I
R. R. IRANI, C. F. CALLIS, and T. LIU Research Department, Inorganic Chemicals Division, Monsanto Chemical Co., St. Louis 66, Mo.
How to Select
Flow Conditioning and Anticaking Agents I
Salt won’t pour, sugar lumpy? Don’t give up. This study determined the effectiveness of several common conditioners in improving flow properties and inhibiting caking and probable mechanisms for their dramatic effects in handling materials
FISELY
divided powders have been used for some time as conditioners to improve handling properties of powdered and granular materials, a phenomenon which logically includes flow conditioning and cake inhibition. A flow conditioner is generally defined as a n additive that will aid a powder in maintaining a steady flow and,’or increase its flow rate through an orifice located in the base of a container. Little information is available about the relative effectiveness of powders used as conditioners or the reasons for the observed dramatic changes in physical nature of a mixture containing only a small percentage by weight of the conditioner (70). Caking and its inhibition have received relatively more attention because of its importance in the fertilizer industry (3-5, 7, 76). Thus, lumpy or caked fertilizer, salt, sugar, and the like are difficult to handle or apply uniformly. No attempt was made to study all possible conditioners, materials, cake evaluation tests, flow conditioning tests, or combinations of these. Flow and caking tests were selected because of their sensitivity to observed differences in the handling properties of the conditioned products. Experimental
Materials to Be Conditioned. Cocoa,
dichlorodiphenyltrichloroethane (DDT), instant powdered milk, niter cake, powdered salt, and Baker’s Special sugar (intermediate sized material) were all unconditioned, proprietary brand products selected a t random from various manufacturers and from grocers’ shelves. Micronized, precipitated sulfur was reagent grade material.
Conditioners. Calcium sulfate and magnesium carbonate were reagent grade materials. Cornstarch, diatomaceous earth, and kaolin were technical grade conditioners. Calcium silicate hydrate was a proprietary brand, conditioner grade material. Santocel C (Monsanto) commercial, finely divided silica. and the tricalcium phosphate (Monsanto) used was a commercial conditioner grade. Commercial tricalcium phosphate was a n orthophosphate with a n apatite x-ray diffraction powder pattern (75). The 100-micron glass beads used in the flow test were obtained from Minnesota Mining and Manufacturing Co. Table I shows particle size distribution and Table I1 chemical and physical properties of the materials used in this work. The tricalium phosphate was classified into different size fractions with a B.A.H.C.O. classifier (H. W. Dietert Co., Detroit, Mich). Flow Test. I n most cases, when
I
powders are mixed with coarsrr I-reeflowing materials, the mixture exhibits better flow than the original powder (70). This effect was used as the basis of the flow test. Two funnels were set up, one directly above the other on a ring stand, with the funnel tip of the upper one 1 inch abovr the lower funnel opening. These were 60’ powder funnels with a 4-inch maximum diameter cone and a 5,’s-inch diamrtrr outlet tube, 0.75 inch long. The lower end of the bottom funnel stem was stoppered. Fifty to one hundred grams of sample (depending on its bulkiness) was gently poured into the top funnel, thus permitting the sample to disperse and settle into the lower funnel. After alloiving 3 minutes for trapped air to escape from the powder bed, the cork was removed without gross disturbance of the sample. If the sample completely flowed out of the funnel, it was classified as “frer flowing.” Such samples were quantita-
Selecting a Conditioner Price Performance: If a conditioner has desirable properties and other factors are unimportant, cost is the determining factor. Ease of Handling, Blending: An obvious expense is blending cost and possible reduction in plant throughput. Adaptability to existing facilities is important. Effects on Chemical and Physical Properties: All such changes must be evaluated. These effects are less important the smaller the amount of additive required. Toxicity: Toxicity in handling or in final usage limits any conditioner. NutritionalValue: A plus value for tricalcium phosphate. Adulteration of Foods: Conditioners should not be used in foods without first satisfying the FDA that they are harmless food additives.
