On Fragmentation and Agglomeration Phenomena in an Ultrafine Dry Grinding Process of the Egyptian Ca

Journal of Mining World Express, Volume 3 2014 www.mwe‐journal.org doi: 10.14355/mwe.2014.0301.02

On Fragmentation and Agglomeration Phenomena in an Ultrafine Dry Grinding Process of the Egyptian Calcium Carbonate: the Role of Oleic Acid Addition K. A. Selim and F. H. Abd El‐Rahiem Central Metallurgical Research and Development Institute (CMRDI) P.O. Box: 87 Helwan, Cairo, Egypt *

rababrashad2003@yahoo.com

Abstract This study deals with ultrafine dry grinding of Egyptian calcium carbonate particles using a micronizer called “Planetary Mill”. As the size decreases during the process, the particles are strongly subjected to attractive interparticle forces leading to an aggregation phenomenon that generates a decrease in the surface area. In order to improve the process efficiency, the use of grinding aids, such as oleic acid, has been tested. The adsorption of the oleic acid on the particle surface is found to prevent the aggregation through an electrostatic effect that allows the surface area to be increased.

Keywords Ultrafine Dry Grinding; Oleic Acid; Agglomeration; Egyptian Calcium Carbonate

Introduction Grinding is one of the most used processes that allow the production of very fine particles. Many industrial applications are concerned by ultra‐fine wet grinding processes. Examples of applications are fillers for paper or plastic, coatings, pigments, ceramics for abrasives and structural applications, pharmaceuticals, agro‐chemicals, etc. However, the prediction and the control of the ground product properties are still difficult, especially related to ultra‐fine grinding processes [Garcia at el.]. Comminution includes all the processes allowing a size reduction or an increase of the specific surface area of particles, but other physico‐ chemical properties are modified [Somasundaran, Lin at el., Dandurand, Yvon]. Indeed, during a fragmenta‐ tion step, the stress energy is converted into elastic energy applied to the lattice lacks (vacancies, dislocations, grain joints, etc.), which gives rise to cracking.

Additionally, plastic deformations also occur. During prolonged mechanical treatments, the further dissipation of energy generates the alteration of superficial and structural properties of materials and some phenomena can occur such as superficial amorphization of crystallized solids, polymorphic phase transformations, surface activation or changes in surface properties and mechano‐chemical solid‐ state reactions. Even moderated mechanical treat‐ ments can influence the superficial energizing of minerals that is put in evidence by adsorption isotherms of molecules having a strong affinity with the solid. Thus, various physical and physico‐chemical aspects of a grinding operation have to be known, which depends on the minerals and mill characteristics, the energy applied to the system, the temperature and the pressure of the environment.

Different studies have been developed for several materials on these points.

Grinding processes are considered as energetically expensive and additives are usually used for fragmentation operations. Such additives are chemical compounds which, incorporated with the powder or into the suspension submitted to grinding, allow obtaining finer particles. The increase of specific surface area may vary from several percent up to 20 times the specific surface area of the particles ground without any additive [Garcia at el., Ding at el, Toraman, Ahmadian at el.]. Common additives often used during grinding processes of minerals (calcite, dolomite, quartz, magnetite, and hematite) are alcohols, ketones, amines, carboxylic acids, esters, electrolytes, surfactants, neutral or charged polymers.

Nevertheless, these additives are expected to be used in small proportion to improve the process and also to make it economically less expensive than without any.

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During a grinding process, the main part of the energy input is dissipated in heat loss instead of being used to create new surfaces. Minimizing losses with an exterior agent may increase the grinding efficiency [Garcia at el., Ding at el, Toraman, Ahmadian at el.].

The efficiency of the additives can be explained by different approaches, strongly dependent on the nature of the material, the size of the ground product and the grinding technology used. This can be described by the prevention of the aggregation phenomenon that may result from interparticle attractive forces, considering the DLVO theory [Rose at el.]. In the case of wet processes, when sufficient additive amounts are used, allowing reducing significantly the suspension viscosity. The rheological behavior of the ground pulp is thus an important parameter that must be controlled during the process in order to obtain finer particles. Moreover, for wet ball milling, when the viscosity of concentrated suspensions becomes too important, the pulp may cause the grinding media to be stuck on the surface of the grinding chamber so altering the fragmentation of the particles [Garcia at el., Ding at el, Toraman, Ahmadian at el.].This phenomenon can also be educed and the hydrodynamics in the grinding chamber is improved.

