Po~ T«lIIIOlogy- Elsevier Sequoia SA.. Lausanne- Printedin the Netherlands 171 I. J. UN. and P. SOMASUNDARAN Co"-'b;a U"iwnity. HIM" Knftb Sdroolof MiM.f. N- York. N.Y. 10027(U.S.A.) (ReceivedD«embcr 30. 1971;in revised(onn February9. 1971) ~. .. Summary INTRODUCTION Samples for the determination of the chemical Comminution techniques such as grinding are composition and physical characteristics of materials often used in laboratories to prepare samples for are usually prepared in laboratories by prolonged determination of chemical and physical character- dry or wet grinding. During such grinding the physical istics of materials of interest The objective of grind- as well as the chemical characteristics of these ing of samples is indeed to produce fine particles materials can. however. undergo significant altera- for further treatment or studies. Very frequently, tions. Our analysis of the past work in the literature however, the physical as well as the chemical .thows that the exact nature of the alterations is characteristics of these materials are found to dependent.among other things. b.v the conditions of undergo significant changes during prolonged grinding as well as the method of grinding that is grinding. There is sufficient evidencein the literature adopted. In this work. samples of t'arious minerals that not only desired changesin physical properties such as quartz and calcite and other materials such such as specific surface area and shape but also as massicot wereground in a pebblemill as afunction changesin chemical properties as catalytic activity of time for several hundred hours and the rhange in can take place during grinding periods. Further- various properties studied b}' grot'imetric. th,.rmo- more, even polymorphic transformations of miner- gravimetric and X -ra}' diffraction analyses. It M'as als and soud state reactions are known to occur found that the densit>.of the particles in the caseof during prolonged comminution. An examination of quartz decreasedas a funrtion of the grinding time some typical examples or the physical and chemical (or partirle size) apparent~vdue to the rreation of changes during dry grinding would clearly show deep amorphous la.verson the particles. In the case that the effects can give rise to deceptive informa- of massicot and calcite. poo'morphic transitions were tion on a sample unless necessary precautions are found to alter its structure to that of litharge and taken to avoid such changesor at least to take them aragonite respectivel.v. Even solid state reactions into account during the final evaluation of the ori- werefound to take plare during grinding as shown ginal material from which the sample was drawn. b.v the formation of galena during the grinding of This paper presents a discussion or various massicot or litharge with sulfur. These changes reports in the literature on the physical and chemi- suggest the importance of recogni:ing the fact that cal alterations during prolonged dry grinding along the properties of small samplesprepared by grinding with the experimental evidences that we have ob- are not necessarily total/}' representativeof the bulk tained to confirm such changes in a quantitative materials. The.v also suggest the importance of manner. flstablishing standard conditions of preparation of samplt's so that tht're is It'ss discrt'panc.vbt'twet'n results obtained from various studit's at different PAST WORK ~ laboratories. ., There are several repons in the literature on - marked physical changesduring prolonged grinding . On leave from Technion-Israel Institute of Technology. of minerals. particularly quanz. F'Jf example. Haifa. Israel. changes including amorphous transformations or l Powd" T«hIIOl~' (1972) 172 I. J. LIN. P. SOMASUNDARAN partly first to the triclinic fonn and then to an amorphous state. The triclinic fonn was always found to convert completely into the amorphous ( state by grinding. Other reports in the literature on polymorphic transition include that of calcite (Sp. Gr. 2.72) into aragonite (Sp. Gr. 2.95)15,vaterite (Sp. Gr. 2.64) into calcite16, wurtzite (Sp. Gr. 3.98) into sphalerite (Sp. Gr. 4.05)17 and massicot (Sp. Gr. 9.64) into litharge (Sp. Gr. 9.75)18- ZO.In the case of HgIz, the transition was manifested by a gradual change in color from red to yellow21. As in the caseof sodium tetrametaphosphate and kaolinite discussedearlier, the structure of minerals has often been found to be disrupted by grinding. It might be noted that this effect is not in general detectedif the grinding is done in a liquid mediumz2. Grinding in these media. however, produces other effects such as decrease in rupture, bending, fracture and fatigue strength of materials. This topic has been critically examined in a recent review by Somasundaranand Linz3 on the effect of the nature of environment on comminution. It might be men- tioned at this point that an active gaseous medium can also cause such changes, possibly through the presenceof physical and chemical adsorption. The rupture of bonds