INFLUENCE OF p H ON T H E KINETICS OF C O M M I N U T I O N OF Q U A R T Z P. H A L A S Y A M A N 1 , I S . V E N K A T A C H A L A M , A N D R . M A L L I K A R J U N A N Department of Metallurgical Engineering, Indian Institute of Technology, Bombay 76, India
The effect of pH on the rate of formation of fines for quartz has been investigated. The initial rate of formation of fines is faster when the grinding is performed in an almost neutral (pH 6.8) rather than weakly acidic or basic medium. The results are discussed on the basis of the effect of pH on the fracture strength of quartz and the state of flocculation in the ball mill, The initial rate of production of fines in a ball mill is shown to be a zero-order rate phenomenon in the pH range investigated-i.e., the amount of fine particles produced is directly proportional to the time of grinding.
HE object of this investigation is to find out whether the T i n i t i a l rate of formation of fine particles is affected by a change in the p H of the grinding medium. Arbiter and Bhrany (1960) observed that the initial rate of production of fine particles in a ball mill is a zero-order rate phenomenon-Le., the amount of fine particles produced is directly proportional to the time of grinding. As pointed out by these authors, if fine particles are produced at a constant rate, comminution of these fine particles themselves must be negligible. These authors confined their experiments to dry and wet grinding (pH 7).
Basic Principles
Gaudin (1955) has suggested that comminution can be considered as a chemical reaction wherein a rupture of a covalent or electrovalent bond takes place. Not much work has been reported on the use of additives in grinding. Frangiskos and Smith (1957) studied the effect of surface-active reagents on the comminution of limestone and quartz, using a stamp mill. They found that, generally, when the crushing was done in the presence of electrolyte, a n optimum concentration for the production of maximum surface area existed. Working on the ball milling of quartz, Gilbert and Hughes (1962) found that the use of silicone as an additive resulted in an improvement far below that obtained by the previous workers using similar additions in a stamp mill. Wet grinding of quartz with cationic wetting agents at various concentrations over a range of p H values actually showed a slight decrease in grinding efficiency. Experiments showed that the use of additives in conventional rod or ball mill practice decreases grinding efficiency or has no effect. Berry and Kamack (1957) indicated that the addition of small amounts of naphthenic acids to ilmenite, in the dry grinding, markedly increased the limiting specific surface that could be produced. Materials and Method
High purity quartz (99.970 SiOz) was used. I n the case of subangular- and angular-shaped quartz, the following size fractions were chosen for the feed: - 1600 1250 microns, - 1250 f 800 microns, - 800 560 microns, and - 560 400 microns.
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Present address, The Indian Smelting and Refining Co., Ltd.,
Bombay, India.
I n the case of subspherical-shaped quartz, the initial size 1600 fractions studied were the same as above except for the 1250-micron fraction. The quartz was subjected to wet grinding in a laboratory ball mill-length, 75 mm.; diameter, 100 m m . ; and speed, 86 r.p.m. (72.6y0 of critical speed). The ball mill contained five 1-inch diameter manganese steel balls, whose total weight was 140 grams. The charge consisted of 100 grams of quartz of a particular fraction and 50 cc. of water at p H 6.8 (pH of distilled water)Le., the ball mill was worked at 66.66yo solids. Batch grinding was carried out for 4, 8, 12, 16, and 20 minutes. T h e same experimental procedure was adopted for grinding a t different p H values. The p H values obtained with a p H meter, ranged from 5 to 9, hydrochloric acid and sodium hydroxide being used to regulate the pH. No conditioning time was given for any of the experiments. After each grinding operation, the material was transferred to a beaker, and quartz particles were allowed to settle. Later the liquid was decanted. The negligible weight of the particles carried away by the decanted liquid was also taken into account. The settled quartz, dried a t 100' C. and cooled, was then subjected to sieving in a Rotap machine, using sieves calibrated in microns, without hand finishing. The different fractions were then weighed. T h e results are averages of three independent experiments conducted under identical conditions and agree within =t1.5%. I n plotting cumulative weight per cent passing of a given size us. time, the entire undersize weight was taken, and the percentage was calculated on the basis of the weight of the original charge.
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Results and Discussion
Figures 1 and 2 show the cumulative percentage of fines us. grinding time. I n general, the initial rate of formation of fines is proportional to time. Deviation of some of the points from the straight line can be attributed only to experimental errors. The results confirm the work of Arbiter and Bhrany (1960). The present investigation shows that in both weakly acidic and weakly basic grinding media, the rate of fines formation is still a zero-order reaction. The proportionality of the initial rate of formation of fines with time in each type of grinding is slightly different. Figures 3 and 4 show that a t p H 6.8 (pH of distilled water), the rate of formation of fines is maximum; whereas, in either VOL. 7 N O . 1
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Figure 1.
