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Pulp dispersion in selective desliming of iron ores
International Journal of Mineral Processing, 12 (1984) 1--13 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
PULP DISPERSION IN SELECTIVE DESLIMING OF IRON ORES
S.V. K R I S H N A N
I and I. I W A S A K I 2
IBatelle, Columbus Laboratories, 505 King Avenue, Columbus, O H 43201 (U.S.A.) 2 Mineral Resources Research Center, 56 East River Road, University of Minnesota,
Minneapolis, MN 55455 (U.S.A.) (Received June 13, 1982; accepted for publication March 28, 1983)
ABSTRACT
Krishnan, S.V. and Iwasaki, I., 1984. Pulp dispersion in selective desliming of iron ores. Int. J. Miner. Process., 12: 1--13. In the upgrading of finely disseminated iron ores, selective desliming is a critical step prior to flotation. A prerequisite for selective desliming is a properly dispersed pulp; sodium silicate is commonly used as a dispersant. The mechanism by which pulp dispersion is attained by means of sodium silicate in the presence of calcium ions and magnesium hydroxide precipitates was examined by settling tests, abstraction tests, selective flocculation tests and electron microscope observations. Flocculation and disperson regions of quartz as functions of calcium, magnesium and sodium silicate levels were delineated. Pulps consisting of artificial mixtures of quartz and goethite dispersed nonselectively on addition of sodium silicate. At low silicate levels, addition of corn starch aided the flocculation of goethite over quartz; at higher silicate levels, addition of starch was ineffective. A model is presented to explain pulp dispersion in the presence of these species.
INTRODUCTION
Process water quality is of major concern affecting the efficiency of desliming in selective flocculation of iron ores (Paananen and Turcotte, 1980; Dicks and Morrow, 1978). Pulp dispersion is influenced by hardness of process water, soluble silicate in the pulp, and temperature. Since a properly dispersed pulp is an essential prerequisite to selective desliming, the manner in which these factors affect pulp dispersion must be clearly understood. In iron ore pulps, presence of Ca 2+ and Mg 2÷ c o m m o n l y occurs from such sources as dissolution of calcium- and magnesium-containing minerals and clays associated with the ore, and hardness of water. The Ca 2+ and Mg 2+ contents of water equilibrated with iron ores have been found to be in the range of 10 -4 to 10 -3 M (Iwasaki et al., 1980). The silicate level in process pulps is contributed not only by the addition of sodium silicate as the dispersant, b u t also by the dissolution of the silicate minerals in the ore. At the pH ranges c o m m o n l y employed in selective flocculation of iron ores, Ca 2÷ is in an ionic form while Mg 2÷ precipitates as magnesium hydroxide. In an earlier
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paper, the mechanism of dispersion of iron ores by sodium silicate in the presence of Ca 2÷ was delineated (Krishnan and Iwasaki, 1982). The present article compares the effect of Ca 2÷ and Mg~÷ on selective desliming and examines the manner by which pulp dispersion is affected in the presence of sodium silicate and magnesium hydroxide precipitates. MATERIALS AND METHODS The quartz sample used in this study was prepared from high-purity Brazilian rock crystal. The goethite sample was prepared from hand-picked pure lumps from the Cuyuna Range, Minnesota. The samples were sized to --60 ~m for flocculation and selective desliming tests. Details on the preparation of the samples have been reported (Iwasaki and Lipp, 1971). N-sodium silicate, obtained from the PQ Corporation, had a SiO~/Na20 ratio of 3.22 and a silica content of 28.7%. Fresh solutions were made by suitably diluting the as-received sample. Magnesium chloride used was of analytical grade. Sodium hydroxide was used to adjust pH. All tests were performed using distilled water. The experimental procedures for flocculation and selective desliming tests have been reported (Heerema and Iwasaki, 1980; Heerema et al., 1982). After settling tests, the suspension was poured on a 270 mesh screen, washed thoroughly with denatured alcohol, and transferred to a beaker. The solids were then dried, weighed, and leached with hydrochloric acid. The leachate was analyzed for Mg by atomic absorption to give abstraction density. A scanning transmission electron microscope {STEM), JEOL Model 100 CX, was used to observe the surface of quartz samples.
EXPERIMENTAL RESULTS The manner by which selective flocculation of goethite-quartz mixtures is affected by the presence of Ca 2+, Mg2÷ and sodium silicate was examined by flocculation tests, abstraction tests, scanning electron microscope observations and selective flocculation tests. Flocculation tests
The influence of calcium ions and magnesium hydroxide precipitates on the settling behavior of quartz, and the effectiveness of sodium silicate in dispersing the suspension in the presence of these species were examined by a series of flocculation tests. The tests were performed in the absence of starch in order to study the interactions between the mineral and the chemical species exclusively and then to correlate the results with those obtained in the presence of starch. Settling behavior of quartz in the presence of Ca 2+ and Mg2+ at pH 11 as a function of sodium silicate addition is shown in Fig. 1. It is observed that, as
a general trend, settling decreased progressively with silicate addition. In the presence of 2.5 × 10-4M and 10-3M Ca 2÷, the suspension was fully dispersed at sodium silicate additions of 215 ppm SiO2 and 2100 ppm SIO2, respectively. Full dispersion in the presence of 10 -3 M Mg 2÷ was attained at a sodium silicate addition of 215 ppm SiO2.
I
==\ ,-- E ¢o
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o 10-3 M Mg 2+ 42.5 x 10"4 M Ca2÷ o 10 "3 M Ca2+ • Mg+Si+Qz 10 -3 M Mg 2+
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~
, ,,I
0
,I
10-5 10 "3 M Mg 2+ "~
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~
-
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2.5 x. 10"4 M Ca2+ , , ,,l , , ,,i , , ,,I , , ,,I 10 ' 1O0 1000 10,000 Sodium Silicate, ppm SiO 2
Fig. I. Flocculation test results o n quartz at pH i i as function of sodium silicate addition and in the presence o f Ca 2÷ or Mg 2+. (No starch added.)
