Crystallization phenomena and the floatability of sylvine and hard salt

COLLOIDS

ELSEVIER

Colloids and Surfaces A: Physicochemical and Engineering Aspects

SUURACES 88 (1994) 91-101

Crystallization phenomena and the floatability of sylvine and hard salt H. Stechemesser

a,

K. Volke

b,

Th. Jung"

Max-Planck-Institut fur Xolloid- and Grenzfldchenforschung, Institutsteil Freiberg, Chemnitzer Str . 40, D-09599 Freiberg, Germany " Freiberg University of Mining and Metallurgy, Akademiestraffe 6, D-09599 Freiberg, Germany c Unstrut-Kieswerk, D-06578 Oldisleben, Germany

Received 20 August 1993; accepted 14 December 1993

Abstract For various reasons, difficulties arise in the flotation of sylvinite and kieseritic hard salt, resulting in considerable quality losses in potash production . High carnallite amounts in the crude salt and as a consequence different MgCl 2 concentrations in the brine are the most important reasons for the difficulties . The aim of this study was to characterize the nature of the surface of sylvine in brines of different MgCl, concentrations, i .e . to prove the existence of time-dependent partial solution effects on the salt surface and/or of crystallization phenomena leading to the formation of new mineral phases as centres of selective adsorption of surfaceactive substances, and to examine the influence of these alterations on floatability . By means of a comparison between laboratory flotation results, adsorption measurements, radiotracer measurements, and measurements of adhesion forces as well as scanning electron microscopy investigations, it was shown that the problems in the kieseritic hard salt (Rossleben, Germany) are mainly caused by the so-called red variety of sylvine . The conclusions of these investigations are, firstly, that improvement of flotation characteristics is possible by conditioning the flotation feed in the brine for a period of time, depending on the MgCI 2 concentration, before the addition of reagents to the brine and before the floatation . Secondly, the flotation-improving effect of the clay depressant Amysed-W is caused by the interaction of this reagent with the red sylvine and with the collector Rofamin by forming an inclusion compound of Amysed-W with the amine .

Keywords : Camallite decomposition; Crystallization ; Flotation ; Inclusion compound ; Potash ores

1 . Introduction

These were the reasons for a systematic characterization of the hard salt flotation process in the Rossleben potash processing plant, Germany, and of the sylvinite flotation process in the Zielitz

Sylvine concentration from hard salt is undoubtedly one of the most difficult tasks in the field of salt flotation . Comparing hard salt flotation and sylvinite flotation, reduced sylvine recoveries and concentrations as well as a smaller upper floatable grain size make the problem obvious .

potash processing plant, Germany, using physical and physicochemical methods [1] . Flotation conditions used in the two potash plants are presented in Table 1 .

* Corresponding author.

The main difference between the two plants is the Mg" ion concentration in the brines as well

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H. Stechemesser et al.lColloids Surfaces A : Physicochem Eng. Aspects 88 (1994) 91-101

Table I h' o ion con t tuns m

e

tsossa en anon ate t zYiz playas

Plant

Deposit

Composition

Sylvine

Brine

Brine concentration (g 1- ')

Rossleben

Kieseritic hard salt (Schoenite field)

20% sylvine 50% halite 15% kieserite 5% carnallite Clayish impurities

White and red

KCI NaCI MgSO 4 MgCl'

90 110 89 113

Ziehtz

Sylvinite (Sylvinite field)

