Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 513
found for estimating the composition for a high-recovery case, and it would seem that in such a case, the composition must be determined rigorously without the use of short-cut methods. From Tables I11 and IV we can see that the short-cut method using the simple approach can be used to obtain reasonably close agreement to a rigorously calculated solution of packing height and outlet composition for problems which do not involve a high recovery. For a high-recovery problem, the packing height can be reliably estimated using the Edmister-type approach t o the short-cut method, but no simple method was found to calculate the outlet composition. Nomenclature a = interfacial surface per unit volume of packing, area/ volume A = absorption factor, L/K,G, dimensionless D = diffusivity, area/time F = mass-transfer coefficient, mol/time (interfacial area) G = gas mass velocity, mol/time (area) h = heat-transfer coefficient, heat/time (area) degree h' = heat-transfer coefficient corrected for mass transfer, heat/time (area) degree H = enthalpy, heat/mol HJ,= partial enthalpy of component J at temperature Ti and concentration x J L ,heat/mol Ht = height of a transfer unit, length K = vapor-liquid equilibrium constant, dimensionless L = liquid mass velocity, mol/time (area) N = flux of mass transfer, mol/time (interfacial area) Nt = number of transfer units, dimensionless P = partial pressure Q = flux of heat transfer, heat/time (interfacial area) R = defined by eq 10 S = stripping factor, K , G / L , dimensionless Sc = Schmidt number,'p/pD, dimensionless T = temperature, deg x = liquid concentration, mole fraction y = gas concentration, mole fraction YJ = gas concentration, mol of J/mol of insoluble carrier gas 2 = packing height, length
A = increment p =
viscosity, mass/length (time)
K
= total pressure
p
= density, mass/length3
Subscripts and Superscripts A = component A, a solute E = effective value G = gas i = interface j = a typical component in a system consisting of a total of n components, ranging from 1 through n; jk is the "key" component J = a typical component in a system consisting of a total of J components, ranging from A through J L = liquid n = component n, last in a system of n components next = condition at next level of packing N P = bottom tray of tray power 0 = overall S = component S,the solvent 1 = at bottom of packed tower; also tray 1 at top of tray tower in eq 19 and 20 2 = at top of packed tower L i t e r a t u r e Cited Bassyoni, A. A., McDaniel, R., Holland, C. D., Chem. Eng. Sci., 25,437 (1970). Dwyer, 0. E.. Ph.D. Dissertation, Yale University, New Haven, Conn., 1941. Edmister, W. C., Ind. Eng. Chem., 35 (8),837 (1943). Feintuch, H. M., Ph.D. Thesis, New York University, New York, N.Y., 1973. Holland, C. D., McMahon, K. S.,Chem. Eng. Sci., 25, 431 (1970). McDaniei, R. Ph.D. Dissertation, Texas A & M University, College Station, Texas,
1970. Oldershaw, C. F., Simenson, L., Brown, T., Radcliffe, F., Chem. Eng. Prog., 43 (7),371 (1947). Othmer, D. F., Gilmont, R., Ind. Eng. Chem., 36 (9),858 (1944). Othmer, D. F., Scheibel, E. G., Trans. A.I.Ch.E., 37, 211 (1941). Raal, J. D., Khurana, M. K., Can. J . Chem. Eng., 51, 162 (1973). Rubac, R. E., McDaniel. R.. Holland, C. D., A .I.Ch.E. J., 15 (4), 568 (1969). Sherwood, T. K., Pgford. R. L., "Absorption and Extraction", 2nd ed,McGraw-Hill, New York, N.Y., 1952. Simon, M. J., Govinda Rau, M. A., Ind. Eng. Chem., 40 (l),93 (1948). Treybal, R. E., "Mass-Transfer Operations", 2nd ed, McGraw-Hill, New York, N.Y., 1968. Treybal, R. E., Ind. Eng. Chem., 61 (7),36 (1969).
