- Email: [email protected]
Influence of dispersants on the solubility of calcined kaolin
Applied Clay Science 13 Ž1998. 137–147
Influence of dispersants on the solubility of calcined kaolin Jun Yuan ) , William L. Garforth, Robert J. Pruett ECC International, P.O. Box 471, SandersÕille, GA 31082, USA Received 4 September 1997; revised 19 March 1998; accepted 19 March 1998
Abstract The solubility of calcined kaolin relative to different aqueous solution chemistries is important to its industrial application. When used in paper-making, a calcined kaolin product is required to have good flow characteristics and viscosity stability, both of which are largely controlled by the type and level of soluble salts in the kaolin suspension. In this study, five common dispersing agents including tetrasodium pyrophosphate, sodium hexametaphosphate, sodium polyacrylate, sodium silicate, and sodium carbonate and one flocculant aluminum sulfate were investigated in terms of their relationship with the concentration of dissolved ions ŽAl 3q, Si 4q, Mg 2q, and Ca2q . in calcined kaolin suspensions. Controlled variables included dispersant dosage, suspension pH, and aging. Results from the study showed that the calcined clays dispersed with tetrasodiumpyrophosphate and sodium hexametaphosphate yielded higher levels of soluble aluminum than those with sodium polyacrylate, sodium carbonate, and sodium silicate at equivalent chemical dosage and pH condition. The variations of other soluble salts ŽSi 4q, Mg 2q, and Ca2q . were not as significant. Mg 2q, and Ca2q ion concentrations were generally low compared to aluminum and silicon. Higher dosage of any dispersant generally raised the concentration of soluble aluminum. Higher pH favored soluble aluminum but lowered soluble silicon. Addition of aluminum sulfate in calcined clay suspension did not raise the soluble aluminum level. The effect of aging on soluble salts concentration was not significant. Overall, the solubility of calcined kaolin seemed largely related to the pH condition in addition to the type and level of dispersantrflocculant. q 1998 Elsevier Science B.V. All rights reserved. Keywords: calcination; kaolin; solubility; dispersant
)
Corresponding author. Tel.: q1-912-553-5747; fax: q1-912-553-5797; e-mail: [email protected]. 0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 1 6 - 7
138
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
1. Introduction Calcined kaolin finds wide use as a filling andror coating component in newsprint and high grade papers because it enhances brightness, opacity, smoothness, and printability of paper. One of its most important properties for paper-making is the general rheology and the change of rheology with time or stability in a dispersed state. The rheological characteristics of calcined kaolin depends largely on soluble salts concentration Ž Bundy and Murray, 1973. . Truitt and Sennett Ž1996. believed that the soluble salts in calcined kaolin suspension are related to the type of dispersant used to deflocculate the clay. In order to understand the influence of dispersant on the solubility of calcined kaolin, we have investigated five common dispersants, namely, tetrasodium pyrophosphate ŽTSPP. , sodium hexametaphosphate Ž SHMP. , sodium polyacrylate ŽSPA., sodium silicate, and sodium carbonate and one flocculant aluminum sulfate Ž representing alum in paper-making usage. in terms of their relationship with the concentration of dissolved ions Al 3q, Si 4q, Mg 2q, and Ca2q in calcined kaolin suspensions. Controlled variables included dispersant dosage, suspension pH, and aging.
2. Materials and methods This study used a calcined kaolin sample, specially produced from GA sedimentary kaolin, with a calcination temperature of about 9008C and a residence time of approximately 40 min. The calcined kaolin, designated as Cal-K, was free of dispersant. X-ray diffraction analysis showed a mineralogical composition of mostly metakaolinite, an amorphous aluminum silicate. Elemental composition of Cal-K obtained using a Perkin Elmer Plasma 40 Inductively Coupled Plasma Atomic Emission Spectroscopy Ž ICP-AES. appears in Table 1. Table 1 Elemental composition of Cal-K Oxide
Wt.%
SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 MgO CaO Na 2 O K 2O LOI
52.1% 43.21% 2.39% 1.16% 0.05% 0.18% 0.13% 0.21% 0.57%
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
139
Cal-K was treated with TSPP, SHMP, SPA, sodium silicate, sodium carbonate, and aluminum sulfate at a 0.1% to 0.3% dose and at pH conditions of 5, 7, and 9 " 0.1. Some of the treated slurry samples were aged, both for 1 day and for 2 weeks and their soluble salts levels re-checked. The preparation of samples for the soluble salts analysis was as follows: Cal-K was first made into aqueous slurry by adding 30 g clay and 70 ml de-ionized water into a 200-ml plastic beaker with continuous stirring. Note that the de-ionized water contained the dispersantrflocculant before the addition of clay. The Cal-K slurry without dispersant appeared to be flocculated and had a pH of about 3.7. The suspensions with dispersant already mixed in were generally fluid, however. Adding small amount of NaOH or H 2 SO4 adjusted the pH. Stirring continued for 30 min after pH adjustment. After stirring, the slurry was poured into a plastic centrifuge tube and centrifuged at 12 000 rpm for 20 min in an International Centrifuge Model SBV. The supernatant was removed with an syringe and filtered through a 0.2 m m Millipore membrane to remove any remaining trace of clay. The clear liquid was analyzed for Al, Ca, Mg, and Si using a Perkin Elmer Plasma 40 ICP-AES, with concentration expressed as mgrl in the supernatant. De-ionized water used in this experiment was analytically pure with a pH of 4.5. TSPP, SHMP, SPA, sodium silicate, and sodium carbonate, and aluminum sulfate were commercial chemicals. Caustic soda and sulfuric acid for pH adjustment were reagent grade, and had a concentration of 25% and 23%, respectively. As control, the five dispersants were also analyzed in the absence of clay, by dissolving in water at a 5% concentration. The solutions were analyzed using the same ICP-AES instrument. This procedure was to understand the contribution of the dispersants to the soluble salts in calcined clay.