It is noted that in the presence of the same amounts o f Ca 2÷ and Mg 2÷, viz., 10 -3 M, considerably more sodium silicate was needed to disperse suspensions with Ca 2÷ than those with Mg 2÷. Also, the settling behavior of quartz in the presence of 10 -3 M Mg 2+ corresponded closely with that in the presence of 2.5 × 10-4M Ca 2÷. Yet, as well be seen subsequently, the abstraction behavior in quartz suspensions and selective flocculation results of mixed suspensions at these conditions differed quite appreciably. These differences can be related to the fact that Ca 2+ is an ionic species and Mg 2÷ is precipitated as Mg(OH)2 at this pH, and to the differences in interaction between these species and sodium silicate. Suspended solids concentrations reflected, more or less, the trends observed in the settling curves. It is noted that at each concentration of the cations, suspended:solids concentration increased sharply above a certain value
of sodium silicate addition. This observation can be related to the abstraction of Ca ~÷ or Mg ~÷ on quartz. At each concentration of Ca 2÷ or Mg ~÷ studied, the abstraction density curves were observed to increase with sodium silicate addition until all the Ca 2÷ or Mg2÷ added was abstracted, and to decrease thereafter. Comparing the abstraction curves with the suspended solids curves, it is seen that the suspension started to disperse beyond the maximum level of Mg2÷ abstraction, but was well-dispersed at or below the m a x i m u m level of Ca 2÷ abstraction. It was noted in tests with Mg2÷ that when the sequence of reagent additions was altered such that sodium silicate was added first to a quartz suspension followed by MgC12, or a mixture of MgC12 and silicate solutions were added to quartz (indicated in Fig. 1 as Mg + Si + Qz), no change in settling rate or abstraction density was observed. However, sequence of addition of sodium silicate and CaC12 resulted in an appreciable change in abstraction density of Ca 2÷ on quartz surface (Krishnan and Iwasaki, 1982). When calcium silicate precipitate was mixed with quartz, abstraction of the precipitates onto quartz was low and remained essentially unchanged with increase in sodium silicate addition. This was attributed to the repulsion between the negatively charged precipitates, due to excess silicate ions, and the likecharged quartz surface. When a quartz suspension was conditioned with Ca ~÷ first and then sodium silicate added, Ca 2÷ abstraction increased until all the Ca 2÷ was abstracted, and then decreased, as noted in Fig. 1. The effects of sequence of reagent addition with Ca 2÷ demonstrates the difference in heterocoagulation and surface precipitation. With Mg 2÷, however, heterocoagulation of Mg(OH)2 precipitates with their surface properties modified by silicate ions must have played a d o m i n a n t role. Figure 2 shows the flocculation and dispersion regions of quartz as functions of Ca ~÷ or Mg2÷ and sodium silicate additions. It is noted that below about 2 × 10 -4 M Ca 2÷ or Mg 2~, quartz suspensions remained dispersed with little or no sodium silicate addition. Also, for the same level of sodium silicate addition, a higher a m o u n t of Mg 2÷ could be tolerated than Ca 2÷, again showing the differences in the interactions of Ca 2÷ and Mg(OH)2 precipitate with sodium silicate. Stability curve for goethite suspensions in the presence of Ca 2÷ reported earlier indicated that a lower concentration of sodium silicate was required to disperse goethite than quartz (Krishnan and Iwasaki, 1982). For selective desliming, quartz must be fully dispersed and goethite flocculated. The results obtained in the absence of starch suggested that starch perhaps held the key to selective desliming in this system. In order to relate the results of Figs. 1 and 2 meaningfully to those of selective flocculation tests with corn starch, settling tests were carried out to study the effect of starch on quartz and on goethite in the presence of Ca 2÷ and Mg2÷. The results are presented in Fig. 3, which shows the suspended solids concentrations of quartz and goethite at pH 11 as a function of sodium silicate additions with and without starch, and at a constant Ca 2÷ or Mg2÷ level. A Ca :÷ level of 2.5 × 1 0 -4 M and a Mg2. level of 10 -3 M have
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1000 Sodium Silicate, ppm SiO 2
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Fig. 2. E f f e c t of Ca 2+ or Mg 2* and s o d i u m silicate additions on flocculation-dispersion p h e n o m e n a o f quartz at pH 11. (No starch added.) 20 Starch
None
Quartz
Goethite
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Fig. 3. F l o c c u l a t i o n test results o n quartz and goethite at pH 11 in the presence o f 2.5 x 10 -4 M Ca 2÷ or 10 -3 M Mg 2+ showing the effect of corn starch at different levels o f sodiu m silicate addition.
been selected because at these levels quartz suspensions exhibited similar settling behavior (Fig. 1). It is observed in Fig. 3 that with Ca 2. and in the absence of starch, goethite was fully dispersed at a silicate addition of 100 p p m SIO2. An addition of starch flocculated t h e goethite suspension and lowered the suspended solids concentration. Quartz under these conditions showed no significant effect of starch on its flocculation behavior. In fact, with the same addition of sodium silicate, goethite in the presence of starch settled more than five times faster than did quartz. With Mg2÷, the results were similar. Quartz dispersed progressively with silicate addition in the absence of starch, and would be expected to behave similarly in the presence
o f starch. However, quartz ul the presence o f Mg 2÷ flocculated m o r e at. l o w e r silicate levels t h a n in the presence o f Ca 2~. The curvu for goethite in the presence of Mg 2+ and s o d i u m silicate was surmised f r o m selective flocculation tests which will be discussed s u b s e q u e n t l y . Just as in the t,resence o f Ca '+, g o e t h i t e flocculated in the presence o f corn starch at low s o d i u m silicate additions, but dispersed at higher levels. Also, for the c o n c e n t r a t i o n s o f Ca 2+ and Mg 2+ e m p l o y e d in these tests, the d i f f e r e n c e in s u s p e n d e d solids o f quartz and g o e t h i t e , at c o n s t a n t silicate levels, was c o n s i s t e n t l y larger in the p r e s e n c e o f Ca 2+ t h a n in the presence o f Mg 2+. This behavior was reflected in selective f l o c c u l a t i o n test results carried o u t u n d e r these conditions. F o r b o t h these systems, it was clear t h a t at low s o d i u m silicate levels, a d d i t i o n o f c o r n starch selectively f l o c c u l a t e d g o e t h i t e over quartz, while at higher silicate c o n c e n t r a t i o n s the addition o f starch was ineffective.
/1 Fig. 4a. Scanning electron micrographs o f quartz at pH 11 in the presence of Mg ~÷ w i t h s o d i u m silicate. A. 10 -3 M Mg 2+ and 4 p p m SiO~. B. 10 -3 M Mg 2÷ and 214 p p m SiO~. C. 3 x 10 -4 M Mg 2+ and 214 p p m SiO~. D. 5 x 10 -3 M Mg ~+ and 2 1 4 p p m SiO:.
STEM observations A small quantity of quartz sample was set aside after flocculation or abstraction tests for scanning electron microscope observations directly on quartz surfaces. These observations are shown in Fig. 4. Photomicrographs 4A and B show quartz surface at constant Mg 2+ addition of 10 -3 M (24 ppm) and silicate addition equivalent to 4 ppm SiO2 and 214 ppm SiO~, respectively. The amount of Mg(OH)2 precipitates dotting the surface is clearly seen to reduced significantly on higher silicate additions. Figures 4C and D show the surface under constant silicate addition and Mg 2+ addition of a b o u t 3 × 10-4M (7 ppm) and 5 × 10-3M (120 ppm), respectively. An increase of Mg(OH)2 precipitates on quartz surface as Mg 2+ was increased as can be seen. At the higher Mg 2+ levels, blebs of gelatinous precipitate, appearing different from Mg(OH)2, were observed on the surface of quartz. These were presumably magnesium silicate precipitates.
II II
0.5u
Fig. 4b. Scanning electron micrographs of quartz at p H 11 in the presence of Ca 2÷ with s o d i u m silicate. E. 10 -3 M C a 2+ and 1 0 0 0 p p m SiO 2. F. 10 -3 M C a 2÷ and 1 5 0 0 p p m SiO 2. G. 10 -3 M C a 2+ and 2 1 0 0 pprn SiO 2. H. 10 -3 M Ca 2+ and 3 0 0 0 p p m SiO~.