VOL. 51, NO. 10
OCTOBER 1959
1285
Conditioner : 0 Tricalcium Phosphate D Calcium Silicate Hydrate 0 Santocel C
-
Conditioner 0 Trlcalclui Phcrphctr
Calclam S l l i c o t r H y d r a t r
E . i
c
0 Dlatomacronr E a r t h
0
-E 900 -D I
0 Santccrl C
.6 0
+
=D
-
'* 0
700
n
z 6
e 0 ly
p 500 Percent C o n d i t i o n e r b y Weight
0 c 0 0 D
A Figure 1 . Improvement in flow properties of micronized sulfur mixed with conditioning agents depends on conditioner and level used
b Figure 2. cocoa
Effect of conditioners on the flow properties of
U
a
n
300 .)
:i m
5
100
0
After reaching oDtimurn level further a d d i t i o n of conditioning a g e n t m a y inhibit flow
tively compared by measuriiq drlivcry rate in weight per unit time. If the sample did not complrtely flow out of the funnel. the minimum weight percentage of 100-micron glass beads necessary to render the mixture "free flowing" was determined. This \vas systematically done in each case, with a 1 to 1 mixture being tested first. Qualitative observations. as !\-ell as measurements of angle of repose and difficulty of flow through variously sized orifices, showed that the poorrr the flow, the more glass beads must be used. Accelerated Caking Test. Tiventy grams of lump-free sample \vert spread evenly in a Petri dish 1.5 inches deep and 3 inches in diameter, Lvhich \vas p1acc.d in a desiccator. Desiccator temperattirr \vas maintained a t 27' 1 O C. and humidity a t 60 or 757, with saturated solutions of ammonium nitrate or sodium c h l o r i d ~ , respectively, in the desiccator base. T h e sample was left for or weeks, depending on its hygroscopicity. Insufficient moisture pickup forms no cake while too much saturation cakes all samples, conditioned and unconditioned, so that no relative differencrs can be shown. After humidification, the sample \vas removed from the desiccator and dried in another desiccator ovcr calcium chloride until no further change in weight occurred. usually 12 hours. The dried sample was carefully transferred onto a screen that Mould normally let all the original sample pass through..
+
1 286
0
s1iakt.n to separatr uncaked matrrial. T h e tkvo fractions \\-ere \veiglied. and the pcrcentage caked (left o n the screen) \\-as then calculated. Particle Size Distribution. Particle sizr distribution riirasurrinenrs (Table I ) ivrrr inadr \\-ith an rlrctronic counting and sizing trchiiquc ( 7 7) described earlier ( I ) . Static Surface Electric Charge. Frictionall>- generated charge \vas measurrd \vith a devicr basrd 011 streaming potential principles (21. Particles of the po\vdrr \ver'e accelerated with knoivn air llo\v rates and allowed tu impingc on the insidc \vall of a brass nozzlr. Charged particles n w r then collected i n a target similar to a Faraday cagy (1-1) made of t\vo thin. concentric cylindrrs insulated \\-ith I 8-inch Plrsiglas. 'I'he 1ir1chargr of the particles collected in r h r targrt \vas measurrd with a high iinprdance rlrctromcter, and iveiglit of rhe particles collected and charge per u n i t \\-right were then calculated Results and Discussion
Flow Conditioning. Data showing comparative flow conditioning effectivrness of tricalcium phosphate, calcium silicate hydrate. diatomacrous earth. Santocel C: or kaolin on micronized sulfur are presented in Figure 1. Similar d a t a are showm in Figure 2 for cocoa treated \vith tricalcium phosphate. calcium silicate hydrate. or Santocel C .