20% sylvine 70% halite 3% carnallite

White

KCI NaCl MgSO4 MgC12

100 165 15 90

as the fact that 50% of the Rossleben sylvine is of the red variety . Owing to the high SO ;' ion concentration in the brine of the Rossleben potash plant, the flotation-effective surface can decrease during the flotation process because of sulphate overgrowth formations on the salt surface . DShler [2] and Schubert [3] have already pointed out that sylvine may react to form sulphate double salts, particularly schoenite, and have recommended high MgC1 2 concentrations and an as short as possible retention time of the feed material in the brine in order to diminish this surface change. The clayish impurities of the Rossleben deposit make necessary the use of a depressant which will be adsorbed by these impurities, thus reducing collector losses . For this purpose, Amysed-W, an oxidized hot-water-soluble potato starch, is used . KShler and Konratzki [4] have studied the influence of some organic polyelectrolytes on the flotation of a sylvinite crude salt. Their role as clay depressants as well as their effect as flocculating agents for sulphate salt sludges are shown and it is emphasized that there is a positive effect on sylvine flotation as well, if the crude salt does not contain clayish impurities or only a small amount of sulphate . It is assumed that the retardation of crystallization is affected [5] . Arsentiev et al . [6] have tested several depressants for the flotation of sylvinite with a high clay content. When combining polyethylene amine and starch, a clear synergistic effect could be observed, but was not interpreted.

The aim of the studies was to characterize the nature of the surface of water-soluble salts, particularly sylvine and halite in brines of different MgCI 2 concentrations, i .e. to demonstrate the existence of time-dependent partial solution effects on the salt surface and/or of crystallization phenomena leading to the formation of new mineral phases, and to examine the influence of these alterations on floatability . Further, the studies should contribute to the establishment of a mechanism of starch action in sylvine flotation .

2. Experimental 2 .1 . Experimental conditions

The experiments were performed under the conditions prevailing in the Zielitz and Rossleben potash plants (Table 1) . It must be pointed out that the brine contains different Mg ion concentrations and that the Rossleben sylvine consists of 50% of the red variety . In order to demonstrate the above timedependent alterations in all the experiments the samples were removed for analysis at intervals of 5, 15, 30 and 60 min after conditioning the salts in the brine . All the experiments were carried out at 23°C . The following reagents were used : 152 g t - t of Rofamin, 14 g t - t of Oktanediol and 232 g t - t of Amysed-W for the Rossleben potash plant ; 64 g

H. Slechemesser et al . lColloids Surfaces A : Physicochem . Eng. Aspects 88 (1994) 91-101

t - ' of Rofamin, 19 g t - ' of Oktanediol and 30 g t - ' of kerosines for the Zielitz potash plant.

2 .2 . Materials 2 .2 .1 . Surfactants

As the collector for sylvinite, Rofamin, a mixture of primary aliphatic fatty amines with a maximum carbon chain length of 16-18 carbon atoms, produced in the chemical plant at Rossleben, Germany, was used . For the depression of the clay components of the feed material the water-soluble starch derivative Amysed-W from the Dallmin starch producing plant, Germany, was applied . Octanediol (Chemische Werke Buna, Germany), a mixture of isomeric, aliphatic, branched C 8 diols, proved to be an effective foaming agent .

2 .3 . Methods 2 .3 .1 . Scanning electron microscopy

Surface analysis was carried out using a scanning electron microscope (REM-Tesla BS 340) and an energy-dispersive X-ray analyser (EDX, type KEVEX) . The sylvine samples (0.1 g ; grain size range 0.315-0 .500 mm) were conditioned in the brine for the periods described above, screened on a 250 gm sieve, washed with ethanol and then analysed by the method of Herrmann et al. [7] .

2 .3 .2. Measurements of the adhesion forces

The adhesive strength of particles to the liquid/gas interface represents a characteristic measure of the stability of bubble-particle aggregates . This quantity was measured using the centrifuge method [8] . In the glass cell of a laboratory centrifuge a particle floats on the brine surface . This particle is exposed to a continuously increasing tensile force. The centrifugal acceleration a. acting at the moment of disrupture (which is many times the gravitational acceleration) is a measure of the force of adhesion . In each case, 100 particles almost isomeric in form and of grain size of 0.5-1 mm were tested .

