I. J. UN. and P. SOMASUNDARAN ~

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 mentioned 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 decomposition 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. the progress of the reaction being indicated by the gradual change in color of the Results for density of various size fractions of mixture. quartz ground in air and ammonia are given in The present study was conducted with the Fig. 1. It can be seen that the smaU size particles purpose of investigating systematically the extent of obtained by grinding in air had a significantly lower physical changes during comminution of homo- density and hence a considerable amount of geneous materials and the extent of solid state amorphous matter in them. The panicles obtained reactions during comminution or heterogeneous by grinding in water were found to be mostly materials under known operating conditions, crystalline. This observation cannot, however, be

Pown T«hIIOl- 6(1972) 174 L J. LIN. P. SOMASUNDARAN interpreted to mean that there was no amor- buffer solution by approximately200/0. this being phousization of quartz during grinding in water. On possiblydue to the removalof the amorphouslayer ( the contrary, it is highly possible that amorphousi- by the water. If therewas water presentat the time zation occurred at an equal or even at a larger of the formation of the amorphouslayer itself and if scale in water than in air but also that the amor. the particleswere continuously washed with water, phous material dissolved in the medium during this as it would be in the caseof wet grinding, it is then change,thus avoiding the reduction in density of the reasonablethat the amorphous layer might be particles. In fact. the observation that the grinding completelydissolved in water. efficiency is higher or that the particle size of the The data for densityof particlesgiven in Fig. 1 for ground product is lower for grinding in a liquid grindingin air yield a straight line whenplotted asa medium than in a gaseousmedium lends support to functionof the natural logarithm of the particle size. this hypothesis. Henderson et al.3O observed that (SeeFig. 2.) The resultant line is desCribedby the the washing of quartz particles five times with water relationship: decreasedthe amount of silica dissolved in a pH 7 PI= 0.032In dl+ 2.150 (1)

0 OJ 0.2 0.3 0.4 0.5 0.. 0.7 o.a 0"' 1.0 PARTICLE SIZE. - FiJ. 1. Plot ol density lIS. particle size for quartZ ground in a laboratory ball mill.

0.1 1.0 10 100 1000 dt .Micr- Fig. 2. Diagram showing the parameten of amorphousization as a function of panicle size for quartz ground in a laboratory ball mill.

Powd,r TtChllOl~ 6 11972' l PROPERTIESOF SAMPLESDURING PREPARATION BY GRINDING .73

(It)

Further substitution of the previously mentioned values of Pc and P. gives Y;.= 1.12-0.072In dj (12) \ A In order to calculatethe thicknessof the amorphous .' layer, Y;. is expressedfurther in the following ~. manner:

y.. - 1 - V/c Y..= ~- JI; 2cS, 3 y~ 1 1 = - ( - d( I (14). .

where 15jis the thickness of the amorphous layer in the spherical particles of diameter dj. Substitution of eqn. (12) into eqn. (14) yields d 15(= 1f1-(O.O72 In dj-O.12)t~ (15)

The above treatment illustrates a method for quantitatively assessing the amorphousization of various fractions in a sample from a knowledge of the density of the ground product. Results obtained in the present case for volume percent and weight percent of amorphous material in various size fractions are given in Table 1 and also in Fig. 2 along with the thickness of the amorphous layer in each case.It can be seen,as an example. that in the caseof grinding in air 14 wt.% of the material of size 50 JJ.n1 is amorphous. The results are certainly significant enough to be remembered carefully when trying to use them to characterize the original material.

TABLE 1 Data showing the amorphous nature of quartz parti-:les of various sizes caused by dry grinding in a laboratory ball mill

41,(.4) P, X, fj 6;(.1) ',/4.

