which affect the flotation process such
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as pH of the pulp , dose of sodium silicate (S.S.) as desprassent agent, conditioning time and dose of oleic acid as collector were studied .
2. Materials and Experimental procedures 2.1. Material
province, Egypt. The sample was crushed in ``5X6``Denver Jaw Crusher . The feed of top size is ~ one inch which is fed to the Attritioner followed by spiral classifier and the overflow was fed to the hydrocyclone 3//, the overflow of which was the feed to flotation tests. The chemical analysis of the bulk sample is shown in Table 1.
A sample of ~ 500kg was kindly provided from Zaweit Sultan locality, hard variety El Minya Table 1. Complete Chemical Analysis of the Sample Constitutents % CaO 44.750 CO2 39.210 Fe2O3 0.948 MgO 1.699 Al2O3 0.623 SiO2 9.300 SO3 0.969 Cl 0.064 ZrO2 0.027 K2O 0.037 P 0.085 Na2O 1.201 SrO 0.176 TiO2 0.089
Analytical grades of sodium hydroxide and sodium silicate are taken from BDH chemicals, UK were used. 2.2. Experimental Procedures All flotation tests were carrird out in the " Denver D12" flotation machaine with a 1.5 liter stain steel container where batches of 250 gm were conditioned with a predetermined dose of sodium silicate for 20 min. adjusting pH was maintained during the couse of process. Conditioned with a predetermined dose of oleic acid for another 20 min. at pulp density of 40 % solid/liquid ratio at 2500rpm. Flotation was commenced after reducing the motor speed to 2000 rpm and the aeration started at lower pulp density. The flotation continued until all the mineralozed froth is skimmed. The floated and unfloated fractions were dried and weighed for chemical analysis. However, the final concentrates at optimum condition were characterized by chemical ananlysis. The degree of whiteness was measured by Whiteness & color Meter.
3. Results and Discussion The results of the quantitative complete chemical analysis of the bulk sample by X-ray flouresence are shown in Table (1). It is obvious that the sample has CaO and CO2 approach that of the stoischiometric percentage in pure CaCO3. The main impurities in
the sample are SiO2 constitute ~ 9 and R2O3 constitute ~ 1.5 %. The source for colouring material is clayey bearing iron oxides. Therefore it is suggested that flotation process be applied to the hydrocyclone overflow product to minimize the impurities. The chemical analysis of this product is 0.948 % Fe, 43 whiteness degree and 7.5 µm D97 size analysis. Inorganic dispersants such as sodium silicate (S.S.), known commercially as water glass was used as desprassent agent and oleic acid as collector. There are many parameters which affect the flotation processes such as pH, dose of dispersing agent, dose of collector and conditioning time.
3.1. Effect of conditioning time of sodium silicate on the floatability The tests were carried out at a constant dose of oleic acid ( 1 kg/t) , pH ~9 and constant dose of sodium silicate ( 1 kg/t). It is clear from these results that the impurities are decreased by increasing the conditioning time from 5 min. to 20 min . The acid insoluble is decresed from ~ 9 in the original sample to 7.71% in the concentrate and iron content is decreased from 0.94 in the original sample to 0.41 % at conditioning time 20 min.
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On the other hand, the tail fraction is enriched by acid insoluble, it reached to 16% and the iron content is ~1.22%. The removal of these impurities raised the degree of whiteness from 43 in the original to ~65.5 in the concentrate after 20 min. conditioning time. This unnoticeable of the effect of the dispersing action of sodium silicate "S.S." may be due to the presence of divalent cations ( unaviodable ion in system), that reduce the action of "S.S." to low extent, because of the formation of calcium silicate (5). The prolonged peroid of conditioning improved the whiteness may be due to disliming of clayey material
from the surface of ground calcium carbonate. The presence of unavoided ions Ca2+ doesn't effect the flotation process, because the precipitated calcium oleate can be converted into adsorbed oleate as concludied by Finkelstein (6). It is stated that the reagent can be precipitated under the action of dissolved calcium ions, which doesn't mean that it will be ineffective as a collector, although it may well mean that its efficiency will be lowered . In line with deduction it has been observed that the results obtained ,when flourspar- quartz mixtures are floated with calcium oleate are very close to those obtained , when sodium oleate is used (7).