93

2 .3 .3. Laboratory flotation test

The floatation tests were carried out in a 11 flotation cell (double-finger agitator, 1400 rev min - ', specific air flow rate, 2m' m - ' min - ' ; solids concentration, 400 g 1 - ' according to the scheme summarized in Table 2. The agitator speed was adapted to the 1 s criterion [9] . As test materials, sylvinite crude salt (the screening number d 95 =1 mm) from the screen underfiow of the dry classification in the Zielitz potash plant, and kieseritic hard salt (d95 =0.6mm) from the flotation feed of the Rossleben potash plant were used . 2 .3 .4 . Radiotracer measurements

Crystallization phenomena in carnallitecontaining brines were demonstrated by means of brines with added 92 KC1 tracer . The 1 .51 MeV y line of 42K was analysed with a y-ray spectrometer (type 20050 from Robotron, Germany) . Rossleben and Zielitz crude salts (400 g of each) were stirred for 1 h in 695 ml of brine which had been mixed with 0.5 ml of an aqueous solution of 42KCI (340 MBq) . After conditioning intervals of 5, 15, 30 and 60 min, solid samples were removed and the finest grains and adhering brine were separated at a cut size of 40 µm by wet screening with alcohol . The high rate of dissolution of carnallite [10] leads to a KCl oversaturation followed by KCI crystallization. The maximum grain size of the fine secondary crystallites formed as a result of overnucleation is 15 pm, thus justifying a cut size of 40 µm. Therefore the `KCl activity detected in the solid material seems to be caused mainly by crystal overgrowth formation, and not so much by the amount of finest grains produced by the nucleation . 2 .3 .5 . Adsorption tests

The estimation of the amines was accomplished photometrically according to the Silverstein method, i .e . by complex formation of the cationic collector with Methyl Orange and extraction using chloroform [11] . The sylvines to be examined (white, red) were obtained by hand sorting and sink-float separation at 2.1 g cm - ' and then cleaned by washing with ethanol. The grain size was within the range



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Table 2 Time sequences for reagent addition and conditioning for the flotation of hard salt and sylvinite in the Rossleben and respectively

Brine

Zielitz brines

Time sequences Conditioning

Flotation test procedure

Rossleben

Before reagent addition

(1) Agitation of the hard salt at 23`C with different conditioning times (5, 15, 30, 60 min) in the brine (2) Addition of reagents in the sequence Amysed-W, Rofamin, Oktanediol over a period of 3 min (3) Aeration and flotation over a period of 3 min

Rossleben

After reagent addition

(1) Agitation for 1 min of the hard salt at 23'C in the brine (2) Addition of reagents in the sequence Amysed-W, Rofamin, Oktanediol at different conditioning times (0, 10, 25, 55 min) in the brine (3) Aeration and flotation over a period of 3 min

Zielitz

Before reagent addition

(1) Agitation for 4 min of the sylvinite at 23`C in the brine (2) Addition of reagents in the sequence Rofamin, Oktanediol, kerosine and agitation at different conditioning times (0, 10, 25, 55 min) in the brine (3) Aeration and flotation over a period of 3 min

0 .1-0 .16 mm and the specific surface area -t. amounted to 0.047 mz g Corresponding to the flotation conditions in the Rossleben potash plant, 2 .16 mg of Rofamin per 25 ml and 3 g of sylvine were used in the adsorption tests, and were shaken for 15 min . After removing the surplus of amine by extracting with chloroform, then separating the sylvine from the aqueous phase and washing with brine, the adsorbed amine can be determined after the dissolution of the sylvine . The Amysed determination was carried out using the method of polarographic maximum damping in highly dilute KCl solutions [12] . After the adsorption (3 g of sylvine per 25 ml of brine with 3 .3 mg of Amysed-W) and separation of the sylvine as described above, the adsorbed amount of Amysed was determined.

Fig . 1 . Scanning electron micrograph of a sylvine crystal with a highly divided surface and with small secondary formation of KCI crystallites. The retention time in Rossleben brine was only 5 min, i .e . the formation of these crystallites was very fast. The size of the crystallites was 5-8 µm . The carnallite content was 8 .6%.