.. x loJ 2.371 0.513 0.623 138.& 0.138 I x 10" 2.44S 0.404 0.457 921.0 0.092 I x 10' 2.S18 0.248 0.292 5435.0 0.054 S x 10' 2.S70 0.139 0.176 15650.0 0.031 I x 106 2.S94 0.094 0.125 21850.0 0.022 2x 1~ 2.616 0.051 0.076 26(XX).0 0.013 3x 106 2.629 0.025 0.047 24000.0 0.008 Pi = (l-1':..)p,+ YtuP.. 4x 106 2.638 0.008 0.026 17400.0 0.004 ,'" (10) s x 106 2.64S 0 0.010 7CXX'./) 0.001 6 x 106 2.651 0 0 0 0 Substitution of eqn. (1) into eqn. (10) yields

L Powdrr T«hnol- 6 (1972) 176 L J. LIN. P. SOMASUNDARAN

.0 1.0

.u. goo! 0.8 \II ~ ~ '" ... I- ~ Z ~ \II I g ~ 880 10.' c 2 \II c ... c ... Z 0 0 Z ~0: 110 0 0.4 ~ u ~ c 2 c 0 ... u ... I- 0 % 840 0,2 !2 r +~ III .

O 82 .:~~::::::::=,~,- , , , ,I , "'" , 0 I ~ 100 TIME. HHn Fig. 3. The decomposition temperature or ground calcite samplesand the fraction or aragoniterormed as a runction or lrinding time. (A indicatesthe decompositiontemperature or purecalcite and 8 that or pure aragonite.)

Results obtained for the polymorphic transition of TABLE 2 calcite into aragonite due to prolonged dry grinding Some solid state reactions takins place during grinding in a are given in Fig. 3. Decomposition temperatures of nitrogen atmosphere samples after grinding for various intervals are plotted in this figure. The amounts of aragonite No. R~action Remarks that were found to be formed according to the X-ray . - diffraction analysis are also shown in this figure. ZnS+AI1S0. -znSo. +Ag2S Pcbble mill. ( At the end often hours of grinding 26% of the calcite (white)(whitc) (whitc)(black) color obs. II CdS+AI2S0. -CdSO. +AI1S Ball mill. has transformed into aragonite, while at the end of (ycllow)(whitc) (whitc)(black) color obs. another 90 hours the transformation is almost 70% III 2PbO+3S 2PbS+SO2 Pebble mill. complete. Aragonite is the denser lower entropy (ycllow)(ycllow)(grey) - color obs. form of the two calcium carbonate polymorphs and X-ray diff. it appears that the lattice disturbances during grinding is sufficiently severe to produce transformation to the denser form. Heating of the above samples to 450°C for 2-3 hours converted all the color of the mixture was usedas a qualitative indi- calcium carbonate back to calcite. It was possible cator .of the progressof the solid state reaction. to repeat the cycle a number of times using one ReactionsI and II are instantaneousand are wholly sample. It might be noted at this point that the completein a few minutesgrinding in dry air or change in heat of solution often observed for calcite vacuum.Reaction III was found to consistof two after some grinding, attributed usually to particle distinct stageslO:continuous polymorphic transi- size change, might at least partly be due to the for- tion (checkedby X-ray diffraction techniqueand mation of some aragonite in the samples. Like color observation)of the oxide from massicotto aragonite, ground PbO samplesalso when annealed litharge up to 20-21 hours followed by a chemical for three hours at 450°C underwent complete re- reaction(identified by X-ray diffraction studies)in verse transformation. which the oxide in both forms is convertedinto The reactions conducted to confirm the oc- galena.In the time interval betweenthe 20th-and currence of solid state reactions during grinding in a 21st hours a distinct changein the mill noise oc- nitrogen atmosphere are listed in Table 2 The mix- curred.Examination of the compositionof samples tures were prepared in stoichiometric proportions from the mill showedthat the changesin the mill and ground for known intervals. The change in noisecoincided with spontaneousgalena formation.

PowderTechllol~ 6 (1912) "'"' PROPERTIESOF SAMPLESDURING PREPARATIONBY GRINDING 177

TIME. Mi.wl.. Fig. 4. Volume fraction growth of litharge and galena liS.time during dry grinding of a stoichiometric mixture or massicot and amorphous sulfur in a laboratory ball mill.