Photomicrographs of quartz surfaces obtained in the presence of constant additions of Ca 2. (10 .3 M) and increasing additions of sodium silicate axe presented in Fig. 4E through H. It is noted that a considerable amount of precipitates occurred on quartz surface at 1000 and 1500 ppm SiO2 increasing to a thick gelatinous coating at 2100 ppm SiO2 at which condition "all the Ca 2÷ added had been abstracted (Fig. 1). Higher additions of sodium silicate showed less precipitates on the surface. Further addition of sodium silicate to 3000 ppm SiO2 reduced the abstraction, showing correspondingly less precipitates on the surface. These photomicrographs thus reflect the results obtained by flocculation tests.
Selective flocculation tests From flocculation and abstraction tests on quartz suspensions, and STEM observations, it was apparent that an addition of sodium silicate resulted in removal of Mg(OH)2 from the surface of quartz leading to dispersion of the suspensions. For selective flocculation of a quartz-goethite mixture to be effective in the presence of corn starch, quartz must be dispersed and goethite flocculated. Selective flocculation tests were, therefore, performed to correlate the results obtained with individual suspensions with those obtained with mixed suspensions. Results of selective flocculation tests on artificial mixtures of quartz and goethite at p H II in the presence of Ca :+ and M g 2÷ are presented in Fig. 5. It is noted that an addition of 10 -3 M M g 2+ in the absence of sodium silicateresulted in indiscriminate flocculation of the entire pulp. W h e n sodium silicate equivalent to 100 p p m SiO2 was added reasonably good grade and recovery were obtained despite the presence of 10 -3 M M g 2+. Further addition of sodium silicate to 215 p p m SiO2 caused indiscriminate dispersion of the pulp; in fact at this silicatelevel individual suspensions of quartz showed full dispersion (see Fig. 1). With an addition of 2.5 × 10 -4 M Ca 2+, very good upgrading was obtained. This was probably because the Ca 2+ adsorbed on goethite surface providing anchor points for enhanced adsorption of starch on these surfaces. In fact, even in the absence of starch, individual suspensions of goethite settled 1.5 times faster than quartz when 2.5 × 1 0 - 4 M Ca 2÷ was added to the suspensions. In these tests, a Ca :+ concentration of 2.5 × 10 -4 M was chosen not only as a comparison against I0 -3 M M g 2+, as mentioned earlier, but also because flocculation tests on quartz showed that considerably higher amounts of sodium silicate, involving higher ionic strength and the presence of colloidal silicate complexes, would be required to disperse quartz suspensions containing more than 2.5 X 10 -4 M Ca 2+. Further addition of sodium silicate decreased grade and recovery, and fully dispersed the pulp. Though in flocculation tests, Ca :+ and M g 2+ levels of 2.5 X 1 0 - 4 M and 10 -3 M, respectively, showed similar effects with addition of sodium silicate, selective flocculation tests on an artificial mixture of quartz and goethite
under the same conditions showed upgrading to be effective at different levels of sodium silicate addition. In both systems, however, sodium silicate acted as a nonselective dispersant. A similar behavior was also reported in natural iron ores (Colombo, 1973).
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~
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Sodium Silicate Ca 2* Mg 2÷
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I
32 36 40 44 Percent Iron in Sands
_
None loo
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,2030
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i
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48
52
56
Fig. 5. Effect of sodium silicate on selective desliming of an artificial mixture o f goethite and quartz in the presence o f 2.5 × 10 -4 M Ca ~+ or 10 -3 M Mg 2+. (pH 11, corn starch 188 g/t.)
DISCUSSION Several investigations have reported on the nature, type and distribution of silicate species in aqueous solutions (Weldes and Lange, 1969; Harman, 1932; Iler, 1979). The aqueous chemistry of soluble silicates is complexly related to several factors, such as concentration, pH, temperature and SiO2/ Na~O ratio, making possible the existence of varying amounts of monomeric and multimeric silicate ions and polymeric ("colloidal") silicate micelles. A dynamic equilibrium exists among the different silicate species. Stumm and Morgan (1970) have summarized in a plot the type and concentration of stable silicate species in aqueous medium as a function of pH. The plot shows that monomeric species of silica are stable at low concentrations at all pH levels, and at high concentrations at higher pH levels. At pH 11, for concentrations up to 600 ppm SiO2, the stable silicate species are monomeric. Above 600 ppm SiO2, multimeric and "colloidal" silicate species also occur. Streaming potential measurements have shown that addition of sodium silicate to quartz suspensions in the absence of CaC12 or MgC12 resulted in decreasing zeta potentials (Krishnan and Iwasaki, 1982). Sodium silicate, therefore, acted more or less as indifferent ions to quartz. Addition of MgC12 to quartz suspensions at pH 11 in the absence of sodium silicate decreased
I0
and even reversed the charge on quartz (Heerema and Iwasaki, 1980l. This is known to be due to the heteroeoagulation of Mg(OH): onto quartz surfaces. In studies with Ca 2+ the zeta potentials were observed to increase in the negative direction as sodium silicate was added. A similar behavior may be expected to occur in the presence of Mg e' as the silicat.e ions in solution are attracted by the Mg ~-÷ and Mg(Otth on quartz s u r f a c e The strong tendency of Mg-'+ and silicate ions to form precipitates has been demonstrated by potentiometric titration results l Falcone, 1981t. [['he precipitates formed are gelatinous with the possible chemical formula of MgO.2SiO2.H~O (Friend, 1926). Hast (1956) identified the precipitate formed by mixing finely ground silica gel and MgO in water to be hydrous magnesium silicate, sepiolite (Meerschaum). The reported ability of Mg(OH)2 to reduce the silica content of water to very low levels shows the strong affinity between the precipitate and silicate species (Vail, 1952). Similar results have also been reported in selective floeculation studies on iron ores where addition of 0 to 26 ppm Mg 2+ at pH 11.0 reduced the soluble silica content of the pulp from 79 to 37 ppm (Colombo, 1973). Evidence of the interaction between Mg(OI-I)2 and silicate ions was also found in the present work where the stability of quartz suspensions depended markedly on the concentration ratio of Mg 2÷ to silicate ions. Quartz and goethite are well dispersed in highly alkaline pulps on account of their high negative charge. Addition of Mg 2÷ lowers the charge by the adsorption of Mg :+ and surface precipitation of Mg(OH)~, and flocculates the suspensions. Introduction of sodium silicate solutions to these suspensions results in the residual Mg 2. in solution precipitating as magnesium silicate, and the Mg(OHh on the surface of the minerals developing a surface coating of magnesium silicate. Freshly formed precipitates are highly reactive due to their nascent, metastable nature, large surface area and to incompletely coordinated ions at the surface (Kolthoff and Sandell, 1952; Salutsky, 1961). Furthermore, the magnesium silicate precipitates had favorable charge characteristics for attachment onto the surface of the minerals. Plausible models of magnesium hydroxide and starch molecules on quartz and goethite surfaces are summarized in Fig. 6. At pH 11, more than 99% of the Mg 2+ added is precipitated as Mg(OH)2, a large portion of which is attached to the surfaces of the minerals (Krishnan, 1982). Also, at this pH, Mg(OH)2 has a positive charge. At low silicate additions, the magnesium hydroxide precipitates on the surfaces of the minerals get coated with silicate. However, at this condition the overall charge on the precipitates is mildly positive enabling the starch molecules to adsorb on the mineral surfaces (Fig. 6a, d ). At pH 11, quartz covered with Mg(OH)2 has a zeta potential of about +30 mV (Iwasaki et al., 1980). When sodium silicate levels in the solution are increased, the silicate coats the Mg(OH)2 precipitates on the surface of the minerals and those in the solution. However, since the surfaces contain a much greater portion of the precipitate, they retain a net positive charge.