Received for review December 7, 1977 Accepted April 18, 1978
Selective Hydrocracking of n-Paraffins in Jet Fuels Nal
Y. Chen' and William E. Garwood
Mobil Research and Development Corporation, Princeton and Paulsboro Laboratories, Princeton, New Jersey 08540 and Paulsboro, New Jersey 08066
Lowering the freeze point of paraffinic jet fuel by the selective conversion of n-paraffins is shown to be feasible over metal loaded molecular sieve zeolites, Ca-A and H-erionite, under hydrocracking conditions. Comparable experiments over large-pore Zeolite X show that in the absence of shape selectivity, the freeze point of the jet fuel is raised instead of lowered on account of the preferential conversion of the more reactive branched molecules. The reaction over Ni/H-erionite is shown to be sensitive to the partial pressure of hydrogen and sulfur. The presence of sulfur, and/or the absence of high hydrogen partial pressure (- 1000 psig) inhibits the conversion of n-paraffins. The decrease in activity is explained by the accumulation of intracrystalline inhibitor concentration which increases with hydrocarbon partial pressure, but decreases with increasing hydrogen partial pressure. Sulfur had the additional effect of promoting the conversion of non-normals. Similar activation by sulfur was reported by Dudzik on Zeolite K-A .
Introduction Normal paraffins are present in all petroleum fractions. Their concentrations depend on the crude source and 0019-7882/78/1117-0513$01.00/0
intervening processing history. Despite the relative inertness of n-paraffins, Weisz and Frilette (1960) demonstrated the selective conversion of n-paraffins over a
0 1978 American Chemical Society
514
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
Table I. Properties of Charge Stock gravity, " M I gravity, specific 25-plate distillation, IBP, "F ASTM dist., O F , IBP 5% 10% 20% 50% 90%
E.P. freeze point, O F pour point, "F composition, wt % (mass spec.) paraffins naphthenes aromatics
53.0 0.7669 358 398 408 410 414 426 460 494 -41 -40 89.8 8.4 1.8
molecular sieve zeolite, 5A. The same catalyst did not convert branched paraffins which cannot diffuse through the intracrystalline pores. Compared to other hydrocarbons in the same boiling range, n-paraffins have higher melting points. In the manufacture of such fuels as jet fuel, kerosene, and home heating oils, the end points of these fuels are often determined by the freeze point and/or pour point specifications which are related to the n-paraffin concentration. In gasoline production, n-paraffins also are the lowest octane component in the product. Thus, the selective conversion of n-paraffins has the promise of establishing a new class of catalytic processes in petroleum refining. Commercial application of selective conversion of n-paraffin boiling in the C5 to C12range to boost octane rating of gasoline was first described by Chen et al. (1968). Robson et al. (1974) reported the selective hydrocracking of C5/C6naphtha over synthetic erionite. Giannetti et al. (1975) studied selective hydrocracking of reformates and a natural gasoline with ferrierite. In this paper we report the shape selective hydrocracking of Clo to CI6n-paraffins present in kerosene type jet fuels over nickel/H-erionite and platinum/Ca-A catalysts. Earlier studies (Chen and Garwood, 1973) on lower molecular weight molecules demonstrated that the cage structure of erionite can influence the reaction rate of the incoming molecules. The cage structure can also force interactions between molecules of varying chain length within the crystalline structure. Thus, one of the objectives of the present study was to extend the investigation of intracrystalline molecular interaction to longer chain molecules. A more practical objective was to explore the potential of the application of shape selective catalysis to the manufacture of low freeze/pour point kerosene and jet fuels.