The progress of the formation of litharge as well as galena is indicated in Fig. 4. Since the results indicated that the galena formation begins only after the complete transformation of massicot to litharge, an additional set of experiments was conducted with a litharge-sulfur mixture. Results given in Fig. 4 show that when litharge is used in the original mixture instead of massicot, the galena formation occurs in 17-17; hours, the three-hour difference being possibly due to the lower energy input required in the absenceof polymorphic transition. In a parallel experiment, the grinding medium was omitted In this caseno galena formation took place over the same interval, the only result being some size reduction. The formation of galena during grinding of massicot and sulfur has been further confirmed by electron microprobe examination of the ground product. Figures S(a) and (b) are the electromicro- probe analysis pictures for lead and sulfur respec- tively of a typical ground sample. The ratio of lead atoms to sulfur atoms as calculated from these results is unity. giving evidencefor the formation ofa compound as galena(PbS) with the rest of the sulfur having been apparently removed from the system during the operation. The ratio of lead to sulfur before grinding was 2: 3. Further evidence of a solid state reaction was found in an experiment involving a mixture of crystalline quartz and crystalline HgI2- On grinding, the red iodide lost its color. possibly due to the Fig. S. Electron microprobe analysis for (a) lead and (b) sulfur decomposition of the Hgl2 at the fresh quartz sur- of an end product obtained by ball-milling a stoichiometric face as follows: mixture of massicot and amorphous sulfur. l Powder Techno/" 6 (1972) 178 J. LIN. P. SOMASUNDARAN 0 0 I I o-Si-o- o-Si.;.o- HJ+ + ( I I 0 +Hg11- 0 I I O-Si + o-Si+ I- I I 0 0 red mixture no color This observationis in agreementwith the results obtained by BenturlS by thennal means,namely mixing a small quantity of HgI1 with silicagel and heatingthe mixture on a bunsenflame. The discolo- ration that wasobserved in the caseof grindingwas gradualand passedthrough an intennediateyellow phase.possibly due to the polymorphictransition of HgIz.

CONCLUSIONS

The discussion presented above clearly shows that severalphysical as well as chemical changescan take place during the grinding of a sample.These include amorphousization of the surface. lattice distortions, polymorphic transformations and even chemical reactions between the components. The feasibility and extent of the alterations are dependent upon grinding conditions such as mill ( speed, grinding medium, grinding time, state and composition of the environment. and indeed temperature and pressure of the surroundings. Even contamination with the grinding media, as reported by Jamieson and Goldsmith27, who found mullite contamination in their sample ground in mortar and pestle, is to be expected while the materials are subjected to prolonged grinding. The implications of these effects on preparation of samples for chemical analysis by grinding must be recognized lest the final results yield misleading information on the actual character of the original material. A detailed knowledge of the various physical and chemical changes and the effect of grinding conditions on thesechanges is essentialfor selecting tests that are least likely to give any false information and for taking into account such effects while interpreting the final results.

REFERENCES

1 R. C. Ray. The effect or long arinding on quartz. Proc. Roy. Soc..A 102.(1923)640-642.

Po,,'dt7' T«hl/O/~ 6 (1972) ~ PROPERTIESOF SAMPLES DURING PREPARATION BY GRINDING 179

28 D. Ocepek, Mechanical and mech.no-cbemical reactions in 140 (1970); l J. Un. Fonnation of galena during comminu- crushing Processes.Rudarskl>-Met. Zbomik. (I) (1969) 5-16; tion process. Israel J. Earth Scj~ 20 (1971) 41-45. E. A. Prodan It at. Dct-omposition oCsodium triphosphate 30 J. H. Henderson. J. K. Syevs and M. L Jackson. Quartz hexahydrate during dry grinding. DokL Akod. Nauk &10- dissolution as influenced by pH and the presenceof a distur- russk.. 14 (6) (1970) S26-9 [C.A.. 73 (1971)72664z]. bed surface layer, Israel J. Chem.,8 (1970)357-372 29 1.-1.Un and A. Metzer. Changes in the state of solids due to 31 A. M. Gaudin, Comminution as a chemical reaction. Mill. comminution, T«hIIiO/I Fac. Cil1il £ng.. r"ternaJ Publ. No. EII9.,7(19S5)S61-S62. ~ , ., .

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