11
Quartz ÷
÷ ÷
©-
~-~
(a)
(b)
(c)
Sodium Silicate Addition Goethite
(d) 0
(el
If)
Magnesium Hydroxide Precipitate
c-~ Starch Molecule
Fig. 6. Models of magnesium hydroxide precipitates and starch goethite surfaces.
molecules
on
quartz and
The silicate-coated Mg(OH)2 precipitate in the solution is attracted to the surface of the minerals by coulombic interaction. This accounts for the increase in abstraction of Mg2. with silicate addition {Fig. 1). At increasing silicate additions, Mg(OH)2 precipitates become more and more negative due to the adsorption of silicate ions on their surface. Starch molecules are, therefore, repelled from quartz surfaces. However, due to their strong affinity to iron oxide surfaces, starch molecules are able to adsorb on goethite surface at lower silicate additions, but are prevented from adsorption by the shielding action of the precipitates at higher silicate additions. Consequently, the suspensions become dispersed due to the combined effect of charge and steric stabilization (Fig. 6b, e). At high silicate additions, especially above 600 ppm SiO2, colloidal aggregates of silicates are present in solutions (Harman, 1932; Iler, 1979). In the case of quartz, as a consequence of the highly negatively charged precipitate on the mineral surfaces of like charge, leading to strong electrostatic repulsion forces, the precipitates get dislodged from the surface. Starch molecules do not adsorb on quartz because quartz surfaces no longer have positively charged Mg(OH)2 precipitates on them (Fig. 6c). In the case of goethite, however, the precipitates remain on the surface of the mineral and prevent adsorption of starch molecules (Fig. 6f). A similar picture has been proposed for Ca2+-sodium silicate (Krishnan
12 a n d Iwasaki, 1982) a n d C a 2 ~ - p o l y p h o s p h a t e s y s t e m ( H e e r e m a et al., 1982). With s o d i u m silicate, c a l c i u m ions readily f o r m e d p r e c i p i t a t e s which c o a t e d the minerals. An a n a l o g o u s situation o c c u r r e d in the case o f calcium ions and p o l y p h o s p h a t e s . T h e shielding action of calcium silicate and c a l c i u m p o l y p h o s p h a t e p r e c i p t a t e s , respectively, on g o e t h i t e surfaces p r e v e n t e d a d s o r p t i o n o f starch o n t o these surfaces, leading to dispersion o f g o e t h i t e suspensions. Selective f l o c c u l a t i o n o f g o e t h i t e u n d e r these ~ o n d i t i o n s was, t h e r e f o r e , adversely a f f e c t e d . Similar b e h a v i o r s were also indicated in the investigations b y C o l o m b o ( 1 9 7 3 ) and R e a d (1971). F l o c c u l a t i o n and selective f l o c c u l a t i o n tests indicated t h a t a higher level o f Mg 2÷ t h a n Ca ~÷ c o u l d be t o l e r a t e d in g o e t h i t e - q u a r t z p u l p s with s o d i u m silicate a d d i t i o n . S u r f a c e c o a t i n g o f silicate o n m a g n e s i u m h y d r o x i d e precip i t a t e s a b s t r a c t e d o n g o e t h i t e p r e v e n t e d a d s o r p t i o n o f starch o n t o t h e iron oxide. D i s p e r s i o n o c c u r r e d d u e to t h e c o m b i n e d a c t i o n o f charge a n d steric stabilization. A t l o w levels o f s o d i u m silicate, and in t h e p r e s e n c e o f Ca 2÷ or Mg 2÷, c o r n starch with its s t r o n g a f f i n i t y f o r iron o x i d e surface, was able t o o v e r c o m e t h e charge o n t h e p r e c i p i t a t e a n d f l o c c u l a t e t h e mineral. A t high silicate a d d i t i o n s , c o r n starch was an ineffective f l o c c u l a n t . E f f e c t i v e desliming in t h e p r e s e n c e o f starch was achieved at l o w Ca 2÷ and Mg 2÷ ranges and at l o w s o d i u m silicate levels. ACKNOWLEDGEMENT T h e financial s u p p o r t p r o v i d e d for this p r o j e c t b y t h e N a t i o n a l Science Foundation under Grant ENG-7605835A02 and by the American Iron and Steel I n s t i t u t e u n d e r G r a n t 2 0 - 4 0 9 is g r a t e f u l l y a c k n o w l e d g e d .
REFERENCES
Colombo, A.F., 1973. An Evaluation of the Factors Affecting the Dispersion-Selective Flocculation -- Flotation of Iron Ores. Ph.D. Thesis, University of Minnesota. Dicks, M.L. and Morrow, J.B., 1978. Application of the selective flocculation-silica flotation process to Mesabi Range ores, Preprint No. 78-8-6-, AIME Annual Meeting, Denver. Falcone, Jr., J.S., 1981. Recent advances in the chemistry of sodium silicates: implications for ore beneficiation. Presented at SME--AIME Fall Meeting, Denver, Preprint No. 81-315. Friend, J.N., 1926. A Textbook of Inorganic Chemistry, Vol. 3. Charles Griffin, London, 86 pp. Harman, R.W., 1932. Aqueous solutions of sodium silicates, part VIII. General summary and theory of constitution. Sodium silicates as colloidal electrolytes. J. Phys. Chem., 32: 44--60. Hast, N., 1956. A reaction between silica and some magnesium compounds at room temperature and at +37°C. Ark. Kemi., 9: 343. Heerema, R.H. and Iwasaki, I., 1980. Chemical precipitation of alkaline earth cations and its effect on flocculation and flotation of quartz. Min. Eng., 32: 1510--1516. Heerema, R.H., Lipp, R.J. and Iwasaki, I., 1982. Complexation of calcium ion in selective flocculation of iron ore. Trans SME--AIME, 272: 1879--1884.