during grinding results in a surface with unsatisfied valencies. This, combined with the high surface energy, favors physico- chemical reactions between the solid and the sur- ( rounding medium31. Pure metals as well as their sulfide minerals have been found to undergo rapid surface ox~dation on grinding24.. Weyl21 reported that quartz on prolonged grinding in a dilute solution of silver nitrate actually reacted with the solute to fonn a monomolecular film of yellow silver nitrate. In addition, the fresh surface acquired hydrophilic properties aRdadsorbed relatively large quantities of water vapor 1°. Quartz has also been found to be more soluble in sodium hydroxide on grinding. Bentur5 noted that the reactive surface of ground quartz is even capable of breaking down oxygen molecules. In fact, grinding of quartz in a ball mill has been known to produce ozone inside the mill. Ground sulfides'are also highly reactive with the atmospheric oxygen. For example, molyb- denum sulfide in water suspension was found to fonn sulfurous and sulfuric acids with the pH of the medium continuously decreasing with grinding time. As mentioned earlier, grinding can lead in some caseseven to solid state reactions. Such rC'dctions are found to be most prominent during the grinding PowderTtchnol~ 6 (1972) <. PROPERTIESOF SAMPLESDURING PREPARATION BY GRINDING 173 of carbonates. For example. zinc carbonate2 and EXPERIMENTAL cadmium carbonate26 with relatively low decom- position temperatures give carbon dioxide by mere Two ball mills (20 cm x 10 cm) we~ used in the grinding at room temperature. In the case of experiments,one with a chrome liner, a brass cover carbonates such as magnesium. with higher de- (with provision for evacuation and variation of the composition temperatures. prolonged dry grinding atmosphere)and six symmetrical lifter bars parallel \ lowered their decomposition temperatures signi- to the shaft, and the other made of porcelain ficantly. Jamieson and Goldsmith21 ground man- material. The grinding media was in the first case ganesecarbonate for up to three days and obtained 6300 g of steel bearing balls of 1-1! in. diam. and in Mn]O. and Mn203 after loss of carbon dioxide. the second case river pebbles. The media occupied Severalmixtures of carbonates are found to react to SOvol. % of the mill capacity in both cases.The mill form heterogeneoussolid solutions. their nature and speedwas 76 r.p.m. (80% of the critical speed)in the magnitude being dictated essentially by their phase fIrst case and 70 r.p.m. in the second case. Except diagrams. In general two types of reactions are for the natural minerals used, all materials were of found to take place-In the first type MCO3-MO+ chemical purity grade and dried for 24 hours at CO2, the oxygen is supplied by the carbonate itself 60°C. and hence the reaction is essentially independent of To investigate the change in physical properties oxygen partial pressureand dependent only on the during grinding. SOO-gbatches of - 7 to + 14mesh partial pressure of carbon dioxide in the mill. An quartz were prepared from a sample of milkycrys- example of this type is the decomposition of talline mineral collected from Eilath pegmatite. smithsonite (ZnCO3~ The second type of decom- leached with hot concentrated hydrochloric acid. position is of the form and washed with distilled water till free of chloride 6 MCO3-2 MJO.+6 CO2 ion!. Grinding was carried out in this casein air and in ammonia. Densities of various size fractions were determined according to the two parallel methods 1J:::~3M2O3 usedby Burton2, one using liquid mixtures of known In this case the oxygen supply is from the mill amounts of tetrabromoethane and carbon tetra- atmosphere and hence the extent of the chemical chloride and the other in a suspension under changes is dependent upon the composition of the vacuum using a pyknometer. environment in which the grinding is done. Another Decomposition temperatures of mineral samples important example of chemical decomposition were determined using a thermogravimctric tech- during grinding is that of NaSPJOlO'6 H2O to nique described elscwhere29.The lattice structures form ortho and pyrophosphates28.Several hydrated of the minerals were determined using a picker salts have been found to decomposeduring grinding. Horizontal X-ray Diffractometer with a CuK~ For example. FeSO. .7H2O transforms on grinding radiation and scanning rate of one degree per first to FeSO..4H2O and later to FeSO.' H2O, minute. The composition of materials and the and BaCI2'2H2O first to BaCI2'H2O and then to natu~ of distribution of relevant clements were BaC1229.These reactions are most prominent if the determined by electron microprobe techniques mill atmosphere is dry, (accelerating potential is I kV, specimen current An interesting complete chemical reaction that 0.05-0.06 ,uA). has beendiscovered to occur due to grinding is that between black lead sulfide and white cadmium sulfate to form white lead sulfate and yellow cad- RESULTS AND DISCUSSIONS mium sulfide.