3. Results 3 .1 . Scanning electron microscopy

With increasing conditioning time, surface alterations were observed such as rounding and partial solution effects as well as secondary formations of KCl and NaCl overgrowth both on sylvine (Fig . 1)

and on halite (Fig . 2) . It is of interest that with increasing conditioning time a secondary formation of KCl covering the surface can be observed, especially on sylvine (Fig . 3) . Figure 4 shows that after a conditioning time of 60 min, schoenite/leonite are detectable too, i .e. the formation of sulphate double salts begins, as sug-

H Stechemesser et al, lColloids Surfaces A : Physicochem. Eng. Aspects 88 (1994) 91-101

Fig. 2 . Scanning electron micrograph of the halite matrix with a good observable growth of KCI crystallites (less than 5 µin) on the surface.

95

Fig . 4. Scanning electron micrograph of a sylvine crystal with secondary schoenite/leonite formation (elongated, prismatic crystallites) and KCI formation after a 60 min conditioning time (crystallite size, approximately 5 pun),

maximum value at a retention time in the brine of 30 min . The greater adhesion forces in the Zielitz brine are caused by kerosine acting as adhesion promoter. 3.3 . Laboratory flotation tests

Fig, 3 . Scanning electron micrograph of a sylvine surface after a retention time of 25 min. The surface is largely covered with secondary formation of crystallites of NaCl and especially of KCI.

gested by Dohler [2] and Schubert [3] . However, this begins at a moment that is of no interest with regard to practical flotation times . 3 .2. Measurements of adhesion forces

The results of the measurements of adhesion forces given in Table 3 show that regardless of the retention time of the sylvine crystals in the brine of both potash plants the adhesion forces of the red sylvine are smaller than those of the white sylvine . The adhesion forces of both the white and red varieties of the Rossleben sylvine reach a

The aim of these tests was to show the effect of the alteration of the sylvine and . halite surfaces caused by the conditioning time . Furthermore, it should be determined whether reagent addition before or after the conditioning exercises an influence on the flotation results . The results show that the sylvine (Fig . 5) and halite (Fig . 6) recoveries are functions of the retention time, with a maximum at 15 min under the Zielitz conditions and at 30 min under the Rossleben conditions . The recovery of sylvine on carrying out the conditioning before the reagent addition is more successful than that obtained on carrying out the conditioning after the reagent addition (see Fig. 5) . Considering the ratio of the KCI and NaCl contents in the concentrate as a characteristic parameter of the selectivity in Table 4, it can be found that the conditioning of sylvine in the brine also leads to an increase in the selectivity . It can be demonstrated that for different contents of carnallite in the Rossleben flotation feed the K 2 0 recovery also depends on the conditioning



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H. Stechemesser et aL/Colloids Surfaces A : Physicochem. Eng. Aspects 88 (1994) 91-101

Table 3 Force of adhesion a, (g) as a function of the retention time of the sylvine crystals in the Zielitz and Rossleben brines (see Table 1) Retention time (min)

Brine

Zielitz sylvine, White

Rossleben sylvine White

Red

Rossleben

15 30 45

36.2 ± 1 .2 51.6 ± 3 .1 42.9 t 3 .1

28 .9±1.2 42 .2+4.6 29 .0+3 .8

37.4±1 .7

Zielitz

15

59.9 ± 3.4

17 .2+1 .3

61 .3+3.2

Sylvine recovery,% 100 60 60 40 20 _ 0 0

10

20

30

40

50

60

70

Retention time, min Fig . 5. Recovery of sylvine as a function of the retention time under the Rossleben and Zielitz conditions : +, retention time before the addition of reagents, under Rossleben conditions ; 11, retention time after the addition of reagents, under Rossleben conditions ;

, retention time after the addition of reagents under Zielitz conditions .

Fig. 6. Recovery of halite as a function of the retention time under the Rossleben (+) and Zielitz (0) conditions.