13 Iler, R.K., 1979. The Chemistry of Silica. Wiley-Interscience, New York, N.Y., 866 pp. Iwasaki, I. and Lipp, R.J., 1971. Flocculation and clarification of mineral suspensions. EPA Water Pollution Control Series No. 14010 DRB. Iwasaki, I., Smith, K.A., Lipp, R.J. and Sato, H., 1980. Effect of calcium and magnesium ions on selective desliming and cationic flotation of quartz from iron ores. In: P. Somasundaran (Editor), Proceedings of the International Symposium on Fine Particles Processing, Vol. 2. Las Vegas, Nevada. Kolthoff, I.M. and Sandell, E.B., 1952. Textbook of Quantitative Inorganic Analysis. Third Edition, MacMillan Company, New York, N.Y. Krishnan, S.V., 1982. Heterocoagulation and Surface Precipitation in Selective Flocculation of Iron Ores. Ph.D. Thesis, University of Minnesota. Krishnan, S.V. and Iwasaki, I., 1982. Sodium silicate as a dispersant in the selective flocculation of iron ores. Presented at SME-AIME Annual Meeting, Dallas, February, 1982, Preprint No. 82-51. Paananen, A.D. and Turcotte, W.A., 1980. Factors influencing selective flocculationdesliming practice at the Tilden Mine. Min. Eng., 32(8): 1244--1347. Read, A.D., 1971. Selective flocculation separations involving hematite. Trans., Inst. Min. Metatl., Sect. C, 80: 24--31. Salutsky, M.L., 1961. Treatise on Analytical Chemistry. Chapter 18. I.M. Kolthoff et al. (Editors), Park I(B) Vol. 1. Interscience, New York. Stumm, W. and Morgan, J.J., 1970. Aquatic Chemistry. Wiley-Interscience, New York, N.Y. 396 pp. Vail, J.G., 1952. Soluble Silicates, Vol. 1. ACS Monograph Series, Reinhold, New York, N.Y. Weldes, H.H. and Lange, R.R., 1969. Properties of soluble silicates. Ind. Eng. Chem., 61: 29--44.
PULP DISPERSION IN SELECTIVE DESLIMING OF IRON ORES
S.V. K R I S H N A N
I and I. I W A S A K I 2
IBatelle, Columbus Laboratories, 505 King Avenue, Columbus, O H 43201 (U.S.A.) 2 Mineral Resources Research Center, 56 East River Road, University of Minnesota,
Minneapolis, MN 55455 (U.S.A.) (Received June 13, 1982; accepted for publication March 28, 1983)
ABSTRACT
Krishnan, S.V. and Iwasaki, I., 1984. Pulp dispersion in selective desliming of iron ores. Int. J. Miner. Process., 12: 1--13. In the upgrading of finely disseminated iron ores, selective desliming is a critical step prior to flotation. A prerequisite for selective desliming is a properly dispersed pulp; sodium silicate is commonly used as a dispersant. The mechanism by which pulp dispersion is attained by means of sodium silicate in the presence of calcium ions and magnesium hydroxide precipitates was examined by settling tests, abstraction tests, selective flocculation tests and electron microscope observations. Flocculation and disperson regions of quartz as functions of calcium, magnesium and sodium silicate levels were delineated. Pulps consisting of artificial mixtures of quartz and goethite dispersed nonselectively on addition of sodium silicate. At low silicate levels, addition of corn starch aided the flocculation of goethite over quartz; at higher silicate levels, addition of starch was ineffective. A model is presented to explain pulp dispersion in the presence of these species.
INTRODUCTION
Process water quality is of major concern affecting the efficiency of desliming in selective flocculation of iron ores (Paananen and Turcotte, 1980; Dicks and Morrow, 1978). Pulp dispersion is influenced by hardness of process water, soluble silicate in the pulp, and temperature. Since a properly dispersed pulp is an essential prerequisite to selective desliming, the manner in which these factors affect pulp dispersion must be clearly understood. In iron ore pulps, presence of Ca 2+ and Mg 2÷ c o m m o n l y occurs from such sources as dissolution of calcium- and magnesium-containing minerals and clays associated with the ore, and hardness of water. The Ca 2+ and Mg 2+ contents of water equilibrated with iron ores have been found to be in the range of 10 -4 to 10 -3 M (Iwasaki et al., 1980). The silicate level in process pulps is contributed not only by the addition of sodium silicate as the dispersant, b u t also by the dissolution of the silicate minerals in the ore. At the pH ranges c o m m o n l y employed in selective flocculation of iron ores, Ca 2÷ is in an ionic form while Mg 2÷ precipitates as magnesium hydroxide. In an earlier
0301-7516/84/$03.00
© 1984
ElsevierScience Publishers B.V.
paper, the mechanism of dispersion of iron ores by sodium silicate in the presence of Ca 2÷ was delineated (Krishnan and Iwasaki, 1982). The present article compares the effect of Ca 2÷ and Mg~÷ on selective desliming and examines the manner by which pulp dispersion is affected in the presence of sodium silicate and magnesium hydroxide precipitates. MATERIALS AND METHODS The quartz sample used in this study was prepared from high-purity Brazilian rock crystal. The goethite sample was prepared from hand-picked pure lumps from the Cuyuna Range, Minnesota. The samples were sized to --60 ~m for flocculation and selective desliming tests. Details on the preparation of the samples have been reported (Iwasaki and Lipp, 1971). N-sodium silicate, obtained from the PQ Corporation, had a SiO~/Na20 ratio of 3.22 and a silica content of 28.7%. Fresh solutions were made by suitably diluting the as-received sample. Magnesium chloride used was of analytical grade. Sodium hydroxide was used to adjust pH. All tests were performed using distilled water. The experimental procedures for flocculation and selective desliming tests have been reported (Heerema and Iwasaki, 1980; Heerema et al., 1982). After settling tests, the suspension was poured on a 270 mesh screen, washed thoroughly with denatured alcohol, and transferred to a beaker. The solids were then dried, weighed, and leached with hydrochloric acid. The leachate was analyzed for Mg by atomic absorption to give abstraction density. A scanning transmission electron microscope {STEM), JEOL Model 100 CX, was used to observe the surface of quartz samples.
EXPERIMENTAL RESULTS The manner by which selective flocculation of goethite-quartz mixtures is affected by the presence of Ca 2+, Mg2÷ and sodium silicate was examined by flocculation tests, abstraction tests, scanning electron microscope observations and selective flocculation tests. Flocculation tests
The influence of calcium ions and magnesium hydroxide precipitates on the settling behavior of quartz, and the effectiveness of sodium silicate in dispersing the suspension in the presence of these species were examined by a series of flocculation tests. The tests were performed in the absence of starch in order to study the interactions between the mineral and the chemical species exclusively and then to correlate the results with those obtained in the presence of starch. Settling behavior of quartz in the presence of Ca 2+ and Mg2+ at pH 11 as a function of sodium silicate addition is shown in Fig. 1. It is observed that, as
a general trend, settling decreased progressively with silicate addition. In the presence of 2.5 × 10-4M and 10-3M Ca 2÷, the suspension was fully dispersed at sodium silicate additions of 215 ppm SiO2 and 2100 ppm SIO2, respectively. Full dispersion in the presence of 10 -3 M Mg 2÷ was attained at a sodium silicate addition of 215 ppm SiO2.
I
==\ ,-- E ¢o
¢n
o 10-3 M Mg 2+ 42.5 x 10"4 M Ca2÷ o 10 "3 M Ca2+ • Mg+Si+Qz 10 -3 M Mg 2+
1if
1
O-- ,
~
, ,,I
0
,I
10-5 10 "3 M Mg 2+ "~
10. 6
~
-
10 "3 M Ca2+
~
g° ~o ~ E10 7 < 10. 8
2.5 x. 10"4 M Ca2+ , , ,,l , , ,,i , , ,,I , , ,,I 10 ' 1O0 1000 10,000 Sodium Silicate, ppm SiO 2
Fig. I. Flocculation test results o n quartz at pH i i as function of sodium silicate addition and in the presence o f Ca 2÷ or Mg 2+. (No starch added.)