80
66 64
75
Distribution %
Fe reovery % Ca recovery% Whiteness A.I. recovery %
65
60 58 56
60
54 52
55
DEgree of Whiteness
62 70
50 50 48 45
46 4
6
8
10
12
14
16
18
20
22
time, min.
Figure 1. Effect of conditioning time of sodium silicate
3.2. Effect of conditioning time of oleic acid on flotation The tests were carried out at constant pH 9, 1kg/t sodium silicate"S.S.", conditioning time of sodium silicate 20 min. and 1kg/t of oleic acid. The conditioning time of collector changed from 5 min. up to 30 min. and flotation is commenced after that , Figure 2. It is clear that the weight percent of floated fraction is increased from 73 % up to 96 % by increasing the conditioning time from 5 min. to 30 min., respectivly. However , the content of impurities in the concentrate are more or less the same for acid insoluble, but the iron increased from 0.4 % to 0.55% by increasing the conditioning time . This increase in the weigh perecnt of floated fraction may be due attributed to increase of the temperature of the pulp , due to the long peroid of stirring and hence rearangment of collector of the ground
calcium carbonate surfaces and the liberated silica is depressed by " S.S." as the non- floated fraction . Its content of acid insoluble recovery increased from ~ 15% to 60% by increaing the conditioning time from 5 min. to 30 min., respectively. However, the increase in recovery of ground calcium carbonate by increasing the conditioning time of the collector , may be attributed to the increase in unavoidable ions released from the solid phase ,which act as activator to echoer the oleate to solid and hence, enhances its floatability , also this improvement of flotation by increasing the conditioning time may be due to the replacement of the depressant "S.S." by sodium oleate on the surface of the solid particles as encountered by Hernainz et al. (8) . As a result of increasing the recovery, the degree of whiteness was decreased, due to the increase of impurities in float fraction with the concentrate. Therefore, the optimum conditioning of collector may be selected as 30 min.
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110
90 Whiteness Fe recovery% Ca recovery % A.I. recovery %
100
80
70
80
70
60
Degree of Whiteness
Distribution %
90
60 50 50
40
40 0
5
10
15
20
25
30
35
Time,min.
Figure 2. Effect of conditioning time of oleic acid on flotation
collector was 30 min. and conditioning time for depressant was 20 min. The results of this parameter were shown in Figure 3.
3.3. Effect of sodium silicate dosage on the floatability
110
69
100
68
90
67
80
66 Fe recovery % Ca recovery % Whiteness Degree A.I. recovery %
70
65
60
64
50
63
40
Whiteness Degree
Distribution %
Where the other parameters were kept constant; collector dose 1 kg/ton , conditionimg time for
62 0
1
2
3
4
Dose Sodium silicate, kg / ton
Figure 3. Effect of sodium silicate dosage on the floatability of GCC
From these results, it is clear that the weight perecent of the floated fraction decreased by increasing "S.S." dose from ~ 96% up to 80 % at 1 kg/t to 3 kg/ t , respectively. This decrease may be due to increasing depression action of "S.S." and competed with the oleate adsorption on the surface as mentioned before as silicate anions. It increased in the medium compared with that constant concentration of oleate anions. Both acid insoluble and iron content in the concentrate decreased by increasing the dose of "S.S." from 7.3% to 5 % A.I. and from 0.6 % to 0.45
% Fe , respectively. This decrease in both impurities that caused colouring of the concentrate , may be attributed to the depressing of ferrogeneous clayey material liberated and unliberated, so that the acid insoluble recovery decreased due to the depressent of unliberated impurities from 60% to~ 40% A.I. The degree of whiteness increased from 43 to ~68 in the original and the final productsat 3kg/ t "S.S.",respectively.