3. Results 3.1. Soluble salts in dispersants Results from the chemical analysis of TSPP, SHMP, sodium silicate, sodium polyacrylate, sodium carbonate, and sodium sulfate solutions at 5% concentration appear in Table 2. TSPP, sodium polyacrylate, and soda ash showed very little soluble aluminum and silicon Ž less than 0.73 mgrl and 6.41 mgrl, respectively.. In contrast, SHMP and sodium silicate contained moderate amount of soluble aluminum and silicon Ž11.64–16.11 mgrl and 16.27 mgrl, respectively. . The concentrations of soluble magnesium and calcium are very low in all chemicals. Because the dispersant dosages used to disperse calcined clay are generally low Žat 0.1–0.3%., the contribution of soluble salts from the dispersant chemi-
140
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Table 2 Concentrations Žmgrl. of soluble salts in the six different chemicals Chemical Žat 5% solution.
Al 3q
Si 4q
Mg 2q
Ca2q
Tetrasodium pyrophosphate Sodium hexmetaphosphate Sodium carbonate Sodium polyacrylate Sodium silicate Aluminum sulfate
0.73 11.64 0.01 0.18 16.11
6.41 16.27 5.62 2.25
0.42 0.54 0.75 0.13 0.51 0.36
0.80 1.24 1.53 0.88 1.43 1.55
a
a
3.95
a
Indicates the element is a primary component of the chemical and its level above applicable limit by ICP-AES.
cals can be considered negligible. For example, the sodium silicate in crystal form contains about 322.2 mgrl soluble aluminum, which is the highest among all dispersing chemicals. Nevertheless, a 0.3% dose of such sodium silicate added to calcined clay will contribute only 0.41 mgrl soluble aluminum to the clay suspension. This calculation assumes that all aluminum from the sodium silicate is soluble in clay suspension. 3.2. Soluble salts in Cal-K Soluble salts concentrations of the dispersant-free Cal-K appear in Table 3. The clay contained very low concentrations of soluble aluminum, 0.74–7.21 mgrl, and moderate concentrations of soluble silicon, 7.46 to 21.40 mgrl. Soluble magnesium and calcium were negligible except at the lowest pH. Within the tested pH range, soluble aluminum generally increased with the pH increase and soluble silicon decreased. This observation indicates that the release of soluble aluminum and silicon from calcined clay is strongly pH-dependent. The change of aluminum solubility was the most dramatic from pH 7 to pH 9, indicating a non-linear relationship between the two parameters.
Table 3 Concentrations Žmgrl. of soluble salts in Cal-K under various pH conditions Cal-K soluble salts concentration Žmgrl. pH
Al 3q
Si 4q
Mg 2q
Ca2q
3.7 Žas-is. 5 7 9
1.29 0.74 1.60 7.21
26.60 21.40 11.61 7.46
0.65 nrd nrd nrd
9.11 0.13 0.02 0.03
The pH of the clay slurries was adjusted using a 25% NaOH solution. nrds Not detected.
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
141
The aqueous suspension of Cal-K without pH adjustment had a low pH, 3.7. A rise of pH from 3.7, to 5, 7, and 9 reflected the addition of more and more NaOH for pH adjustment. Because NaOH is a strong alkali, it reacted with anhydrous aluminum silicate in calcined clay. The reaction released aluminum and silicon from calcined clay into solution. In the case of aluminum, higher pH or higher level of NaOH meant more aluminum released by the reaction. However, as aluminum became available in solution, it began to inhibit the release of silicon from calcined kaolin. Al 3q ion can greatly reduce the dissolution rates of opal and amorphous silica Ž Okamoto et al., 1957; Iler, 1973. . Okamoto et al. Ž 1957. showed that increasing the Al concentration from 20 mgrl to 100 mgrl could reduce molecularly dispersed Si from 7 mgrl to 0.5 mgrl. This reduction of silica solubility is thought to be caused by the formation of alumino-slicate gel:
According to this reaction, formation of alumino-silicate coatings will increase as pH increases, since the reaction will shift to the right in response to the decreasing wHxq; this slows the dissolution of surface layers. 3.3. Soluble salts in dispersed Cal-K Table 4 lists the concentrations of soluble cations in the Cal-K slurries under different pH conditions and with different dispersantrflocculant types. Figs. 1 and 2 illustrate the variation of soluble aluminum. Under the same pH and dispersant dosage, the two phosphates ŽTSPP and SHMP. produced more soluble aluminum at 10.75–130.37 mgrl than the other three dispersants. This observation is in agreement with the report of Truitt and Sennett Ž1996.. When TSPP, sodium carbonate, or sodium silicate was used as the dispersant, a more alkaline condition appeared to favor the dissolution of aluminum. However, when SHMP was present, the reverse seemed to be true. The fact that TSPP, sodium carbonate, and sodium silicate are themselves strong bases while a pure SHMP is itself acidic in solution can explain this behavior. SPA showed a different kind of effect on the solubility of calcined kaolin in that the soluble aluminum level was highest at neutral pH. The strong affinity of phosphorous ions to Al 3q is the reason that polyphosphates tend to associate with higher level of soluble aluminum than others ŽHsu, 1989.. Bidwell et al. Ž 1970. showed that the reaction between TSPP and
142
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Table 4 Concentrations of soluble salts in Cal-K with different dispersants and under various pH conditions Chemical type
Dose
pH
Soluble salts concentration Žmgrl. Al 3q
TSPP
0.1%
0.3%
SHMPP
0.1%
0.3%
Soda Ash
0.1%
0.3%
SPA
0.1%
0.3%
Silicate
0.1%
0.3%
Alum
0.1%
0.3%
nrds Not detected.