3.4 . Radiotracer measurements time (see Fig. 7) . The highest recovery values were reached at a carnallite content of 8 .6% in the flotation feed material . However, with increasing The aim of the experiments was to demonstrate carnallite content from 3 .2% to 8 .6%, the selectivity gradual reduction of KCl oversaturation caused decreased from 5 .2 to 4.0. by agitating the carnallite-containing flotation feed



H Stechemesser et al.lColloids Surfaces A : Physicochem . Eng. Aspects 88 (1994) 91-101

97

Table 4 Comparison of the selectivities in sylvine flotation after retention times of 5 and 30 min Conditions

Brine

c4a /cNaa in the concentrate 5 min

30 min

Rossleben

Retention time after reagent addition Retention time before reagent addition

3 .3 4 .1

3 .5 5 .2

Zielitz

Retention time after reagent addition

5 .1

5 .3

The carnallite content was 3 .2% . ° KCI concentration . b NaCI concentration .

Sylvine recovery,% 80

60 40

20

I 0 0

10

20

30 40 Retention time, min

50

60

70

Fig . 7. Recovery of sylvine as a function of the retention time, with respect to the grade of carnallite in the Rossleben feed : •, 3 .2% camallite ; +, 8 .6% camallite; D, 18.9% carnallite.

material in the saturated brine, as well as the subsequent retention-time-dependent secondary crystallization of KCI on the salt surfaces . Seidel et aI. [13] has carried out investigations concerning the decomposition reaction along the 25°C equilibrium isotherms of the KC1MgCl2H2O system up to the two-salt-point KCl/KCIMgC12 6H 2 O . The findings demonstrated that the carnallite decomposition generally takes place during complete dissolution before a secondary KCI crystallization occurs owing to KCl oversaturation . Therefore the investigations were carried out with 42K-labelled brines . Figure 8 shows the relative 42 KC1 activity as a function of the conditioning time, both for the Zielitz and the Rossleben brines . The increasing activity with rising conditioning time indicates secondary formation of KCI crystallite overgrowths on the surfaces of the primary crystals, as

had already been observed by scanning electron microscopy studies . It should be taken into consideration that the KCI activity increases more slowly under Rossleben conditions than under the conditions prevailing at the Zielitz plant . 3 .5 . Influence of Amysed on the adsorption and flotation results

The influence of the hydrophilic Amysed-W used for depressing the clay impurities of the Rossleben crude salt on the hard salt flotation process proved to be of interest . As shown in Table 5, red sylvine absorbed less amine than white sylvine . As expected, white sylvine does not adsorb Amysed. The Amysed adsorption by red sylvine, however, was unexpected . In contrast to the white sylvine, it contains up to 0 .34% of Fe 2 0 3 impurities . Table 5



H. Stechemesser et aL/Colloids Surfaces A : Physicochem. Eng. Aspects 88 (1994) 91-101

98

K-42 activity, relat . 1,2 1 0,8 0,6

.. . . . .. .. .. . ..

0,4 0,2

0 0

10

20

30

40

50

60

70

Retention time , min Fig. 8 . Relative 02 K activity as a function of the retention time under Rossleben (+) and Zielitz (t) conditions .

Table 5 The influence of Amysed and amine adsorption (mg g - ') on hard salt on the forces of adhesion and the K 2O recovery Reagent

Without Amysed Amysed/amine Amine/Amysed Amysed-amine-mixture

Sylvine

Hard salt

White

R . . o° from hard salt

Adhesive force

Red

(%)

(g)

Amysed

Amine

Amysed

Amine

53 68 63 56

17 29 -

0.040

0 .106 0.157

<0 .005

0 .183 0.183

K2O recovery .

shows also that both the force of adhesion and the hard salt recovery are, surprisingly, influenced positively by the hydrophilic Amysed . In order to examine this finding, hard salt flotation tests were carried out with different sequences of reagent addition . Whereas the use of an amine-Amysed mixture leads only to a small increase in the recovery, as can be seen from Table 5, the highest recovery can be achieved under normal flotation conditions with Amysed added before the amine addition, i .e . the Amysed adsorption layer influences favourably the subsequent adsorption of the amine .