Experimental Section A. Apparatus. The experiments were conducted in a high-pressure microreactor capable of operating up to 3000 psig. The reactor, enclosed by a three-zone heater, had an isothermal reaction zone holding up to 10 cm3 of catalyst. Accurate material balance was facilitated by a liquid withdrawal system which consisted of two highpressure collection vessels arranged in series with a valving arrangement so that while the first vessel was collecting the liquid product, the second vessel could be isolated from the system, depressurized, drained, flushed, and repressurized and could rejoin the system without upsetting its pressure. B. Charge Stocks. Charge stock A was a highly paraffinic jet fuel made by reforming a selected kerosene, followed by SO2 extraction. Some of the properties and the chemical composition of the feed are summarized in
Table 11. Carbon Number Distribution of Feedstock A (Freeze Point -40 OF) wt % normal/ n-paraffins non-normal
wt % feed 1.2 10.5 31.6 33.3 14.6 6.2 2.6
-
0.4 3.3 9.4 7.4 3.1 1.2 0.2
0.54 0.45 0.42 0.29 0.27 0.23 0.11
25.0
0.33
-
100.0
Table 111. Carbon Number Distribution of Feedstock B (Freeze Point - 30 F) ~
wt % feed
wt % n-paraffins
0.8 5.8 16.1 20.8 18.5 21.4 16.6
0.3 1.8 4.8 4.6 3.9 4.0 1.6
100.0
21.0
Table IV. X-Ray Diffraction Pattern of Nevada Erionite Sample
hkl
d
l ~ l l l oxo 100
100 101 002 110 102 200 201 202 21 0 21 1 300 21 2 104 302 220 21 3 310 204 311 312 400 21 4 401 402 41 0 322
11.39 9.12 7.55 6.60 6.30 5.74 5.36 4.59 4.32 4.16 3.81 3.75 3.58 3.41 3.30 3.28 3.18 3.15 3.11 2.93 2.86 2.84 2.81 2.68 2.50 2.48
100 4 4 55 3 21 15 9 71 24 42 47 15 2 32 10 14 12 6 7 58 36 36 14 20 13
Table I. A detailed analysis of the distribution of normal paraffins and non-normal hydrocarbons (isoparaffins and cyclic hydrocarbons) according to their carbon number (Table 11) showed that the feedstock contained 25.0 w t % n-paraffins, or about three non-normal molecules to one normal paraffin, and the carbon number ranged from Clo to C16 with more than 96% in the range of Cll-Ci5. Feedstock B which had a pour point of -30 O F was prepared by a 1:l blend of feedstock A with its highest boiling fraction. The carbon number distribution of feedstock B is shown in Table 111. The n-paraffin concentration in feedstock B was lower, about 3.8 non-normal molecules to 1 normal paraffin, but the total concentration of C14+ n-paraffins was increased from 4.6 to 9.5 w t 70. C. Catalysts. Natural erionite was obtained from Jersey Valley, Nevada. Its X-ray diffraction pattern as shown in Table IV is similar to that reported by Robson
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
Table V. Chemical ComDosition of Catalysts wt % Ni/Hbrionite
Pt/Ca-A
Ni/RE-X
68.6 16.5
SiO,
--
Na, 0
0.1
KZO
3.9 2.8 5.6 0.2 2.3
CaO NiO MgO
--
Pt
46.5 20.8 26.7 0.8
--
--
5.2
--
---
et al. (1974). It was first converted into the ammonium form by ion-exchanging with a 5 N ammonium chloride solution at room temperature and then exchanged with a refluxing 0.5 N nickel acetate solution. The finished catalyst has the composition as shown in Table V. The platinum containing Ca-A catalyst was prepared by crystallizing the zeolite Na-A in a mixture containing [Pt(NH3)4]2+cations (Weisz et al., 1962). The resulting Pt/Na-A zeolite was then calcium exchanged in an alkaline medium (Chen, 1968). The finished catalyst contained 0.2 wt 70 platinum. Its composition is also shcwn in Table V. Both erionite and zeolite A have intracrystalline channel systems with openings made of eight-membered oxygen rings (Breck, 1974). In the cationic forms as prepared, both catalysts have the ability to adsorb n-paraffin hydrocarbons and exclude branched hydrocarbons. The Ni/RE-X catalyst was prepared by first ion exchanging Na-X powder with a 5 wt % solution of mixed rare earth chlorides (cerium/lanthanum) solution at 180 O F . The dried catalyst was steamed at 1200 O F , 15 psig for 20 h, then impregnated with nickel nitra1;e solution at room temperature. Zeolite X is a large-pore zeolite having a three-dimensional channel system consisting of cavities separated by 12-membered oxygen rings. It absorbs both normal and branched paraffin hydrocarbons. These catalysts were dried, pelleted, and sized to 14/25 mesh, calcined at 1000 OF in air, and reduced in hydrogen a t 900 O F before use.