It is noted that in the presence of the same amounts o f Ca 2÷ and Mg 2÷, viz., 10 -3 M, considerably more sodium silicate was needed to disperse suspensions with Ca 2÷ than those with Mg 2÷. Also, the settling behavior of quartz in the presence of 10 -3 M Mg 2+ corresponded closely with that in the presence of 2.5 × 10-4M Ca 2÷. Yet, as well be seen subsequently, the abstraction behavior in quartz suspensions and selective flocculation results of mixed suspensions at these conditions differed quite appreciably. These differences can be related to the fact that Ca 2+ is an ionic species and Mg 2÷ is precipitated as Mg(OH)2 at this pH, and to the differences in interaction between these species and sodium silicate. Suspended solids concentrations reflected, more or less, the trends observed in the settling curves. It is noted that at each concentration of the cations, suspended:solids concentration increased sharply above a certain value
of sodium silicate addition. This observation can be related to the abstraction of Ca ~÷ or Mg ~÷ on quartz. At each concentration of Ca 2÷ or Mg ~÷ studied, the abstraction density curves were observed to increase with sodium silicate addition until all the Ca 2÷ or Mg2÷ added was abstracted, and to decrease thereafter. Comparing the abstraction curves with the suspended solids curves, it is seen that the suspension started to disperse beyond the maximum level of Mg2÷ abstraction, but was well-dispersed at or below the m a x i m u m level of Ca 2÷ abstraction. It was noted in tests with Mg2÷ that when the sequence of reagent additions was altered such that sodium silicate was added first to a quartz suspension followed by MgC12, or a mixture of MgC12 and silicate solutions were added to quartz (indicated in Fig. 1 as Mg + Si + Qz), no change in settling rate or abstraction density was observed. However, sequence of addition of sodium silicate and CaC12 resulted in an appreciable change in abstraction density of Ca 2÷ on quartz surface (Krishnan and Iwasaki, 1982). When calcium silicate precipitate was mixed with quartz, abstraction of the precipitates onto quartz was low and remained essentially unchanged with increase in sodium silicate addition. This was attributed to the repulsion between the negatively charged precipitates, due to excess silicate ions, and the likecharged quartz surface. When a quartz suspension was conditioned with Ca ~÷ first and then sodium silicate added, Ca 2÷ abstraction increased until all the Ca 2÷ was abstracted, and then decreased, as noted in Fig. 1. The effects of sequence of reagent addition with Ca 2÷ demonstrates the difference in heterocoagulation and surface precipitation. With Mg 2÷, however, heterocoagulation of Mg(OH)2 precipitates with their surface properties modified by silicate ions must have played a d o m i n a n t role. Figure 2 shows the flocculation and dispersion regions of quartz as functions of Ca ~÷ or Mg2÷ and sodium silicate additions. It is noted that below about 2 × 10 -4 M Ca 2÷ or Mg 2~, quartz suspensions remained dispersed with little or no sodium silicate addition. Also, for the same level of sodium silicate addition, a higher a m o u n t of Mg 2÷ could be tolerated than Ca 2÷, again showing the differences in the interactions of Ca 2÷ and Mg(OH)2 precipitate with sodium silicate. Stability curve for goethite suspensions in the presence of Ca 2÷ reported earlier indicated that a lower concentration of sodium silicate was required to disperse goethite than quartz (Krishnan and Iwasaki, 1982). For selective desliming, quartz must be fully dispersed and goethite flocculated. The results obtained in the absence of starch suggested that starch perhaps held the key to selective desliming in this system. In order to relate the results of Figs. 1 and 2 meaningfully to those of selective flocculation tests with corn starch, settling tests were carried out to study the effect of starch on quartz and on goethite in the presence of Ca 2÷ and Mg2÷. The results are presented in Fig. 3, which shows the suspended solids concentrations of quartz and goethite at pH 11 as a function of sodium silicate additions with and without starch, and at a constant Ca 2÷ or Mg2÷ level. A Ca :÷ level of 2.5 × 1 0 -4 M and a Mg2. level of 10 -3 M have
10-2, o Ca 2+ o Mg 2+ "0 /
~10-3
•
.'/
,/
Flocculati on~ ispersion
U Monomeric ~
10"10
,
i
~
~l
i
J
M ultimeric ~ - ~ P o l y m e r i c J
,
I
,
i
100
1000 Sodium Silicate, ppm SiO 2
•
,
I
i
i
10 , 0 0 0
Fig. 2. E f f e c t of Ca 2+ or Mg 2* and s o d i u m silicate additions on flocculation-dispersion p h e n o m e n a o f quartz at pH 11. (No starch added.) 20 Starch
None
Quartz
Goethite
Ca 2÷ Mg 2+ Ca 2+ Mg2 + ~ - ~ - - - O O /x ~" /
v~lO
~=E
~,.0.
"
///e s
A
/
/
100 Sodium
/
"/ 200
Silicate,
ppm
300 SiO 2
Fig. 3. F l o c c u l a t i o n test results o n quartz and goethite at pH 11 in the presence o f 2.5 x 10 -4 M Ca 2÷ or 10 -3 M Mg 2+ showing the effect of corn starch at different levels o f sodiu m silicate addition.
been selected because at these levels quartz suspensions exhibited similar settling behavior (Fig. 1). It is observed in Fig. 3 that with Ca 2. and in the absence of starch, goethite was fully dispersed at a silicate addition of 100 p p m SIO2. An addition of starch flocculated t h e goethite suspension and lowered the suspended solids concentration. Quartz under these conditions showed no significant effect of starch on its flocculation behavior. In fact, with the same addition of sodium silicate, goethite in the presence of starch settled more than five times faster than did quartz. With Mg2÷, the results were similar. Quartz dispersed progressively with silicate addition in the absence of starch, and would be expected to behave similarly in the presence
o f starch. However, quartz ul the presence o f Mg 2÷ flocculated m o r e at. l o w e r silicate levels t h a n in the presence o f Ca 2~. The curvu for goethite in the presence of Mg 2+ and s o d i u m silicate was surmised f r o m selective flocculation tests which will be discussed s u b s e q u e n t l y . Just as in the t,resence o f Ca '+, g o e t h i t e flocculated in the presence o f corn starch at low s o d i u m silicate additions, but dispersed at higher levels. Also, for the c o n c e n t r a t i o n s o f Ca 2+ and Mg 2+ e m p l o y e d in these tests, the d i f f e r e n c e in s u s p e n d e d solids o f quartz and g o e t h i t e , at c o n s t a n t silicate levels, was c o n s i s t e n t l y larger in the p r e s e n c e o f Ca 2+ t h a n in the presence o f Mg 2+. This behavior was reflected in selective f l o c c u l a t i o n test results carried o u t u n d e r these conditions. F o r b o t h these systems, it was clear t h a t at low s o d i u m silicate levels, a d d i t i o n o f c o r n starch selectively f l o c c u l a t e d g o e t h i t e over quartz, while at higher silicate c o n c e n t r a t i o n s the addition o f starch was ineffective.