TABLE 1 COMPLETE CHEMICAL ANALYSIS OF CALCIUM CARBONATE SAMPLE

Constitute CaO SiO2 Al2O3 Fe2O3 Na2O MgO K2O P2O5 MnO TiO2 L.O.I.

% 58.50 0.72 0.16 0.02 0024 0.47 0.01 0.05 0.005 0.028 40.00

FIG. 1 X‐RAY DIFFRACTION PATTERNS OF CALCITE

The aim of this work is to produce ground calcium carbonate by dry grinding using a micronizer called “Planetary Mill” under various operating conditions with and without addition of oleic acid. The effect of oleic acid concentration on the grinding efficiency and results has been studied.

Materials and Methods Materials A sample of pure calcium carbonate (99.9%) was kindly delivered from Egyptian‐Jordanian for Mining Industries Co. Laboratory grade oleic acid, supplied

by Adwic Co., Egypt, was used. Analytical grade NaOH and HCl were used for pH regulation.

Methods The sample was sent as primary crushed lumps of about 3ʺ in size. It was secondary crushed using a “Denver” pilot jaw crusher to less than one inch and then in a “Wedag” roller to less than 3.36 mm.The crushed sample was then thoroughly mixed and quartered. The tertiary crushed material was sampled into batches and subjected to ultrafine grinding in a micronizer called “Planetary Mill”.

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FIG. 2 SEM IMAGE OF CALCIUM CARBONATE

The material was primarily ground by the high energy impact of grinding ball and in part by friction between the balls and the wall of the grinding bowl. The grinding bowls with material and balls rotate around their own axis in a counter‐ rotating planetary disc.

The centrifugal forces are caused by rotation of the grinding bowls and supporting discs work in the contents of the grinding bowls, from which it is clear that the force resulting from rotation of the grinding bowl when the mill is started‐ causes the rotating balls to rub against the inside wall of the bowl thus grinding the material. The chemical analysis of this sample is shown in Table 1. Size distribution of the ground products was measured using “Fritsch” Particle Size Analyzer manufactured by GAMBH, Germany. Complete chemical analysis of the bulk sample was carried out using ARL – XRF/Uniquant.

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Results and Discussion Optimization of Ultrafine Dry Grinding without Additives 1) Effect of Rational Speed The rational speed was changed from 200 to 360 rpm at constant grinding time, 20 minute, Fig. 3. It is shown that the surface area after grinding rang from 7.044 m2/cc to 7.35 m2/cc. Also, on increasing the rational speed from 200 to 360 rpm, D50, mean particle size, of the ground sample has been changed from 1.18_ μm at 200 rpm to 1.20_ μm at 360 rpm and D97, top cut size, has been changed from 3.99_ μm to 11.50 _μm. Bradley found that the centrifugal acceleration is proportional to the square of the speed, and the rate of grinding should be proportional to 1.5 power of centrifugal acceleration. It is perhaps of interest to point out that this result is also encountered by vibration milling [Zhao at el].

Cumulative, Wt % Passed

100 200rpm 300rpm 360 rpm

80

 2 D ( x, t ) tx

  S (x)

D ( x, t )  x

xm

 x

 D ( , t )   (  , x ) (1) S ( )  x

Where:‐ D(x,t) is the cumulative under size of particles, x is the particle size, γ is the size of a single particle to be broken, xm is the maximum particle size present. S(x) is the selection function which represents the probability for a particle of size x to be selected for grinding, and is specified by empirical relationship as;