Si 4q
Mg 2q
Ca2q
5 7 9 5 7 9
10.75 33.48 39.78 88.64 130.3 68.11
26.75 19.98 14.50 35.65 23.82 12.38
nrd nrd 0.20 0.05 0.04 0.72
0.06 0.06 1.31 0.83 0.68 5.25
5 7 9 5 7 9
35.12 37.93 12.34 94.21 82.45 20.85
22.33 17.72 8.57 24.45 16.04 7.09
0.06 0.08 0.60 0.27 0.51 1.59
1.93 3.89 14.14 6.69 13.88 21.93
5 7 9 5 7 9
0.34 0.77 2.34 0.11 0.89 2.98
18.40 12.16 6.12 22.68 7.01 5.11
nrd nrd nrd 0.28 nrd nrd
0.29 0.02 0.02 4.06 0.13 0.09
5 7 9 5 7 9
3.27 18.58 9.98 32.23 53.06 16.82
20.48 12.79 7.88 23.10 17.20 8.44
nrd nrd 0.08 nrd 0.01 0.42
0.11 0.50 3.79 0.27 1.10 12.17
5 7 9 5 7 9
0.76 1.92 9.18 4.09 11.17 38.33
69.41 39.60 32.96 175.93 135.09 133.71
nrd nrd nrd nrd nrd nrd
0.24 0.02 0.01 0.09 0.01 0.05
5 7 9 5 7 9
0.16 0.41 4.21 0.85 0.51 3.19
16.45 11.17 5.29 17.47 7.59 3.34
0.03 nrd nrd 0.16 0.02 0.01
0.96 0.05 0.02 2.16 0.28 0.07
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
143
Fig. 1. Chart showing the variation of the soluble aluminum levels in Cal-K suspensions with different pH and TSPP doses.
kaolinite leads to the release of aluminum and silicon from kaolinite into solution. The mole ratio Al:Si found in the solution of TSPP-dispersed kaolinite always exceeds unity implying a preferential dissolution of aluminum from the kaolinite. Chromatographic experiments suggested that part of the aluminum in solution is present as a complex with phosphate. Similar reaction also takes place in calcined kaolin. Once in solution, the phosphorous ions will compete with OHy for Al 3q to form soluble complex ions. They can displace the H 2 O or OHy ligand in the first coordination sphere of Al 3q and thus interfere with or simply inhibit AlŽOH. 3 crystallization and precipitation. The phosphates’ ability to form stable soluble complexes with cations such as Al 3q and Ca2q is so strong that crystalline AlŽOH. 3 cannot develop even at a NaOHrAl molar ratio of 3 to 4 ŽpH 9–10. ŽHsu, 1989. . This means that almost all Al 3q released from the dissolution of calcined kaolin in water will stay in dissolved form. Surprisingly, sodium polyacrylate produced more soluble aluminum, at 3.27– 53.06 mgrl, than sodium silicate and soda ash, though still less than that of either phosphates at equal doses. The function of polyacrylate polymer is probably similar to phosphate in terms of interfering with the AlŽ OH. 3 dissolutionrprecipitation process. Aluminum may be incorporated with the polymer in a soluble form in solution. Sodium silicate produced 0.76–38.33 mgrl soluble aluminum under variable conditions. Higher pH clearly favors the dissolution of aluminum when sodium silicate is used as the dispersant. Sodium carbonate produced the lowest soluble aluminum level, at 0.11–2.98 mgrl, which is even lower than the control samples at equivalent pH conditions. This is because sodium carbonate is a weaker base than NaOH which was used in the ‘control’ samples to adjust pH condition.
144
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Fig. 2. Diagrams showing the variation of the soluble aluminum levels in Cal-K suspensions with different pH and dispersing chemicals Žat 0.1% and 0.3% doses..