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Percent Conditioner 2 b3y Weipht
4
5
1.01 tsach cc~nditionrr-iiiatcl.id~ s) st h e w is an optirnum conditiont.~ 1 br);ond which fio\v inil). i i r ~ t chxrigc. significantly or becointas ~ I ~ ~ J I Y ~ I ~ . Flo\vability a t this Irvel may IX markrdl) difkrrnt for diflerrnt condirioiicrs. Strraming potential rneasurcinf'nts oi nrt static surfact. charqx of cc)nditioncw a n d iintreated and c o n d i t i o n e r - t i ~ r ~ ~ t i ~ ~ rriaterials arr siuriniarizrd in ?'al)lr I 1 1 . .4lrhough thtrrr is n c ) general relationship hc~twreri How conditioning and frictionally grncrated surfricc. charge. Tht. nc't c m a i n rrrnds art> apparen'. static surface charge of tht: conditionc~rs is highly negative rompared with That of conditioned matrrials: prrsuniably brc,iiisc> of relatively small particle. size ol' thc conditioners i?'ablc 1 i , Also. both sign and magnirude of tht. t~liargr diflrr for different matrrials. it conclusion in aqrrcinent with otht,r rrsl!lts i72) Strraming potential ~ n r a s u r c ~ n e also n~s s h o w that particles of conditioner and of matrrial hriris conditionrd in t('rd(.l lricxionall~-. I n thc case o f sugat' ;ind sulfiir conditioned with tricalcium pliosphatr. negative cliargr o f the conditioned material is usually greatrr than that of eithcr conditioner or unconditioncd material. If there \+.we 110 interaction. charge of the mixture ~vouldhave h r e n intrrrnrdiate. Tlir alteration of powder fio\vability on addition of a conditioner can tie explained in gcnrral tcrms. For a conditioner to pcrfurm rffrctiv must adhere t o t.hr surface o f thr mat [TITI.
FLOW CONDITIONING AGENTS to be conditioned. Otherwise, it will merely mix with it and not be properly distributed. Microscopic examinations (70, 73j of conditioned material have shown that particles are uniformly coated with conditioner. Calculations of the number of particles of conditioner per particle of material are consistent with this observation. For example, if a hypothetical conditioner has 1-micron particles and is used to the extent of 1% (weight) in a hypothetical powder having 20-micron particles, there will be 80 conditioner particles per powder particle, assuming equal particle densities. From a practical point of view, the conditioner must be much finer than the material to be conditioned. When a material of a larger particle size is used as a flow-improving agent additive weight is generally exceedingly high, up to 1 0 0 0 ~ oof the material. In such cases, fines of the material being conditioned adhere to the surface of the large particles of flow-improving agent (6).
Table 1.
Adherence of conditioner to the surface of particles being conditioned must alter their gross surface characteristics, including frictional properties in languid motion. Thus, if a powder possesses rough edges that cause particle interlocking, the conditioner would probably produce a smoother and less frictional surface by filling voids and acting as ball bearings. Because particles of flow conditioners readily adhere to each other, as evidenced by poor flow of the conditioner itself, it seems reasonable to explain the existence of an optimum concentration of flow conditioner in terms of a "saturation" effect. Thus, after &e point of "saturation" is reached flocs of conditioner particles will form and retard over-all flow. Nonadhering particles of conditioner will also contribute to the dustiness of the material as they are usually small in size. Cake Inhibition. Only values a t 75y0 relative humidity are reported, as results a t 60y0 relative humidity always
Particle Size Distributions of Materials and Conditioners" Most of the conditioners had relatively small particle size Distribution, % Diameter, Microns 0-2 2-3 3-4 4-5 5-6 6-7 7-8
Materials to be conditioned 1 3 1 8 1 9 8 5 Sugar 3 0 1 8 1 9 7 6 Sulfur 6 8 8 7 7 Salt 13 17 19 13 8 Cocoa Conditioners Tricalciumphosphate 38 27 16 9 4 Santocel C 100 CaSiOa hydrate 82 8 4 2 1.0 Diatomaceous earth 56 16 10 6 4 a By number. To convert to weight see ( I ) .
Av. Size, Micronsb 8-9
9-10
10
5
5
7
4
4 3
6 6
5
5
5
3
3 2 4 3
2.5
1.5
0.8
0.2
1.0
2.4
0.6 2
0.3 2
0.2 1.5
0.1 1
0.2 1.5
1.9
20 4 44 13
3.0 3.1 8.5 4.0
. . . . . . . . . ... ... ... ... ...