D. Analytical Procedures. The chemical compositions of both liquid and gas products were determined by mass spectrometry. The normal paraffinic concentration in the liquid products and their carbon number distribution were determined by a subtractive gas chromatographic method. The sample was analyzed first with a 2-ft silicon gum rubber (SE-30) column and then analyzed again with the same column attached behind a 3-in. Ca-A zeolite column which adsorbed all n-paraffins These same chromatograms also gave a measure of lower boiling products in the liquid. Boiling range values obtained this way agreed well with that obtained from a 25-plate Piros-Glover spinning band distillation. Standard ASTM methods were used to determine pour point and/or freeze point of the product. Results and Discussion A. Comparison of Various Catalysts. Three catalysts, Ni/H-erionite, Pt/Ca-A, and Ni/RE-X were compared under hydrocracking conditions of 2000 psig and 27/1 hydrogen to hydrocarbon ratio. The experimental results are summarized in Table VI. Ni/RE-X was considerably more active than the other two catalysts and, as expected from the crystal structure of the zeolite, it did not preferentially hydrocrack the straight chain paraffin hydrocarbons. Both erionite and zeolite A showed selective hydrocracking of n-paraffins. Freeze point measurements of the hydrocracked jet fuel products clearly show that hydrocracking over a large pore zeolite catalyst led to an increase in freeze point, while selective conversion of n-paraffins over the small-pore zeolite catalysts led to lowering the freeze point of the jet fuel. From Figure 1we note that 2-methyl branched paraffins in the Cl0-Cl6 carbon range are 40 to 60 O F lower in melting point than the n-paraffins, and the 3-methyl paraffins, n-alkyl cyclopentanes, and n-alkylbenzene even lower. Thus, unless substantially more normal paraffins than the chemically more reactive non-normal hydrocarbons are converted in the process, it would not be possible to lower the freeze point of the jet fuel to any significant degree. It is also interesting to note that in the case of zeolite A and erionite the freeze point was lowered by 15 O F with a better than 80% yield of the jet fuel product. Although the overall conversion to 358 O F minus products is less than 2070, it is misleading to infer from the overall conversion
Table VI. Comparison of Catalysts at 2000 psig, 27/1 H,/Hydrocarbon LHSV, h-l temperature, "F feedstock wt % conversion (to 358 "F minus) n-paraffin converted, wt % non-normals converted, wt % jet fuel yield, wt % A freeze point, "F freeze point depression efficiency, O F / % conversion distribution of cracked products normalized to 100% C.-C,
c;-c:
C,-358 "F composition of 180-358 "F naphtha paraffins naphthenes aromatics composition of jet fuel paraffins naph thenes aromatics
Ni/H-erionite
Pt/Ca-A
30 750 A 18.2 42.3 10.2 81.8 -15 0.8
0.5 800
5 49 46 75.2 19.2 5.6 91.2 8.0 0.8
515
B 13.5 41.8 6.1 86.5 -15 1.1
3 30 67
Ni/RE-X 2 480 A 12.3 3.0 11.6 87.7 +10 - 0.8
20 650
A 78.8 69.0 82.0 21.2
+ 10
-0.8
13 87
2 25 73
-
75.9 23.7 0.4
87.4 12.1 0.6
-
93.0 6.9
97.3 2.7
-
Clz n-paraffins can be attributed to the diffusional effect imposed by the cage structure of erionite. Although parallel diffusion studies have not been made on zeolite A, in view of the three-dimensional openness of its pore structure, the result of the present study would suggest that a similar kind of cage effect does not exist in zeolite A. Examination of the carbon number distribution of the cracked products also provided evidence of the cage effect of erionite in producing less C,+ products and more propane than either Ca-A or RE-X. A more detailed comparison of the distribution of C6- products is shown in Figure 3. Although Pt/Ca-A in several respects was superior to Ni/H-erionite, considerations of activity and stability-two of the key requirements of a practical catalyst-led to the selection of Ni/H-erionite as the catalyst for further process study. B. Variables in Processing Jet Fuel over Ni/Herionite. 1. Residence Time. The effect of residence time on the conversion of individual components in the jet fuel was studied at 2000 psig, 27/1 H,/hydrocarbon mole r,atio, and 14 to 30 LHSV. The rate of disappearance of individual n-paraffins and branched paraffins of each carbon member and the disappearance of cyclics followed that of a first-order reaction. The relative reaction rate constants obtained for each group of hydrocarbons, with