Soluble silicon levels generally decrease with increasing pH in the Cal-K suspensions Ž Fig. 3. , regardless of which dispersantrflocculant was used. Except in the case of sodium silicate, the levels of soluble silicon in the calcined kaolin suspensions with different dispersantrflocculant are similar when pH and chemical dose are the same. Sodium silicate produced excessively high level of soluble silicon because of its own contribution. In general, the levels of soluble magnesium and calcium in calcined kaolin are very low. This is because kaolin naturally contains very little magnesium and calcium. The kinetics of the reactions between dispersants and calcined kaolin were studied by analyzing the clay slurries after 2 weeks of aging. Only three
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
145
Fig. 3. Diagrams showing the variation of the soluble silicon levels in Cal-K suspensions with different pH and dispersing chemicals Žat 0.1% and 0.3% doses..
dispersants including TSPP, SHMP, and soda ash were examined. Table 5 lists the soluble salts concentrations after aging. Aging did not significantly change the soluble aluminum levels in all cases including the control sample. 3.4. Effect of aluminum sulfate Aluminum sulfate is a common flocculant used in kaolin industry and paper-making. The concentrations of soluble salts in Cal-K suspensions after additions of aluminum sulfate appear in Table 4. Interestingly, the addition of
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
146
Table 5 Soluble salts concentration and pH of the Cal-K suspensions without aging vs. aging for 2 weeks Samples
Concentration Žmgrl., no aging Al
Ž1. Cal-K Ž2. With 0.1% TSPP Ž3. With 0.1% SHMPP Ž4. With 0.1% soda ash
3q
Si
4q
Mg
2q
Ca
2q
Concentration Žmgrl., aging 2 weeks pH
Al 3q
Si 4q
Mg 2q
Ca2q
pH
1.29 9.35
26.60 20.39
0.65 0.01
9.11 0.07
3.67 5.53
1.20 7.67
11.61 20.22
nrd nrd
0.03 0.08
3.70 5.10
26.86
24.86
0.06
1.80
4.69
24.12
22.30
0.06
1.75
4.48
0.94
10.91
nrd
0.03
6.05
0.71
13.43
nrd
0.05
5.94
No pH adjustment was made for these suspensions. nrds Not detected.
aluminum sulfate to Cal-K hardly produced any more soluble aluminum than the control itself. Apparently, the aluminum from aluminum sulfate formed an attachment with the clay particles, which were settled and removed with centrifugation. The attachment may be due to the formation of hydrolyzed aluminum species and complexes on the surface of the clay particles. Examples of hydrolyzed aluminum are AlŽH 2 O. 33q and Al 13wO4ŽOH. 24 x 7q Ž Ottewill, 1996, oral communication.. The aluminum ion can also precipitate as aluminum oxide on the kaolin surface ŽBundy and Murray, 1973. . The positively charged aluminum hydroxide ions can flocculate the negatively charge calcined clay particles. They are removed along with the clay particles during centrifugation, leaving the supernatant nearly free of aluminum ion.
4. Discussion and conclusions The five types of dispersing agents have various degrees of contribution to the dissolved ions concentrations in calcined kaolin. The calcined clays dispersed with tetrasodiumpyrophosphate and sodium hexametaphosphate yielded higher levels of soluble aluminum than those with sodium polyacrylate, sodium carbonate, and sodium silicate at equivalent chemical dosage and pH condition. Soluble Mg 2q, and Ca2q ion concentrations were generally low compared to aluminum and silicon. Higher dosage of any dispersant generally raised the concentration of soluble aluminum. Higher pH favored soluble aluminum but lowered soluble silicon. The effect of aging on the soluble salts in calcined kaolin was not significant. Addition of aluminum sulfate to the calcined kaolin did not yield higher dissolved aluminum than the control. Overall, the solubility of calcined kaolin seemed largely related to the pH condition in addition to the type of dispersantrflocculant.
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
147
Acknowledgements The authors are grateful to Dr. W.M. Bundy and Dr. W.F. Moll, Jr. for reviewing the manuscript. Their insightful suggestions enhanced the content of the paper. E.W. Andrews, J.R. May and J. Stephens at ECC International provided sample preparation and analytical support for which the authors are thankful.
References Bidwell, J.I., Jepson, W.B., Toms, G.L., 1970. The interaction of kaolinite with polyphosphate and polyacrylate in aqueous solutions—some preliminary results. Clay Miner. 8, 445–459. Bundy, W.M., Murray, H.H., 1973. The effect of aluminum on the surface properties of kaolinite. Clays Clay Miner. 21, 295–302. Hsu, P.H., 1989. Aluminum hydroxides and oxyhydroxides. In: Dixon, J.B., Weed, S.B. ŽEds.., Minerals in Soil Environments, 2nd edn. Soil Science Society of America Book Series No. 1, pp. 354–356. Iler, R.K., 1973. Effect of adsorbed alumina on the solubility of amorphous silica in water. J. Colloid Interface Sci. 43, 399–408. Okamoto, G., Okura, T., Goto, K., 1957. Properties of silica in water. Geochim. Cosmochim. Acta 12, 123–132. Truitt, R.E., Sennett, P.S., 1996. Wood pitch deposition: aluminum ions from kaolin pigments. 82nd Annual Meeting, Technical Section, CPPA: A239-A245.