Table II.
...
* Geometric mean diameter.
Properties of Materials and Conditioners
%
Bulk Density, Lb./Cu. Ft. Loose Packed Materials to be conditioned Cocoa DDT Milk Niter cake (NaHSOI) Salt Sugar Sulfura
...
15.5 44.2 17.1 83.0 37.2 51.3 18.1
Conditioners CaSOP 30.7 Cornstarch 30.6 Diatomaceous earth* 11.2 Kaoline 14.9 MgCOs" 6.2 Santocel Cd 3.5 CaSiOa hydrate 12.7 Tricalcium phosphatee 21.8 a Reagent grade. a 4% AlzOa, 90% Sios. PZOsratio -1.3.
Weight Loss at 110' C.
39.5 59.2 28.5 85.2 67.5 58.1 42.2
7.2 1.8 5.7 2.5 0.04
71.3 50.6 27.3 41.6 10.0 11.0 25.4 40.8
0.53 10.2 7.9 0.25 1.7 0.91 6.3 2.1
Hydrated Mz(Si0a)r.
0.05 0.41
d
93% SiOn.
s
CaO:
Table 111. Static Charge of Powders" Particles of conditioner and material being conditioned interact fractionally Air Flow, Liters/ Min. Powder TCPh CaSiOa Diatomaceous earth Sugar
+ +
25 39 Measure Charge, Coulomb/Gram -0.054 -0.402 8.0 -0.068 -0.068 -0.053 -0.074 -0.123
-
Sugar 3% TCP Sulfur Sulfur 3% TCP Sulfur 3% CaSiOa hydrate -0.069 NaCl +0.172 NaCl 2y0 TCP -0.023
+
+
+
-0.182 -0.932 12.0 -0.044 -0.041 -0.313 -0.153 -0.311
-
-0.098 fO.343 +0.141
-0.022
NaCl 2% CaSiOa hydrate $0.237 t0.278 Cocoa $0.711 +0.835 Cocoa+ 0.5% TCP $0.512 +0.705 Cocoa 0.5% CaSiOa hydrate +0.360 +0.477 Streaming potential measurements (23' i '1 C.,R.H. = 37 =k 0.5%) with nozzle having 7-mm. inside diameter. b Monsanto's commercial tricalcium phosphate with Ca0:PsOs ratio 1.3.
+
paralleled those a t 75y0 and were less discriminative. Results are shown in Table IV. For samples of D D T stored 1 week at 60 or 7570 relative humidity, 40% of the unconditioned D D T was retained on a 10-mesh screen. Tricalcium phosphate, calcium silicate hydrate, and diatomaceous earth reduced the percentage caked to 30, 20, and lo%, respectively, but beyond 0.570 (weight) conditioners had no further effect. Caking experiments showed optimum conditioner levels, and conditioners differed in optimum level performance. The mechanism of caking and its inhibition have been fairly well established (3-5, 7-9, 73, 76). Important variables in caking experiments are moisture level, temperature, pressure, impurities, and time of storage. Thus, many combinations of variables can be selected, and, depending on the combination, different caking behavior can be demonstrated. I n these experiments, pressure and temperature were fixed (1 atm., 27' C.) while relative humidity and time of exposure were varied. In all experiments samples were redried because in normal handling it is the alternate forming of a liquid phase and its drying that contributes significantly to caking (3). The presence of moisture, whether in the sample or artificially introduced, is necessary for caking to take place. If a material is relatively water insoluble, the effect of water on its caking characVOL. 51, NO. 10
OCTOBER 1959
1287
Figure 3. Caking of milled salt when mixed with 1% tricalcium phosphate of different particle size distribution The finer the preparation, the more effective it is
Tricolcium Phosphate A v e r a g e Number-Particle
Table IV.
Effect of Conditioners on Caking
Conditioners differed in optimum performance
level
.\111l111111
(
t-.