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 517
Table VIII. Effect of Pressure on Freeze Point of Jet Fuel (Ni/H-erionite, 750 OF, 14 LHSV, 27/1 H,/Hydrocarbon ) pressure, psig conversion, wt % n-paraf f ins non-normals jet fuel yield,owt % freeze point, F A freeze point, “F freeze point depression efficiency OF/% conversion
200
500
15.6 16.2 18.2 12.1 82.3 87.0 -38 -36 +2 t 4
-
-
1000
58.2 21.8 69.1 -52 -12 0.4
2000
73.8 22.7 64.6 -64 -24 0.7
k of n-CI1taken as 100, are shown in Table VII. The rate constant for n-paraffin conversion decreased with increasing chain length, while the rate constant for isoparaffins did not follow the same pattern. The conversion of branched paraffins and that of cyclics are attributed to catalytic sites external to the erionite crystals or to the natural impurities associated with zeolite ore body. The major reaction of the cyclics appears to be the scission of aliphatic side chains. Of the total cyclics converted, more than 50% appeared as cyclics in the naphtha fraction. It is also interesting to note that the naphtha from Ni/H-erionite was significantly more aromatic than that from the non-shape selective Ni/RE-X (Table VI). Under the reaction conditions used, aromatics are expected to be essentially completely hydrogenated. The preservation of aromatics is an indication of the selective nature of the hydrogenation function in Ni/Herionite. 2. Effect of Pressure. Pressure has an unexpected effect on conversion. In the first series of experiments the pressure was varied from 200 to 2000 psig. Space velocity, temperature, and hydrogen/hydrocarbon mole ratio were kept constant. The results are summarized in Table VIII. The most unexpected finding was that at or below 500 psig total pressure, the jet fuel products had a higher freeze point than the original feed indicating that the catalyst under these conditions was no longer shape selective. The conversion data indicate that there was a sharp decrease in the rate of conversion of n-paraffins as the pressure was lowered from 1000 psig to 500 psig. The decrease in conversion is much higher than could be accounted for by the change in residence time due to the difference in pressure. On the other hand, increasing pressure, and thereby increasing residence time, had almost no effect on the conversion of non-normal hydrocarbons above 500 psig. Thus, the net effect of decreasing pressure is the loss of shape selectivity for the n-paraffins. This is consistent with the freeze point data. In the next series of runs, the partial pressure of hydrogen was varied by (1) replacing hydrogen with a 1/1 mole ratio of hydrogen/nitrogen and (2) increasing the hydrogen to hydrocarbon mole ratio from 27 to 200. The results are summarized in Table IX. They show that high partial pressure of hydrogen appears to be necessary for the selective conversion of n-paraffins. Replacing hydrogen by nitrogen/ hydrogen had a disastrous effect on catalyst activity. It is not expected that pressure should have any effect on intracrystalline diffusion of hydrocarbon molecules in a zeolite. The fact that the nature of the gas had such a strong effect on catalyst activity suggests that the experimental results cannot be explained on the basis of simple diffusional effects. Instead, the competition for active sites between reactants and “unsaturated products” acting as inhibitors could provide a likely explanation. It is plausible to expect that the concentration of “inhibitors” would increase with the partial pressure of