Influence of dispersants on the solubility of calcined kaolin Jun Yuan ) , William L. Garforth, Robert J. Pruett ECC International, P.O. Box 471, SandersÕille, GA 31082, USA Received 4 September 1997; revised 19 March 1998; accepted 19 March 1998
Abstract The solubility of calcined kaolin relative to different aqueous solution chemistries is important to its industrial application. When used in paper-making, a calcined kaolin product is required to have good flow characteristics and viscosity stability, both of which are largely controlled by the type and level of soluble salts in the kaolin suspension. In this study, five common dispersing agents including tetrasodium pyrophosphate, sodium hexametaphosphate, sodium polyacrylate, sodium silicate, and sodium carbonate and one flocculant aluminum sulfate were investigated in terms of their relationship with the concentration of dissolved ions ŽAl 3q, Si 4q, Mg 2q, and Ca2q . in calcined kaolin suspensions. Controlled variables included dispersant dosage, suspension pH, and aging. Results from the study showed that the calcined clays dispersed with tetrasodiumpyrophosphate and sodium hexametaphosphate yielded higher levels of soluble aluminum than those with sodium polyacrylate, sodium carbonate, and sodium silicate at equivalent chemical dosage and pH condition. The variations of other soluble salts ŽSi 4q, Mg 2q, and Ca2q . were not as significant. Mg 2q, and Ca2q ion concentrations were generally low compared to aluminum and silicon. Higher dosage of any dispersant generally raised the concentration of soluble aluminum. Higher pH favored soluble aluminum but lowered soluble silicon. Addition of aluminum sulfate in calcined clay suspension did not raise the soluble aluminum level. The effect of aging on soluble salts concentration was not significant. Overall, the solubility of calcined kaolin seemed largely related to the pH condition in addition to the type and level of dispersantrflocculant. q 1998 Elsevier Science B.V. All rights reserved. Keywords: calcination; kaolin; solubility; dispersant
)
Corresponding author. Tel.: q1-912-553-5747; fax: q1-912-553-5797; e-mail: [email protected]. 0169-1317r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 8 . 0 0 0 1 6 - 7
138
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
1. Introduction Calcined kaolin finds wide use as a filling andror coating component in newsprint and high grade papers because it enhances brightness, opacity, smoothness, and printability of paper. One of its most important properties for paper-making is the general rheology and the change of rheology with time or stability in a dispersed state. The rheological characteristics of calcined kaolin depends largely on soluble salts concentration Ž Bundy and Murray, 1973. . Truitt and Sennett Ž1996. believed that the soluble salts in calcined kaolin suspension are related to the type of dispersant used to deflocculate the clay. In order to understand the influence of dispersant on the solubility of calcined kaolin, we have investigated five common dispersants, namely, tetrasodium pyrophosphate ŽTSPP. , sodium hexametaphosphate Ž SHMP. , sodium polyacrylate ŽSPA., sodium silicate, and sodium carbonate and one flocculant aluminum sulfate Ž representing alum in paper-making usage. in terms of their relationship with the concentration of dissolved ions Al 3q, Si 4q, Mg 2q, and Ca2q in calcined kaolin suspensions. Controlled variables included dispersant dosage, suspension pH, and aging.
2. Materials and methods This study used a calcined kaolin sample, specially produced from GA sedimentary kaolin, with a calcination temperature of about 9008C and a residence time of approximately 40 min. The calcined kaolin, designated as Cal-K, was free of dispersant. X-ray diffraction analysis showed a mineralogical composition of mostly metakaolinite, an amorphous aluminum silicate. Elemental composition of Cal-K obtained using a Perkin Elmer Plasma 40 Inductively Coupled Plasma Atomic Emission Spectroscopy Ž ICP-AES. appears in Table 1. Table 1 Elemental composition of Cal-K Oxide
Wt.%
SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 MgO CaO Na 2 O K 2O LOI
52.1% 43.21% 2.39% 1.16% 0.05% 0.18% 0.13% 0.21% 0.57%
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
139
Cal-K was treated with TSPP, SHMP, SPA, sodium silicate, sodium carbonate, and aluminum sulfate at a 0.1% to 0.3% dose and at pH conditions of 5, 7, and 9 " 0.1. Some of the treated slurry samples were aged, both for 1 day and for 2 weeks and their soluble salts levels re-checked. The preparation of samples for the soluble salts analysis was as follows: Cal-K was first made into aqueous slurry by adding 30 g clay and 70 ml de-ionized water into a 200-ml plastic beaker with continuous stirring. Note that the de-ionized water contained the dispersantrflocculant before the addition of clay. The Cal-K slurry without dispersant appeared to be flocculated and had a pH of about 3.7. The suspensions with dispersant already mixed in were generally fluid, however. Adding small amount of NaOH or H 2 SO4 adjusted the pH. Stirring continued for 30 min after pH adjustment. After stirring, the slurry was poured into a plastic centrifuge tube and centrifuged at 12 000 rpm for 20 min in an International Centrifuge Model SBV. The supernatant was removed with an syringe and filtered through a 0.2 m m Millipore membrane to remove any remaining trace of clay. The clear liquid was analyzed for Al, Ca, Mg, and Si using a Perkin Elmer Plasma 40 ICP-AES, with concentration expressed as mgrl in the supernatant. De-ionized water used in this experiment was analytically pure with a pH of 4.5. TSPP, SHMP, SPA, sodium silicate, and sodium carbonate, and aluminum sulfate were commercial chemicals. Caustic soda and sulfuric acid for pH adjustment were reagent grade, and had a concentration of 25% and 23%, respectively. As control, the five dispersants were also analyzed in the absence of clay, by dissolving in water at a 5% concentration. The solutions were analyzed using the same ICP-AES instrument. This procedure was to understand the contribution of the dispersants to the soluble salts in calcined clay.