S( x )  kx n (2) Fig. 4 represents the effect of the grinding time at constant rational speed 360 rpm. It is obvious that the increment in grinding time increases the surface area of the ultrafine product from 7.35 m2/cc at 20 min. to 8.34 m2/cc at 40 min. and then decreases sharply at 60 min. to 2.75 m2/cc. This is due to the fact that fine particles start to be agglomerated at the highest speed. Also, it can be noticed that the product at 40 min. grinding time has the highest surface area. Its distribution curve shows a bend to coarser sizes at about 90%. At 60 min. grinding time, the shift to coarser sizes started from the beginning, where the medium size is about 2.75_μm, higher than that at 20 min. and 40 min., 1.10_μm. It is noticed that the rate of grinding in the mill is so fast [Zhao at el]. Therefore, the best‐ condition; amongst the tested; of ultrafine grinding by “Planetary Mill” is 20 min. and 360 rpm rational speed.

100

80

Cumulative wt % passed

XRD was used to identify the mineral components in the calcium carbonate sample and to determine, in part, the proportion of the calcite minerals present.

Randomly mounted material was scanned using PW 1170 Phillips diffractometer with CoKα radiation, Fig. 1. SEM observation was performed using Jeol 5410 unit, and the sample was coated by Au. 200 mg samples were analyzed with Netzsch STA 409 C/CD simultaneous thermal analysis apparatus between room temperature and 1100°C at a heating rate of 10°C/min. Alumina was used as a reference material, Fig. 2.

60

40

20 min.

40 min.

60 min.

20

60

0 0.01

40

0.1

1

10

100

1000

10000

Size, m 20

FIG. 4 EFFECT OF THE GRINDING TIME 0 0.1

1

3) The Effect of Fractional Ball Filling

10

Size, m

FIG. 3 EFFECT OF ROTATIONAL SPEED

2) The Effect of the Grinding Time

The kinetics of comminution is modeled by using a classical batch grinding equation based on selection and breakage functions. The change in particle‐size distribution of ground material is described as a function of grinding time, t, as follows [Zhao at el]:

It was found that; the moving distance, frequency and the contact force of the Suring appear to be important quantities affecting the rate of grinding.

It is predicted that these quantities will be determined by the fractional ball filling of the mill [Houssa]. A schematic picture is given in Fig.5, showing the motion state of ball in the mill based on photographic analysis.

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2% at constant grinding time, 60 min. and 360 rpm rational speed, as shown as Fig. 7. It can be seen that by increasing oleic acid dosage, the mean particle size, D50 has been changed ranging from 5.78_ μm at 0.5% to 8.5 _μm at 2 %. At the same time, the top cut size, D97, increased from 11.9 μm to about 17.52_ μm respectively.

FIG. 5 SCHEMATIC REPRESENTATION OF THE MOTION OF BALLS IN THE MILL

At the same time, the cascading of balls does not occur in the mill. It is also known from the analysis result that the entire charge of the mill, as a coherent mass, surges on the mill wall. Thus, for one part of the cyclical motion, the charge moves around the axis of the mill in the same direction as the mills shell; while during the second part of the motion, it moves in the opposite direction. It could be expected that most of the comminution; under this condition, occurs between the mill wall and the ball. The compressive, friction, abrasive and shear stresses of the ball mass applied on the particle layer that adhered to the mill wall and it partly occurs within the ball mass by the shear stress, due to the speed gradient of moving balls. The results in Fig.6 showed that the product in all ratios is more or less the same properties, where; surface area increased from 7.7 m2/cc to about 8 m2/cc.

However, mean particle size, D50, has been changed from 1.0_μm to 0.93_μm as the ratio changed from 50% to 25% and the top cut size, D97, has been changed from 4_μm to 3_μm, respectively.

Therefore, the optimum ratio is 50% as the changes in the product‐ properties are not distinguished all over the different ratios.

100 25 % 30 % 40 % 50 %

Cumulative wt % passed

80

60

40

20

0 0.1

1

10

Size, m

FIG. 6 EFFECT OF FRACTIONAL BALL FILLING

Dry Grinding With Addition of Oleic Acid 1) Effect of Dosage of Oleic Acid

The dose of oleic acid was changed from 0.5% to

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FIG. 7 EFFECT OF DOSE OF OLEIC ACID

Grinding process without additive indicated that a strong aggregation of the particles was revealed particularly for long grinding times. The influence of oleic acid concentration has been studied with regards to the size distribution of the final product.