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However, the decrease in the calcite recovery from 98% to ~ 86% by increasing the "S.S." dose may be attributed to the depressing action of " S.S." to calcite as well as ferrogeneous clayey impurities.This conclusion is similar to that reached by Heraniz et al (8) . The recovery of both calcite and iron content decreased sharply by increasing the "S.S."dose up to 2.5 kg/t, then reached more or less constant values by further increasing of " S.S."dose up to 4 kg / t. As 4 kg/ t dose may lead to higher inorganic electrolyte concentration and resulted in coagulation state,
where selectively decreased and hence increased the acid insoluble in floated concentrate. Therefore, the optimum dose of "S.S." could be taken between 2.5 kg/t and 3 kg/t, where minimum impurities and higher degree of whiteness, and reasonable calcite recovery. 3.4. Effect of pH levels on the floatability At the fixed optimum values of other parameters, the change in pH levels was studied. The results of change pH levels are shown in Figure 4.
100
70
90
Distribution %
80 66 70 64 60 62 50 Fe reccovery % Ca recovery % Whiteness A.I.recovery %
40
Degree of Whiteness %
68
60
30
58 6
7
8
9
10
11
12
pH
Figure 4. Effect of pH levels on the floatability of GCC
From which it is obvious from these results that the floatability is higher at pH 9, which decreased if it is lowered to pH 7 or it is raised to pH 11. The impurities contents are minimum, the weight percent is higher, the recovery is also higher, as well as the degree of whiteness is higher . The higher flotation at pH 9 may be due to the higher concentration of unavoidable Ca2+ & Mg2+ cations , which act as activator to anchor the collector to the negative surface of calcite and hence improved the floatability and selectivity of calcite. At pH 7, which is very close to ZPC of GCC, where the negative centers are minimum, so the readsorption of unavoidable cations is minimum and activation action is mininmum and lower floatability and selectivity. In experiments conducted at pH 11. Giesekke and Harris(9; 10) found that the relative proportions converted to calcium oleate and precipitated remained constant as the total amount of oleate increased. It was found that flourite adsorbed more, and calcite very much less, oleate than that was precipitated. Therefore, the optimum pH level is taken as pH 9.
3.5 . Effect of the oleic acid dosage on the floatability The change in the dose of oleic acid as collector was investigated at fixed optimum values of the other parameters.The results of the change in the dosage are shown in Figure 5. It is clear as expected that the increase of collector dose increases the floated fraction from 70 % up to ~ 92 % as weight percentage at 0.5 kg/t up to 3 kg / t, respectively . However, the assay of acid insolule and iron percent decreased at 1 kg/t, then increased gradually by increasing the dose of oleic acid and the degree of whiteness at 1 kg/t increased to ~ 68 from 43 of the original sample. On increasing the collector dose, the degree of whiteness decreases to 64 % at 2 kg/ t and dropped to 59 % at 3 kg/ t of oleic acid. Also the distribution and recovery of A.I. , Fe2+ and Ca2+ increased by increasing the collector dose. This may be due to increase of adsorption and calcium oleate formation on the solid surface and hence increased the floated fraction with some selectivity at 1 kg/t dose, then the selectivity failed due to the adsorption of collector on the impurities , which
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were flocculated and not dispersed. Finkelestein (6) found that the extent of abstraction by calcite is greater than that by fluorite and increases monotonically with the concentration of oleate in solution , showing no sign of levelling off or peaking within the limit of the experimental conditions – up to 35 monolayers. Gutierrez (11) stated that at all events, it is clear that precipitated calcium oleate can be converted into adsorbed oleate. An important corollary to this finding should be noted, that the
reagent can be precipitated under the action of dissolved calcium ions does not mean that it will be ineffective as a collector, although it may well mean that its efficiency will be lowered (7) .Therefore, the optimum dose of oleic acid may be taken as 1 kg/t, from economical point of view, since oleic acid is the operational cost determinig factor , where the recovery of calcite is high and the distribution of both impurities are ~ 40 % A.I. and ~ 60% of Fe.
100
70
90
Distribution %
80 66 70 64 Fe recovery % Ca recovery % Whiteness A.I.recovery %
60
50
62
Degree of Whiteness %
68
60
40
30
58 0
1
2
3
4
Oleic acid dose,kg/ton
Figure 5. Effect of of the oleic acid dosage on the floatability of GCC
3.6. Cleaning the rougher flotation concentrate The floated fraction ( the rougher concentrate) at the aforementioned parameters was cleaned by another
flotation test at the following parameters of oleic acid cocentration 0.25 kg/t and 0.5 kg/ t " S.S." as despressant of the impurities. The results of cleaning step compared are shown in Table 2.