3. Results 3.1. Soluble salts in dispersants Results from the chemical analysis of TSPP, SHMP, sodium silicate, sodium polyacrylate, sodium carbonate, and sodium sulfate solutions at 5% concentration appear in Table 2. TSPP, sodium polyacrylate, and soda ash showed very little soluble aluminum and silicon Ž less than 0.73 mgrl and 6.41 mgrl, respectively.. In contrast, SHMP and sodium silicate contained moderate amount of soluble aluminum and silicon Ž11.64–16.11 mgrl and 16.27 mgrl, respectively. . The concentrations of soluble magnesium and calcium are very low in all chemicals. Because the dispersant dosages used to disperse calcined clay are generally low Žat 0.1–0.3%., the contribution of soluble salts from the dispersant chemi-
140
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Table 2 Concentrations Žmgrl. of soluble salts in the six different chemicals Chemical Žat 5% solution.
Al 3q
Si 4q
Mg 2q
Ca2q
Tetrasodium pyrophosphate Sodium hexmetaphosphate Sodium carbonate Sodium polyacrylate Sodium silicate Aluminum sulfate
0.73 11.64 0.01 0.18 16.11
6.41 16.27 5.62 2.25
0.42 0.54 0.75 0.13 0.51 0.36
0.80 1.24 1.53 0.88 1.43 1.55
a
a
3.95
a
Indicates the element is a primary component of the chemical and its level above applicable limit by ICP-AES.
cals can be considered negligible. For example, the sodium silicate in crystal form contains about 322.2 mgrl soluble aluminum, which is the highest among all dispersing chemicals. Nevertheless, a 0.3% dose of such sodium silicate added to calcined clay will contribute only 0.41 mgrl soluble aluminum to the clay suspension. This calculation assumes that all aluminum from the sodium silicate is soluble in clay suspension. 3.2. Soluble salts in Cal-K Soluble salts concentrations of the dispersant-free Cal-K appear in Table 3. The clay contained very low concentrations of soluble aluminum, 0.74–7.21 mgrl, and moderate concentrations of soluble silicon, 7.46 to 21.40 mgrl. Soluble magnesium and calcium were negligible except at the lowest pH. Within the tested pH range, soluble aluminum generally increased with the pH increase and soluble silicon decreased. This observation indicates that the release of soluble aluminum and silicon from calcined clay is strongly pH-dependent. The change of aluminum solubility was the most dramatic from pH 7 to pH 9, indicating a non-linear relationship between the two parameters.
Table 3 Concentrations Žmgrl. of soluble salts in Cal-K under various pH conditions Cal-K soluble salts concentration Žmgrl. pH
Al 3q
Si 4q
Mg 2q
Ca2q
3.7 Žas-is. 5 7 9
1.29 0.74 1.60 7.21
26.60 21.40 11.61 7.46
0.65 nrd nrd nrd
9.11 0.13 0.02 0.03
The pH of the clay slurries was adjusted using a 25% NaOH solution. nrds Not detected.
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
141
The aqueous suspension of Cal-K without pH adjustment had a low pH, 3.7. A rise of pH from 3.7, to 5, 7, and 9 reflected the addition of more and more NaOH for pH adjustment. Because NaOH is a strong alkali, it reacted with anhydrous aluminum silicate in calcined clay. The reaction released aluminum and silicon from calcined clay into solution. In the case of aluminum, higher pH or higher level of NaOH meant more aluminum released by the reaction. However, as aluminum became available in solution, it began to inhibit the release of silicon from calcined kaolin. Al 3q ion can greatly reduce the dissolution rates of opal and amorphous silica Ž Okamoto et al., 1957; Iler, 1973. . Okamoto et al. Ž 1957. showed that increasing the Al concentration from 20 mgrl to 100 mgrl could reduce molecularly dispersed Si from 7 mgrl to 0.5 mgrl. This reduction of silica solubility is thought to be caused by the formation of alumino-slicate gel:
According to this reaction, formation of alumino-silicate coatings will increase as pH increases, since the reaction will shift to the right in response to the decreasing wHxq; this slows the dissolution of surface layers. 3.3. Soluble salts in dispersed Cal-K Table 4 lists the concentrations of soluble cations in the Cal-K slurries under different pH conditions and with different dispersantrflocculant types. Figs. 1 and 2 illustrate the variation of soluble aluminum. Under the same pH and dispersant dosage, the two phosphates ŽTSPP and SHMP. produced more soluble aluminum at 10.75–130.37 mgrl than the other three dispersants. This observation is in agreement with the report of Truitt and Sennett Ž1996.. When TSPP, sodium carbonate, or sodium silicate was used as the dispersant, a more alkaline condition appeared to favor the dissolution of aluminum. However, when SHMP was present, the reverse seemed to be true. The fact that TSPP, sodium carbonate, and sodium silicate are themselves strong bases while a pure SHMP is itself acidic in solution can explain this behavior. SPA showed a different kind of effect on the solubility of calcined kaolin in that the soluble aluminum level was highest at neutral pH. The strong affinity of phosphorous ions to Al 3q is the reason that polyphosphates tend to associate with higher level of soluble aluminum than others ŽHsu, 1989.. Bidwell et al. Ž 1970. showed that the reaction between TSPP and
142
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Table 4 Concentrations of soluble salts in Cal-K with different dispersants and under various pH conditions Chemical type
Dose
pH
Soluble salts concentration Žmgrl. Al 3q
TSPP
0.1%
0.3%
SHMPP
0.1%
0.3%
Soda Ash
0.1%
0.3%
SPA
0.1%
0.3%
Silicate
0.1%
0.3%
Alum
0.1%
0.3%
nrds Not detected.