4. Discussion The results reveal an unexpected influence of the retention time of the flotation feed material in the

brine on the recovery of the valuable raw material . The floatability of sylvine increases with the conditioning time and shows a maximum, the position of which depends on the MgCl, concentration in the brine . Maximum recovery is reached at an MgCl2 concentration of 1 .1 mol kg - ' H 2O after 15 min in the Zielitz plant, and at an MgCl 2 concentration of 1 .6 mol kg-' H 2 O after 30 min in the Rossleben plant . These maxima correspond to the results of the measurement of forces of adhesion carried out on the Rossleben sylvine . These also show a maximum for both the white and the red variety at a retention time of 30 min . Scanning electron microscopy investigations have demonstrated that initially the formation of secondary KCI and NaCl crystallites takes place on the surfaces of the matrix crystals and later on the formation of schoenite/leonite . This fact coincides with the observation of Autenrieth [ 14] that



H. Stechemesser et aL/Colloids Surfaces A : Physicochem. Eng. Aspects 88 (1994) 91-101

the sylvine :halite:schoenite :leonite crystallization times under the same conditions are in the ratio 1 :2 :600 :3000. This may be an explanation of the fact that equilibrium values of the KCI recovery are established with increasing conditioning time, because then, apart from secondary KCl formation, the formation of NaCl on the matrix crystals begins . NaCl does not float with amine and therefore exercises a negative influence on the recovery values. The secondary crystal growth could be confirmed directly by radiometric 42 K measurements . The intensity of radiation increases according to the retention time . This means that in fact a timedependent KC1 crystallization on the mineral particles of the salt sample can be established . Therefore the reduction of KCl oversaturation is not only caused by the secondary formation of fine-grained crystallites, since fine grains smaller than 40 µm were not contained in the solid samples for the radiometric measurements . Surface changes for sylvine and halite can be explained on the basis of the two-layer concept of crystal growth . According to this concept it is known that very imperfect crystals are formed under conditions of reaction-controlled crystal growth . The high relative velocity at the interface between crystal and solution during conditioning leads to a reduction in the thickness of the diffusion layer, so that the driving forces of the growth process are shifted into the adsorption layer. Since secondary KCl crystal growth on the sylvine and halite matrices obviously occurs due to carnallite decomposition in the range of low KCI oversaturation, secondary crystal growth takes place at the lattice defects of the matrix crystals according to the KosselStranski model [15,16] . As is known, coarser crystals grow in a very imperfect way in agitated multiparticle systems . This is confirmed by both scanning electron images and radiotracer measurements . This means that a new KCI surface is formed, rich in growth stages . for the white Rossleben and Zielitz sylvines . In particular, the red variety of the Rossleben sylvine shows a fundamental alteration of its surface properties due to the formation of secondary KCI crystallites . It

99

becomes similar to the white sylvine and shows a better floatability. In general, new KCI surfaces are formed, which according to the KosselStranski model represent energetically preferred phase boundaries, and therefore are centres of selective collector adsorption [17,18] . Increased amine adsorption takes place, this being reflected in an augmentation both of the adhesion force and the K 2 0 recovery with increasing conditioning time. The different conditioning times required for reaching maximum K20 recoveries in the Rossleben and Zielitz brines can be explained in terms of the effect of electrolytes on the crystallization kinetics of slightly soluble salts . Ulrich and Konig [19] have shown, both by single-crystal tests and batch experiments, that the linear growth rate of KC1 decreases with increasing MgCl2 concentration in the brine . This fact explains the crystal-growth-reducing effect of the MgC12 concentration in the brine, i .e . the different conditioning times for the Rossleben and Zielitz brines being necessary for the achievement of maximum recovery values . The MgCl2 concentration of the Rossleben brine of 1 .6 mol kg - ' H2O exhibits a lower growth rate compared with the Zielitz brine with 1 .1 mol kg - ' H20 and therefore needs a longer conditioning time to reach secondary KCl formation on the sylvine surfaces, thus improving the floatability . The influence of the MgCl 2 concentration on the KCl growth rate can be directly confirmed by radiotracer measurements . Compared with the Zielitz brine, the growth rate in the Rossleben brine is lower because of the higher MgC1 2 concentration in the brine. Therefore it shows a low 42 K activity depending on the conditioning time . The activating effect of the clay depressant Amysed-W on both the adhesion force and the flotation behaviour of the Rossleben sylvine was surprising, but it has been demonstrated clearly . On the basis of our considerations we concluded that the Amysed forms an inclusion compound with the amine. The main constituent of Amysed is amylose, which, in turn, consists of glucose molecules bound by a-(1.4)glucosidic links . A characteristic property of the spirally arranged amylose chains is the