/1 Fig. 4a. Scanning electron micrographs o f quartz at pH 11 in the presence of Mg ~÷ w i t h s o d i u m silicate. A. 10 -3 M Mg 2+ and 4 p p m SiO~. B. 10 -3 M Mg 2÷ and 214 p p m SiO~. C. 3 x 10 -4 M Mg 2+ and 214 p p m SiO~. D. 5 x 10 -3 M Mg ~+ and 2 1 4 p p m SiO:.
STEM observations A small quantity of quartz sample was set aside after flocculation or abstraction tests for scanning electron microscope observations directly on quartz surfaces. These observations are shown in Fig. 4. Photomicrographs 4A and B show quartz surface at constant Mg 2+ addition of 10 -3 M (24 ppm) and silicate addition equivalent to 4 ppm SiO2 and 214 ppm SiO~, respectively. The amount of Mg(OH)2 precipitates dotting the surface is clearly seen to reduced significantly on higher silicate additions. Figures 4C and D show the surface under constant silicate addition and Mg 2+ addition of a b o u t 3 × 10-4M (7 ppm) and 5 × 10-3M (120 ppm), respectively. An increase of Mg(OH)2 precipitates on quartz surface as Mg 2+ was increased as can be seen. At the higher Mg 2+ levels, blebs of gelatinous precipitate, appearing different from Mg(OH)2, were observed on the surface of quartz. These were presumably magnesium silicate precipitates.
II II
0.5u
Fig. 4b. Scanning electron micrographs of quartz at p H 11 in the presence of Ca 2÷ with s o d i u m silicate. E. 10 -3 M C a 2+ and 1 0 0 0 p p m SiO 2. F. 10 -3 M C a 2÷ and 1 5 0 0 p p m SiO 2. G. 10 -3 M C a 2+ and 2 1 0 0 pprn SiO 2. H. 10 -3 M Ca 2+ and 3 0 0 0 p p m SiO~.
Photomicrographs of quartz surfaces obtained in the presence of constant additions of Ca 2. (10 .3 M) and increasing additions of sodium silicate axe presented in Fig. 4E through H. It is noted that a considerable amount of precipitates occurred on quartz surface at 1000 and 1500 ppm SiO2 increasing to a thick gelatinous coating at 2100 ppm SiO2 at which condition "all the Ca 2÷ added had been abstracted (Fig. 1). Higher additions of sodium silicate showed less precipitates on the surface. Further addition of sodium silicate to 3000 ppm SiO2 reduced the abstraction, showing correspondingly less precipitates on the surface. These photomicrographs thus reflect the results obtained by flocculation tests.
Selective flocculation tests From flocculation and abstraction tests on quartz suspensions, and STEM observations, it was apparent that an addition of sodium silicate resulted in removal of Mg(OH)2 from the surface of quartz leading to dispersion of the suspensions. For selective flocculation of a quartz-goethite mixture to be effective in the presence of corn starch, quartz must be dispersed and goethite flocculated. Selective flocculation tests were, therefore, performed to correlate the results obtained with individual suspensions with those obtained with mixed suspensions. Results of selective flocculation tests on artificial mixtures of quartz and goethite at p H II in the presence of Ca :+ and M g 2÷ are presented in Fig. 5. It is noted that an addition of 10 -3 M M g 2+ in the absence of sodium silicateresulted in indiscriminate flocculation of the entire pulp. W h e n sodium silicate equivalent to 100 p p m SiO2 was added reasonably good grade and recovery were obtained despite the presence of 10 -3 M M g 2+. Further addition of sodium silicate to 215 p p m SiO2 caused indiscriminate dispersion of the pulp; in fact at this silicatelevel individual suspensions of quartz showed full dispersion (see Fig. 1). With an addition of 2.5 × 10 -4 M Ca 2+, very good upgrading was obtained. This was probably because the Ca 2+ adsorbed on goethite surface providing anchor points for enhanced adsorption of starch on these surfaces. In fact, even in the absence of starch, individual suspensions of goethite settled 1.5 times faster than quartz when 2.5 × 1 0 - 4 M Ca 2÷ was added to the suspensions. In these tests, a Ca :+ concentration of 2.5 × 10 -4 M was chosen not only as a comparison against I0 -3 M M g 2+, as mentioned earlier, but also because flocculation tests on quartz showed that considerably higher amounts of sodium silicate, involving higher ionic strength and the presence of colloidal silicate complexes, would be required to disperse quartz suspensions containing more than 2.5 X 10 -4 M Ca 2+. Further addition of sodium silicate decreased grade and recovery, and fully dispersed the pulp. Though in flocculation tests, Ca :+ and M g 2+ levels of 2.5 X 1 0 - 4 M and 10 -3 M, respectively, showed similar effects with addition of sodium silicate, selective flocculation tests on an artificial mixture of quartz and goethite
under the same conditions showed upgrading to be effective at different levels of sodium silicate addition. In both systems, however, sodium silicate acted as a nonselective dispersant. A similar behavior was also reported in natural iron ores (Colombo, 1973).
"o
9o
C
~11/
~
80 z l "~,
g
~
Sodium Silicate Ca 2* Mg 2÷
\
1
28
s,o
~
k
I
I
I
32 36 40 44 Percent Iron in Sands
_
None loo
; o
,2030
[]
× •
-
•
i
[
48
52
56
Fig. 5. Effect of sodium silicate on selective desliming of an artificial mixture o f goethite and quartz in the presence o f 2.5 × 10 -4 M Ca ~+ or 10 -3 M Mg 2+. (pH 11, corn starch 188 g/t.)