Si 4q
Mg 2q
Ca2q
5 7 9 5 7 9
10.75 33.48 39.78 88.64 130.3 68.11
26.75 19.98 14.50 35.65 23.82 12.38
nrd nrd 0.20 0.05 0.04 0.72
0.06 0.06 1.31 0.83 0.68 5.25
5 7 9 5 7 9
35.12 37.93 12.34 94.21 82.45 20.85
22.33 17.72 8.57 24.45 16.04 7.09
0.06 0.08 0.60 0.27 0.51 1.59
1.93 3.89 14.14 6.69 13.88 21.93
5 7 9 5 7 9
0.34 0.77 2.34 0.11 0.89 2.98
18.40 12.16 6.12 22.68 7.01 5.11
nrd nrd nrd 0.28 nrd nrd
0.29 0.02 0.02 4.06 0.13 0.09
5 7 9 5 7 9
3.27 18.58 9.98 32.23 53.06 16.82
20.48 12.79 7.88 23.10 17.20 8.44
nrd nrd 0.08 nrd 0.01 0.42
0.11 0.50 3.79 0.27 1.10 12.17
5 7 9 5 7 9
0.76 1.92 9.18 4.09 11.17 38.33
69.41 39.60 32.96 175.93 135.09 133.71
nrd nrd nrd nrd nrd nrd
0.24 0.02 0.01 0.09 0.01 0.05
5 7 9 5 7 9
0.16 0.41 4.21 0.85 0.51 3.19
16.45 11.17 5.29 17.47 7.59 3.34
0.03 nrd nrd 0.16 0.02 0.01
0.96 0.05 0.02 2.16 0.28 0.07
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
143
Fig. 1. Chart showing the variation of the soluble aluminum levels in Cal-K suspensions with different pH and TSPP doses.
kaolinite leads to the release of aluminum and silicon from kaolinite into solution. The mole ratio Al:Si found in the solution of TSPP-dispersed kaolinite always exceeds unity implying a preferential dissolution of aluminum from the kaolinite. Chromatographic experiments suggested that part of the aluminum in solution is present as a complex with phosphate. Similar reaction also takes place in calcined kaolin. Once in solution, the phosphorous ions will compete with OHy for Al 3q to form soluble complex ions. They can displace the H 2 O or OHy ligand in the first coordination sphere of Al 3q and thus interfere with or simply inhibit AlŽOH. 3 crystallization and precipitation. The phosphates’ ability to form stable soluble complexes with cations such as Al 3q and Ca2q is so strong that crystalline AlŽOH. 3 cannot develop even at a NaOHrAl molar ratio of 3 to 4 ŽpH 9–10. ŽHsu, 1989. . This means that almost all Al 3q released from the dissolution of calcined kaolin in water will stay in dissolved form. Surprisingly, sodium polyacrylate produced more soluble aluminum, at 3.27– 53.06 mgrl, than sodium silicate and soda ash, though still less than that of either phosphates at equal doses. The function of polyacrylate polymer is probably similar to phosphate in terms of interfering with the AlŽ OH. 3 dissolutionrprecipitation process. Aluminum may be incorporated with the polymer in a soluble form in solution. Sodium silicate produced 0.76–38.33 mgrl soluble aluminum under variable conditions. Higher pH clearly favors the dissolution of aluminum when sodium silicate is used as the dispersant. Sodium carbonate produced the lowest soluble aluminum level, at 0.11–2.98 mgrl, which is even lower than the control samples at equivalent pH conditions. This is because sodium carbonate is a weaker base than NaOH which was used in the ‘control’ samples to adjust pH condition.
144
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
Fig. 2. Diagrams showing the variation of the soluble aluminum levels in Cal-K suspensions with different pH and dispersing chemicals Žat 0.1% and 0.3% doses..