Table IX. Effect of Partial Pressure of Hydrogen and Hydrocarbon (Ni/H-erionite, 750 “F, 1 4 LHSV) total pressure, psig gas
gas/ hydrocarbon mole ratio conversion, wt % n-paraffins non-normals jet fuel yield, wt % freeze point, “F A freeze point, “F freeze point depression efficiency OF/% conversion
1000 H* 27 58.2 21.8 69.1 - 52 -12 0.4
1000
1000
500
HJN, 27 21.3 10.0
85.1 -46 -6 0.4
1.2 5.3 95.7 -40 0
-
16.2 18.2 82.3 -38 +2
-
Table X. Effect of Sulfur (Ni/H-erionite, 750 OF, 1 4 LHSV, 27 H,/Hydrocarbon) total pressure, psig wt % S in feed conversion, wt % n-paraffins non-normals jet fuel yield,owt % freeze point, F
2000 none 73.8 22.7 64-6 -64
2000
1000
1000
1
none
1
28.1 34.4 67.2 -35
58.2 21.8 69.1 -52
6.5 21.2 82.5 -34
hydrocarbons. This would account for the conversion behavior of the non-normals with the changes in pressure. It is also plausible to believe that the “inhibitors” could have an even larger inhibitory effect on the intracrystalline sites, because the inhibitor molecules not only reduce the concentration of available sites but could also impede the diffusional process by blocking the intracrystalline channel through competitive adsorption. The lack of shape selective conversion of n-paraffins at below 500 psig of hydrogen pressure and the disastrous loss of activity when nitrogen replaced hydrogen are consistent with this postulated mechanism. The recovery of intracrystalline activity by increasing the partial pressure of hydrogen to 1000 psig and above suggests that the concentration of intracrystalline inhibitor is in dynamic equilibrium with the hydrogen. The lack of similar hydrogen effect on the conversion of non-normals is another indication of the shape selective nature of the hydrogenation function in Ni/H-erionite. The experimental results at 1000 psig and 200 H,/hydrocarbon can also be explained by the postulated role of hydrogen. Compared to the results at 27 H,/hydrocarbon a sevenfold difference in residence time affected conversion by only about a factor of 2, indicating that the higher concentration of hydrogen and lower concentration of hydrocarbon led to lower product inhibition and consequently higher conversion. 3. Effect of Sulfur. The addition of 1 wt % sulfur as thiophene in the feed also had a significant effect on conversion. Of the thiophene added more than 95% was converted to hydrogen sulfide and less than 1%remained in the liquid product. Since thiophene is too large a molecule to enter the pores of erionite, the reaction must have taken place external to the erionite crystals. From the data shown in Table X, the H,S generated inhibited the conversion of n-paraffins and at the same time promoted the conversion of non-normals particularly at 2000 psig. The activation of Na-X by SO3complex was reported by Miale and Weisz (1971). Dudzik et al. (1968) and Dudzik (1970) reported activation of zeolite K-A by sulfur containing free radicals in cracking 2&dimethylbutane, a molecule too large to enter the pores in K-A. Therefore, the observed activity may be attributed to external surface activation effects. However, the inhibition of intracrys-
518
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978
talline activity by sulfur was unexpected. It is possible that the sulfur complexes formed inside the pores hehave like the “inhibitors” described earlier and impede the passage Of the reactants through the pores. On the other hand, the inhibition effect could be the indirect result of sulfur poisoning the hydrogenation sites leading to the buildup of inhibitor concentration. Literature Cited Breck, D. W., “Zeolite Molecular Sieves”, Wiley, New York, N.Y., 1974. Chen, N. Y. (to Mobil Oil CorD.). U S . Patent 3373 110 (Mar 12. 1968). Chen, N. Y., Maziuk, J., Schwa&, A. B., Weisz, P. B., Oii Gas J., 66 (47),154
(1968). Chen, N. Y., Lucki, S. J., Mower,
E. B., J. Catal.,
13, 329 (1969).