The evolution of the size versus mass fraction is presented in Fig. 7. Concerning the run without any additive, the median diameter of the ground particles strongly decreased during the first minutes of grinding. Then, a minimum value has been reached and increased later, when aggregation of fine particles has occurred. After 60 min, a constant median size has been reached again.

Indeed, during an ultrafine grinding process, the particle size and the interparticle distance decreased whereas the number of particles increased. As the size became submicron, the particles may be submitted to strong attractive forces (Van der Waals forces). During the first 3 minutes, the addition of the oleic acid, whatever its concentration, slightly enhanced the particle size reduction. For longer grinding periods and for the two lowest concentrations, the median diameter decreased first before increment then due to particles aggregation. This aggregation occurred earlier and to a larger extent with the lowest concentration (0.5%). Although the effect of the oleic acid was not clearly put in evidence looking at the evolution of the median diameter versus time, it was found that the number of fine particles was significantly increased in the presence of the additive. In that case, the quantity of additive was not sufficient enough after a certain period to

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disperse fragments and it can be assumed that the numerous fines produced during the first minutes promote aggregation.

The Adsorption Mechanism The measurement of zeta potential indicated charge properties of calcite particles and in turn can suggest what can be adsorbed, penetrate, and adhere. Fig. 8 displays the zeta potential of calcite in absence and presence of oleic acid as a function of pH. It is clear that in distilled water the calcite has an isoelectric point at about pH 9. Above it, the surface charge is positive. After conditioning with oleic acid, calcite became negatively charged with a minimum at pH=8‐ 10, indicating that the maximum oleic acid occurred in this pH range.

The FTIR spectra in Fig.9 shows the characteristic absorption bands of calcite (CaCO3) located at 1421, 875 and 711 cm‐1, corresponding to the asymmetric stretching (ν3); out‐of‐plane bending (ν2), and the in‐ plane‐bending (ν4) modes of the carbonate CO32‐ ion group are found to be active [Kajima and Tonaka, Zhao at el]. The FTIR spectra of calcite treated with oleic acid, in Fig.10, shows a band at 1423_cm‐1. This suggests the formation of a metal complex involving the COO‐ group (mono‐coordinated surface calcium oleate COO‐Ca+). The absorption band at 1133 cm‐1 is related to C‐O bonds and the band at 1296 cm‐1 is assigned to the bending vibration of (‐CH2‐). Two bands at 2956 and 2853 cm‐1 are characteristic of (‐CH) asymmetric and the symmetric stretching respectively.

The band at 3612 cm‐1 is characteristic of O‐H stretching. From these results, it can be concluded that oleate anions are chemisorbed on the calcite surface; which agrees with data on the dependence of the COO‐frequency on the metal nature [Kumar at el., Labidi , N.S. and Djebaili], as shown in Fig.9 and Fig. 10.

Conclusions

FIG. 8 ZETA POTENTIAL OF CALCITE IN ABSENCE AND PRESENCE OF OLEIC ACID

Investigations about the influence of additives on ultrafine grinding of calcite particles were carried out.

Ground without additive; calcite has a strong tendency to aggregate under the attractive Van der Waals forces. After some minutes of grinding, the number of fine particles became significant and aggregates appeared. The grinding efficiency of the process was altered due to a great increase of the surface area of the particles. By preventing the aggregation of the particles, an additive allowed increasing the surface area of the particles. The required amount of additive to be added depended on the fineness of the ground product to be obtained. The mechanism of the adsorption of oleic acid on the surface of calcite has been suggested in Fig.11.

FIG. 9 FTIR OF ORIGINAL CALCIUM CARBONATE

FIG. 11 SCHEMATIC REPRESENTATION OF OLEIC ACID ‐ CALCITE ADSORPTION AT THE CALCITE/WATER INTERFACE REFERENCES

FIG. 10 FTIR OF CC TREATED WITH OLEIC ACID

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