Soluble silicon levels generally decrease with increasing pH in the Cal-K suspensions Ž Fig. 3. , regardless of which dispersantrflocculant was used. Except in the case of sodium silicate, the levels of soluble silicon in the calcined kaolin suspensions with different dispersantrflocculant are similar when pH and chemical dose are the same. Sodium silicate produced excessively high level of soluble silicon because of its own contribution. In general, the levels of soluble magnesium and calcium in calcined kaolin are very low. This is because kaolin naturally contains very little magnesium and calcium. The kinetics of the reactions between dispersants and calcined kaolin were studied by analyzing the clay slurries after 2 weeks of aging. Only three
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
145
Fig. 3. Diagrams showing the variation of the soluble silicon levels in Cal-K suspensions with different pH and dispersing chemicals Žat 0.1% and 0.3% doses..
dispersants including TSPP, SHMP, and soda ash were examined. Table 5 lists the soluble salts concentrations after aging. Aging did not significantly change the soluble aluminum levels in all cases including the control sample. 3.4. Effect of aluminum sulfate Aluminum sulfate is a common flocculant used in kaolin industry and paper-making. The concentrations of soluble salts in Cal-K suspensions after additions of aluminum sulfate appear in Table 4. Interestingly, the addition of
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
146
Table 5 Soluble salts concentration and pH of the Cal-K suspensions without aging vs. aging for 2 weeks Samples
Concentration Žmgrl., no aging Al
Ž1. Cal-K Ž2. With 0.1% TSPP Ž3. With 0.1% SHMPP Ž4. With 0.1% soda ash
3q
Si
4q
Mg
2q
Ca
2q
Concentration Žmgrl., aging 2 weeks pH
Al 3q
Si 4q
Mg 2q
Ca2q
pH
1.29 9.35
26.60 20.39
0.65 0.01
9.11 0.07
3.67 5.53
1.20 7.67
11.61 20.22
nrd nrd
0.03 0.08
3.70 5.10
26.86
24.86
0.06
1.80
4.69
24.12
22.30
0.06
1.75
4.48
0.94
10.91
nrd
0.03
6.05
0.71
13.43
nrd
0.05
5.94
No pH adjustment was made for these suspensions. nrds Not detected.
aluminum sulfate to Cal-K hardly produced any more soluble aluminum than the control itself. Apparently, the aluminum from aluminum sulfate formed an attachment with the clay particles, which were settled and removed with centrifugation. The attachment may be due to the formation of hydrolyzed aluminum species and complexes on the surface of the clay particles. Examples of hydrolyzed aluminum are AlŽH 2 O. 33q and Al 13wO4ŽOH. 24 x 7q Ž Ottewill, 1996, oral communication.. The aluminum ion can also precipitate as aluminum oxide on the kaolin surface ŽBundy and Murray, 1973. . The positively charged aluminum hydroxide ions can flocculate the negatively charge calcined clay particles. They are removed along with the clay particles during centrifugation, leaving the supernatant nearly free of aluminum ion.
4. Discussion and conclusions The five types of dispersing agents have various degrees of contribution to the dissolved ions concentrations in calcined kaolin. The calcined clays dispersed with tetrasodiumpyrophosphate and sodium hexametaphosphate yielded higher levels of soluble aluminum than those with sodium polyacrylate, sodium carbonate, and sodium silicate at equivalent chemical dosage and pH condition. Soluble Mg 2q, and Ca2q ion concentrations were generally low compared to aluminum and silicon. Higher dosage of any dispersant generally raised the concentration of soluble aluminum. Higher pH favored soluble aluminum but lowered soluble silicon. The effect of aging on the soluble salts in calcined kaolin was not significant. Addition of aluminum sulfate to the calcined kaolin did not yield higher dissolved aluminum than the control. Overall, the solubility of calcined kaolin seemed largely related to the pH condition in addition to the type of dispersantrflocculant.
J. Yuan et al.r Applied Clay Science 13 (1998) 137–147
147
Acknowledgements The authors are grateful to Dr. W.M. Bundy and Dr. W.F. Moll, Jr. for reviewing the manuscript. Their insightful suggestions enhanced the content of the paper. E.W. Andrews, J.R. May and J. Stephens at ECC International provided sample preparation and analytical support for which the authors are thankful.
References Bidwell, J.I., Jepson, W.B., Toms, G.L., 1970. The interaction of kaolinite with polyphosphate and polyacrylate in aqueous solutions—some preliminary results. Clay Miner. 8, 445–459. Bundy, W.M., Murray, H.H., 1973. The effect of aluminum on the surface properties of kaolinite. Clays Clay Miner. 21, 295–302. Hsu, P.H., 1989. Aluminum hydroxides and oxyhydroxides. In: Dixon, J.B., Weed, S.B. ŽEds.., Minerals in Soil Environments, 2nd edn. Soil Science Society of America Book Series No. 1, pp. 354–356. Iler, R.K., 1973. Effect of adsorbed alumina on the solubility of amorphous silica in water. J. Colloid Interface Sci. 43, 399–408. Okamoto, G., Okura, T., Goto, K., 1957. Properties of silica in water. Geochim. Cosmochim. Acta 12, 123–132. Truitt, R.E., Sennett, P.S., 1996. Wood pitch deposition: aluminum ions from kaolin pigments. 82nd Annual Meeting, Technical Section, CPPA: A239-A245.