Chen, N. Y.. Lucki, S. J.. Anal. Chem., 42, 508 (1970). Chen, N. y., b r w o O 4 w. E.. Adv. Chem SW., NO. 121, 575 (1973). Dudzik, Z.,Preston, K. F., J. Colloid Interface Sci., 26, 374 (1968). Dudzik, 2.. us. Patent 3516947 (June 23, 1970). Giannetti, J. P.. Perrotta, A. J., Ind. Eng. Chem. Process Des. Dev., 14, 86
(1975). Gorring, R, L,, J , catal,, 31, 13 (1973), Miale, J. N., Weisz, P. B., J . catal., 20, 288 (1971). Robson, H. E., Hamner, G. P., Arey, W. F., Jr., Adv. Chem. Ser., No. 102, 417 119741 .,. Weisz, P. B., Frilette, V. J., J. Phys. Chem., 64, 382 (1960). Weisz, P. B., Frilette, V. J., Maatman, R. W., Mower, E. B., J. Catal., 1, 307 I . _ .
(1962).
Received f o r review December 7, 1977 Accepted April 17, 1978
Chemical Grinding Aids for Increasing Throughput in the Wet Grinding of Ores Richard R. Kllmpel*’ and Wllly Manfroy2 Physical Research Laboratory and Functional Products Department, The Dow Chemical Company, Midland, Michigan 48640
The effects of chemical grinding aids for wet ore grinding have been analyzed in both batch laboratory ball mills and continuous industrial scale ball and rod mills. Two important industrial results have been achieved: first, increased feed rate at constant product size; and second, the production of a finer product at constant feed rate. All data have been characterized using the concepts of specific rate of breakage, S,and breakage product distribution, B. The engineering mechanism involved with the use of selective chemical additives is one of allowing more dense slurries to interact with the tumbling media while still maintaining the slurry fluidity and mass transport characteristics of less dense slurries.
Introduction Laboratory and industrial grinding tests have shown that the process of size reduction can be significantly influenced by chemicals added to the powder or slurry being ground. The terms grinding aid or grinding additive refer to a substance which when mixed into the mill contents causes an increase in the rate of size reduction. The increased rate can be used to grind a higher feed rate to the desired product size or it can be used to produce a finer product size a t fixed feed rate. Whether the use of a grinding aid is justified in any given situation depends on the cost of the substance vs. the improvement of output or product quality obtained with its use. Obviously, an expensive chemical must be effective in very small concentrations if it is to be economically justifiable; the cost criteria is calculated on the basis of the cost of the grinding additive per ton of material ground. Although there is direct experimental verification of the advantageous effect of grinding additives, no sound engineering explanation has yet been offered which explains or predicts the general behavior of additives for general mineral processing usage. Rose and Sullivan (1958) have listed most of the work undertaken prior to 1950, Snow (1973) has summarized the implications of selected references, and Hartley et al. (1976) have recently prepared an updated synopsis of the grinding additive literature. Many of the studies reported consist of subjecting mal
Physical Research Laboratory. Functional Products Department. 0019-7882/78/1117-0518$01.0010
terials with simple geometric shapes to some type of hardness or controlled single fracture test. On the other hand, a number of the studies were carried out on operating industrial scale mills, with little control or precise monitoring of the effect of the additives. Out of this work has come a bewildering array of hypotheses to explain the action of grinding aids. The prevention of particle agglomeration and grinding media coating as well as altering the strength of macro porous rocks by liquid presoaking have been suggested. Another mechanism which is often quoted is attributed to Rehbinder and Kalinkovskaya (1932), who suggested that the adsorption of additive on the surface of solid particles lowers the cohesive force which bonds the material of the particles together. In particular, adsorption on the surfaces of a flaw in the surface of a solid could affect the bonding forces and surface energy at the point where fracture initiates as discussed by Griffith (1920) and Austin and Klimpel (1964). Westwood (1966) has demonstrated in a series of articles the effect of adsorbed molecules on various surface mechanical properties and he refers to the phenomena in general as chemomechanical effects. This phenomenon suggests that the adsorbed molecules may “pin” dislocations near the surface thus preventing easy movement of dislocations under stress gradients. Since plasticity is due to the movement of the dislocations, the region near the surface of the solid is thus rendered more brittle. The surrounding molecular environment can certainly affect the critical stress-strain required to produce fracture under conditions where the fracture initiates from a flaw 0 1978 American Chemical Society