Table 2. Chemical analysis of roughing and cleaning concentrates Roughing test Cleaning test Fe % 0.454 0.259 A.I. % 4.75 4.27 Whiteness 68.05 73.25
It is clear from that the iron content decreased to half its value of that in the roughing step and hence the degree of whiteness increased to ~ 73.
4. CONCLUSIONS To improve the degree of whiteness of GCC and minimize the colouring material, flotation process of beneficiation was adapted using sodium silicate "S.S." as depressant and oleic acid as collector. The different parameters that may affect the flotation were investigated as a single factor at a time. The optimum values of these parameters at fixed constant values of the others are: 1 kg / t oleic acid , 3 kg/ t depressant "S.S.", followed by 20 min.,30
min. conditioning time for the depressant and the collector at pH 9 respectively ; yeild a concentrate of minimum iron 0.45 % and acid insoluble 4.78 % content , and with reasonable grade of whiteness ~ 68 and after cleaning the iron content decreased to 0.26 % Fe , and the degree of whiteness increased to 73 . The high reagent consumption is due to the ultra fine particles , which has high surface area. This parameter is the main drawback of the flotation process. Inspite of interior quality of the product concerning the low grade of whiteness, it could be suitable as filler in the Egyptian coloured pulp for paper as well
The Journal of ORE DRESSING ® 2005
as in PVC products and its size distribution is suitable.
REFERENCES 1. The Industrial Minerals, Hand book II, Peter W. Harben In.; pp.37- 43, 1995 . 2. P.I.Prescott and R.J.Pruett, “ Ground Calcium Carbonate : Ore mineralogy, processing and market”, Min. Engineering , pp. 79- 84 , June 1996. 3. M.M. Farhat, " Processing Technology for Beneficiation and Grinding of Egyptian Ground Calcium Carbonate As Added Value for Filler in Some Industries", B.Sc., Minufiya university , Minufiya, Egypt , 2002. 4. P.T.Koh and L.T.Warren, " Flotation of an Ultrafine Scheelite Ore and the Effect Sheer – Flocculation", Int.J.Min.Process. 60 , pp. 163 168,2000 . 5. F.H.Abd El-Rahiem and M.A. Arafa, " Some Aspects on the Selective Dispersion of Fine Particles with Special Emphasis in Egyptian Ground Calcium Carboante”, The Journal of the South Africa Institute of Mining and Metallurgy,vol.4, pp. 218- 223, may 2004.
13 6. N.P.Finkelstein, " Review of Interactions in Flotation of Sparingly Soluble Calcium Minerals with Anionic Collector", Trans. Inst. Min. Metall. Sec.C: Mineral Process.Extr. Metall. pp.157 – 177, sept- December, 1989. 7. K.A.Seleem “ Beneficiation of some Egyptian Flourpar Ores for Different Industrial Uses” , B.Sc., Zagazig University, Zagazig, Egypt, 2000. 8. Hernainz Bermudez de Castro and M. Calero de Hoces," Influnce of Quebracho and Sodium Silicate on Flotation of Celestite and Calcite with Sodium Oleate" , Int. Miner. Process.J.37, pp.283290 , 1993. 9. Giesekke E. W. and Harries P.J.A., " Study of Selective Flotation of Florite from Calcite by Use of a Single Bubble- Stream Micro Flotation Cell" , Int. Confer. on Miner. Scie. Vol.1, pp. 269 – 277 , 1984 b . 10. Giesekke E. W. and Harries P.J.A.," The Influnce of Polyoxyethylated Nonylphenols on the Adsorption of NaOI on Calcite and Florite", JI S.Afr.Chem. Inst., Vol.37 , pp. 96 – 102 , 1984. 11. Gutierrez C., " The Presence of a Maximum in the Adsorption Isotherm of Oleate on Fluorite, Influnce of Calcium and Quebracho", Proceedings 7th Inter.Conf.on Surface Active Substances, Moscow, pp. 638 – 647, 1976.