DISCUSSION Several investigations have reported on the nature, type and distribution of silicate species in aqueous solutions (Weldes and Lange, 1969; Harman, 1932; Iler, 1979). The aqueous chemistry of soluble silicates is complexly related to several factors, such as concentration, pH, temperature and SiO2/ Na~O ratio, making possible the existence of varying amounts of monomeric and multimeric silicate ions and polymeric ("colloidal") silicate micelles. A dynamic equilibrium exists among the different silicate species. Stumm and Morgan (1970) have summarized in a plot the type and concentration of stable silicate species in aqueous medium as a function of pH. The plot shows that monomeric species of silica are stable at low concentrations at all pH levels, and at high concentrations at higher pH levels. At pH 11, for concentrations up to 600 ppm SiO2, the stable silicate species are monomeric. Above 600 ppm SiO2, multimeric and "colloidal" silicate species also occur. Streaming potential measurements have shown that addition of sodium silicate to quartz suspensions in the absence of CaC12 or MgC12 resulted in decreasing zeta potentials (Krishnan and Iwasaki, 1982). Sodium silicate, therefore, acted more or less as indifferent ions to quartz. Addition of MgC12 to quartz suspensions at pH 11 in the absence of sodium silicate decreased
I0
and even reversed the charge on quartz (Heerema and Iwasaki, 1980l. This is known to be due to the heteroeoagulation of Mg(OH): onto quartz surfaces. In studies with Ca 2+ the zeta potentials were observed to increase in the negative direction as sodium silicate was added. A similar behavior may be expected to occur in the presence of Mg e' as the silicat.e ions in solution are attracted by the Mg ~-÷ and Mg(Otth on quartz s u r f a c e The strong tendency of Mg-'+ and silicate ions to form precipitates has been demonstrated by potentiometric titration results l Falcone, 1981t. [['he precipitates formed are gelatinous with the possible chemical formula of MgO.2SiO2.H~O (Friend, 1926). Hast (1956) identified the precipitate formed by mixing finely ground silica gel and MgO in water to be hydrous magnesium silicate, sepiolite (Meerschaum). The reported ability of Mg(OH)2 to reduce the silica content of water to very low levels shows the strong affinity between the precipitate and silicate species (Vail, 1952). Similar results have also been reported in selective floeculation studies on iron ores where addition of 0 to 26 ppm Mg 2+ at pH 11.0 reduced the soluble silica content of the pulp from 79 to 37 ppm (Colombo, 1973). Evidence of the interaction between Mg(OI-I)2 and silicate ions was also found in the present work where the stability of quartz suspensions depended markedly on the concentration ratio of Mg 2÷ to silicate ions. Quartz and goethite are well dispersed in highly alkaline pulps on account of their high negative charge. Addition of Mg 2÷ lowers the charge by the adsorption of Mg :+ and surface precipitation of Mg(OH)~, and flocculates the suspensions. Introduction of sodium silicate solutions to these suspensions results in the residual Mg 2. in solution precipitating as magnesium silicate, and the Mg(OHh on the surface of the minerals developing a surface coating of magnesium silicate. Freshly formed precipitates are highly reactive due to their nascent, metastable nature, large surface area and to incompletely coordinated ions at the surface (Kolthoff and Sandell, 1952; Salutsky, 1961). Furthermore, the magnesium silicate precipitates had favorable charge characteristics for attachment onto the surface of the minerals. Plausible models of magnesium hydroxide and starch molecules on quartz and goethite surfaces are summarized in Fig. 6. At pH 11, more than 99% of the Mg 2+ added is precipitated as Mg(OH)2, a large portion of which is attached to the surfaces of the minerals (Krishnan, 1982). Also, at this pH, Mg(OH)2 has a positive charge. At low silicate additions, the magnesium hydroxide precipitates on the surfaces of the minerals get coated with silicate. However, at this condition the overall charge on the precipitates is mildly positive enabling the starch molecules to adsorb on the mineral surfaces (Fig. 6a, d ). At pH 11, quartz covered with Mg(OH)2 has a zeta potential of about +30 mV (Iwasaki et al., 1980). When sodium silicate levels in the solution are increased, the silicate coats the Mg(OH)2 precipitates on the surface of the minerals and those in the solution. However, since the surfaces contain a much greater portion of the precipitate, they retain a net positive charge.
11
Quartz ÷
÷ ÷
©-
~-~
(a)
(b)
(c)
Sodium Silicate Addition Goethite
(d) 0
(el
If)
Magnesium Hydroxide Precipitate
c-~ Starch Molecule
Fig. 6. Models of magnesium hydroxide precipitates and starch goethite surfaces.
molecules
on
quartz and
The silicate-coated Mg(OH)2 precipitate in the solution is attracted to the surface of the minerals by coulombic interaction. This accounts for the increase in abstraction of Mg2. with silicate addition {Fig. 1). At increasing silicate additions, Mg(OH)2 precipitates become more and more negative due to the adsorption of silicate ions on their surface. Starch molecules are, therefore, repelled from quartz surfaces. However, due to their strong affinity to iron oxide surfaces, starch molecules are able to adsorb on goethite surface at lower silicate additions, but are prevented from adsorption by the shielding action of the precipitates at higher silicate additions. Consequently, the suspensions become dispersed due to the combined effect of charge and steric stabilization (Fig. 6b, e). At high silicate additions, especially above 600 ppm SiO2, colloidal aggregates of silicates are present in solutions (Harman, 1932; Iler, 1979). In the case of quartz, as a consequence of the highly negatively charged precipitate on the mineral surfaces of like charge, leading to strong electrostatic repulsion forces, the precipitates get dislodged from the surface. Starch molecules do not adsorb on quartz because quartz surfaces no longer have positively charged Mg(OH)2 precipitates on them (Fig. 6c). In the case of goethite, however, the precipitates remain on the surface of the mineral and prevent adsorption of starch molecules (Fig. 6f). A similar picture has been proposed for Ca2+-sodium silicate (Krishnan
12 a n d Iwasaki, 1982) a n d C a 2 ~ - p o l y p h o s p h a t e s y s t e m ( H e e r e m a et al., 1982). With s o d i u m silicate, c a l c i u m ions readily f o r m e d p r e c i p i t a t e s which c o a t e d the minerals. An a n a l o g o u s situation o c c u r r e d in the case o f calcium ions and p o l y p h o s p h a t e s . T h e shielding action of calcium silicate and c a l c i u m p o l y p h o s p h a t e p r e c i p t a t e s , respectively, on g o e t h i t e surfaces p r e v e n t e d a d s o r p t i o n o f starch o n t o these surfaces, leading to dispersion o f g o e t h i t e suspensions. Selective f l o c c u l a t i o n o f g o e t h i t e u n d e r these ~ o n d i t i o n s was, t h e r e f o r e , adversely a f f e c t e d . Similar b e h a v i o r s were also indicated in the investigations b y C o l o m b o ( 1 9 7 3 ) and R e a d (1971). F l o c c u l a t i o n and selective f l o c c u l a t i o n tests indicated t h a t a higher level o f Mg 2÷ t h a n Ca ~÷ c o u l d be t o l e r a t e d in g o e t h i t e - q u a r t z p u l p s with s o d i u m silicate a d d i t i o n . S u r f a c e c o a t i n g o f silicate o n m a g n e s i u m h y d r o x i d e precip i t a t e s a b s t r a c t e d o n g o e t h i t e p r e v e n t e d a d s o r p t i o n o f starch o n t o t h e iron oxide. D i s p e r s i o n o c c u r r e d d u e to t h e c o m b i n e d a c t i o n o f charge a n d steric stabilization. A t l o w levels o f s o d i u m silicate, and in t h e p r e s e n c e o f Ca 2÷ or Mg 2÷, c o r n starch with its s t r o n g a f f i n i t y f o r iron o x i d e surface, was able t o o v e r c o m e t h e charge o n t h e p r e c i p i t a t e a n d f l o c c u l a t e t h e mineral. A t high silicate a d d i t i o n s , c o r n starch was an ineffective f l o c c u l a n t . E f f e c t i v e desliming in t h e p r e s e n c e o f starch was achieved at l o w Ca 2÷ and Mg 2÷ ranges and at l o w s o d i u m silicate levels. ACKNOWLEDGEMENT T h e financial s u p p o r t p r o v i d e d for this p r o j e c t b y t h e N a t i o n a l Science Foundation under Grant ENG-7605835A02 and by the American Iron and Steel I n s t i t u t e u n d e r G r a n t 2 0 - 4 0 9 is g r a t e f u l l y a c k n o w l e d g e d .
REFERENCES
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