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I h E C PROCESS DESIGN A N D DEVELOPMENT
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Rate of formation of quartz fines at various pH. Fraction size, 71 microns
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a weakly acidic or weakly basic medium, the rate is lower than that at p H 6.8. This may lead to the conclusion that the fracture strength of quartz is lowest at p H 6.8 and that in weakly acidic or basic medium its fracture strength is more than that at pH 6.8. T h e interaction of a quartz particle with water or an aqueous solution of a salt is governed by the need to Screen Si+4ions. If the quartz particle is large, the distortion of the surface can
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improve the screening to a considerable degree. If the particle is small, this method becomes less effective. Hence, the surface of small quartz particles depends to a large extent on the chemisorption of polarizable ions for the improved screening of its cations. The reactant, activated complex, and products in the wet grinding of quartz as shown in Figure 5 would indicate that the reaction must definitely be favored by polar solvents; VOL. 7
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Figure 4.
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100 71 microns Fraction size, 1 2 5 0 microns A. Initial size, - 1600 E. Initial size, - 1250 800 microns 5 6 0 microns C. Initial size, - 8 0 0 400 microns D. Initial size, - 5 6 0
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Rate of formation of quartz fines at various pH
Subangular quartz
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but this is not the case. According to Engelhardt (1946) and Gomer and Smith (1953), 5.8 X 106 ergs are required for producing 1 sq. cm. of quartz surface in water, but only 3.2 X 106 ergs per sq. cm. if the grinding operation is carried out in octanol. His results on the abrasion hardness of quartz in different media indicate that the screening power of a liquid cannot be expressed by its dipole moment or by its dielectric constant. T h e higher alcohols apparently are the best 82
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screeners with respect to Si+* ions in the surface of silica. Since a liquid which can wet and screen a freshly formed surface will lower the energy of that surface, such a liquid will decrease the relative abrasion of quartz. Consideration of the foregoing facts may lead to a conclusion that a weakly basic or weakly acidic solution can not screen the Si+4ions in the surface of silica to the extent that water alone does.
An alternative explanation of the results obtained must take into consideration the state of flocculation in the ball mill. This is very likely because of the influence of acidity or alkalinity on the state of flocculation. I t is only a special case of the effect of electrolytes and is an important one. The ions of water, H’ and OH-, are more effective control ions than other monovalent ions. Hence, the increased production of fines during grinding in a neutral medium compared with an acidic or alkaline medium is probably due to the fact that the greater flocculation in a neutral medium makes the flocculated material more amenable to receiving the impacts. From the present work, it is not possible to say which explanation holds good. Probably both are true to a varying degree.
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Conclusions
The initial rate of formation of fine particles smaller than a given size is a function of the given size The kinetics of comminution of quartz is a zero-order reaction. The same is true for wet grinding (pH 5 to 9). The initial rate of formation of fines is faster when the grinding is performed in a medium which is almost neutral (pH 6.8). In weakly acidic and basic medium, the rate is lower compared with that a t p H 6.8.
OH
OH
literature Cited
Arbiter, N.,Bhrany, U. N., Trans. A I M E 217, 245 (1960). Berry, C. E., Kamack, H. J., Proc. Second Intern. Congr. Surface Actioity IV,196 (1957). Engelhardt, W. V., h’aturwissen. 33, 195 (1946). Frangiskos, A. Z., Smith, H. G., Progr. Mineral Dressing, Trans. Intern. Mineral Dressing Congr. Stockholm 1957, p. 67. Gaudin, A. M., Trans. A I M E 202, 561 (1955). Gilbert, I,. A., Hughes, T. H., Vortr. Diskussionen Europaeischen Symp. Zerkleinern, I , Frankfurt am Main 1962, p. 170. Gomer, R., Smith, C. S., Ed., “Structure and Properties of Solid Surfaces,” p. 164, University of Chicago Press, Chicago, Ill., 1953.
0PRODUCTS
RECEIVED for review J u n e 6, 1966 RESUBMITTED March 9, 1967 ACCEPTEDSeptember 25, 1967
Figure 5. Reactant, activated complex, and products in the wet grinding of quartz
THERMAL BEHAVIOR OF AN EXHAUST GAS CATALYTIC CONVERTOR JOSEPH V A R D I AND W I L L I A M F. B I L L E R
Products Research Division, ESSOResearch and Engineering Co., Linden, .V. J .
aim of this work was to examine the transient thermal behavior of a fixed-bed catalyst reactor for treating the exhaust gases of an internal combustion engine. The analysis and the original data presented will predict the behavior of the reactor and, therefore, provide an approach for an effective reactor design. Although in most chemical processes the temperature and the flow rate of the feed are adjusted to produce optimal process conditions, the exhaust gas conversion process provides a unique situation in which the feed cannot be modified easily and thus feed conditions predetermine process operation. The operation of the exhaust gas reactor HE
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includes continuous variation of the temperature and the flow rate of the feed in the range of 0’ to 1600’ F. and 0 to 200 cu. feet per minute, respectively. This results in variable physical properties of gas and catalyst and a changing heat transfer coefficient. The changing feed conditions impose a specialized empirical boundary condition in the mathematical description of the behavior of the reactor. Therefore, the problem a t hand is far more complicated than the simplified situations of constant properties, a constant heat transfer coefficient, and a constant temperature feed: the unsteady-state operation of a packed bed with the assumption of a step change in the VOL. 7
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