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formation of channel-like hollow spaces with eight glucose units [20] . These hollow spaces can be detected by means of the "starch-iodide reaction" (blue inclusion compound) [22] . This Amysed detection method can be applied to the Rossleben brine as well . It has been stated, however, that in the presence of amine the blue coloration weakens, turns violet and totally fails to appear with a surplus of amine . This finding indicates that the hollow spaces are blocked by the amines, and that the formation of an inclusion compound of amine and Amysed occurs . To examine this statement, the detection reaction has also been carried out in the presence of a branched amine (2-ethylhexylamine; M=129 .3g mol - ') showing the same molar concentration as Rofamin . It turned out that the starch-iodide reaction was positive, i .e . a blue coloration appeared, thus proving that the amine was unable to block the hollow spaces of Amysed . The geometric parameters are able to verify this finding . The diameter of the hydrocarbon chain of the C 18 amine (the main constituent of Rofamin) is 0.43 nm [21] and the width of 2-ethylhexylamine at the branch point is 0.68 nm . The hollow spaces of amylose have a diameter of 0 .60 nm, having nearly the same diameter as those of a-dextrin [22] . It is therefore in a position to form an inclusion compound with iodine . The hollow space diameter of Amysed is sufficient to include the Rofamin, but not the branched amine . Therefore the formation of an inclusion compound between Amysed and Rofamin has indirectly been proved . The hydrophobizing effect of the inclusion compound can be explained by the different chain lengths. The spiral starch molecule forms rods with a length of 1 .5-2.0 nm [23], and the C 1s amine molecule has a length of 2 .57 nm with part of its hydrocarbon chain not included. Thus the hydrophobic properties of the inclusion compound can be explained by this part of the hydrocarbon chain jutting out from the starch rod .

5. Conclusions During the flotation of sylvine from sylvinite and hard salt, secondary KC1 layers are formed on

the matrix crystals by the dissolution and recrystallization processes, having a positive influence on the recovery and selectivity . These processes display a time dependence on the MgCI 2 concentration, i.e . on the grade of carnallite in the flotation feed . The reason for the poor floatability of the Rossleben sylvine was found to be due to the presence of the red variety . It has been shown that Amysed-W does not only act as a clay depressant but also promotes the floatability of the red sylvine by forming mixed films of amine and inclusion compounds with Amysed . Nevertheless, it was not possible to achieve the same flotation characteristics for the red variety as for the white variety . This can only be reached by longer retention times of the hard salt in the brine . The retention time dependence on the MgC1 2 concentration leads to surface alterations of sylvine, which are especially apparent with the red component . Owing to secondary KCl formation the red sylvine becomes similar to the white variety with respect to its surface properties . These secondary KCl coatings act as preferred centres of collector adsorption, resulting in increasing adhesion forces and, consequently, in better recoveries with increasing conditioning time, provided that the collector is added after the conditioning. For the industrial application of the results it can be concluded that an improvement in the flotation characteristics is possible by introducing a retention time of the feed in the brine before the addition of reagents and before flotation .

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