Dispersion and Grinding of Pesticides - ACS Publications - American

5 Dispersion and Grinding of Pesticides LEO A. DOMBROWSKI Chicago Boiler Company, Chicago,IL 60614

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch005

MARTIN SCHIERITZ W. A. Bachofen Company, 17 Utengasse Street, Basel, Switzerland The incorporation of insouluble, powdered solids into liquid media i s generic to the chemical coatings, printing ink and pigment dispersions and likewise to the preparation of dispersion concentrates of pesticides. Lessons learned from paints and inks in achieving complete wetting of powders and separation of particles to micron levels with dispersion s t a b i l i t y to forestall s e t t l ing and caking are quite applicable to aqueous and nonaqueous flowables. Available simple mixing and dispersing systems w i l l be discussed relative to producing stable wet concentrates of pesticides. Agricultural chemicals known as pesticides include herbicides, insecticides, and fungicides. These have been marketed as granules or powders and today there is a large market for water based suspensions. The suspensions contain in addition to the active ingredients, additives like surfactants, emulsifiers, and sometimes antifreeze. Suspensions ease filling and handling problems in the f i e l d , as well as storage problems in humid areas. Conditions for a good quality suspension require that at least 90% by weight of the active ingredient have a particle size below 10 microns and preferably be less than 5 microns in diameter, (1) Pesticidal suspensions are most efficient if they can be produced with the active ingredient i n a finely divided state with no agglomerates and with sufficient dispersion stability to prevent undo settling and compaction while in storage. The dry ingredients could be dry ground, but a wet grinding system would avoid the heat elimination problem and dusting that attends dry grinding to below 20 microns(2). Micron sized particles have strong cohesive forces which promote reagglomeration in 30 to 50 micron clumps i n the 0097-6156/ 84/ 0254-0063506.00/ 0 © 1984 American Chemical Society

Scher; Advances in Pesticide Formulation Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


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dry s t a t e i ^ j ) . I n p r a c t i c e , wet g r i n d i n g has an e x t r a b e n e f i t because d u r i n g the comminution, the a c t i v e i n g r e d i e n t can b l e n d w i t h t h e d i s p e r s a n t s n e c e s s a r y f o r s t a b i l i t y and homogenity* H i g h s h e a r wet g r i n d i n g equipment can r e d u c e t h e p a r t i c l e s i z e o f t h e a c t i v e i n g r e d i e n t t o t h e needed s i z e range and a l s o a c t i v a t e t h e c l a y s used i n many f o r m u l a t i o n s to s t a b i l i z e the s u s p e n s i o n . H i g h s h e a r equipment has n o t a l w a y s been used f o r p r o d u c t i o n o f d i s p e r s i o n s . The b a l l m i l l i s one o f t h e e a r l i e s t machines t h a t made d i s p e r s i o n s by t h e i m p a c t a c t i o n o f c a s c a d i n g s t e e l b a l l s o r round p e b b l e s i n a c y l i n d r i c a l drum t h a t s l o w l y t u r n e d on i t s l o n g a x i s , ( f i g u r e 1) The drum's a x i s o f r o t a t i o n was h o r i z o n t a l and t h e s l u r r i e d m i x t u r e s l o s h e d i n s i d e under t h e a c t i o n o f t h e b a l l s . The b a l l s c r e a t e i m p a c t and s h e a r a c t i o n a s t h e y s l i d e and c a s c a d e o v e r one a n o t h e r a s t h e drum r o t a t e s . B a l l d i a m e t e r i s from 12 t o 50 mm. S p h e r e s o f t h i s s i z e have a tremendous mass, comp a r e d t o a s i n g l e c r y s t a l o r g r a i n o f c l a y t h a t might be 1 mm. o r 1000 m i c r o n s i n s i z e . The b a l l m i l l i s c a p a b l e crushing strong c r y s t a l s or aggregates. I t i s a simple e f f e c t i v e m i l l , h u t n o t a h i g h speed p r o c e s s . I t i s also a b a t c h m i l l and l a r g e volume b a t c h e s w i l l n e e d a v e r y s t r o n g s e c u r e machine. I t was n o t e d t h a t s m a l l e r media was more e f f e c t i v e a f t e r some p r e m i x i n g , i n g e t t i n g p a r t i c l e s i z e s down i n the m i c r o n r a n g e , b u t t h a t t h e r o t a t i o n o f t h e drum had t o be slowed t o p r e v e n t media from f r e e z i n g t o c y l i n d e r s i d e s and l o s i n g t h e c a s c a d i n g a c t i o n , (k) A method o f a g i t a t i n g the media w i t h a s t i r r i n g s h a f t was d e v e l o p e d and much s m a l l e r media c o u l d then be employed. The A t t r i t o r , t h e i n v e n t i o n o f t h e l a t e D r . Andrew S z e g v a r i , i s such a device (£), C l o s e l y r e l a t e d to the b a l l m i l l , b u t w i t h a v e r t i c a l c y l i n d e r , i t has a c e n t r a l r o t a t i n g s t e e l s h a f t and p r o j e c t i n g s t e e l r o d s w h i c h s t i r the media i n t h e s t a t i o n a r y drum. ( F i g u r e 2) Originally a b a t c h p r o c e s s l i k e t h e b a l l m i l l , r e c i r c u l a t i n g systems have been added t o t h e g r i n d i n g chambers t o pump s l u r r y i n t o t h e bottom and then draw o f f t h e s u s p e n s i o n from the t o p o f t h e column. The m i l l i n g a c t i o n i s s t i l l p r e d o m i n a n t l y i m p a c t , b u t t h e r e i s an element o f s h e a r f r o m t h e r o d s p u s h i n g t h r o u g h t h e media bed w h i c h c o n t r i b u t e s t o t h e dispersion process. These m i l l s a r e used t o produce p e s t i c i d e s and have t h e a b i l i t y t o cope w i t h lumpy s l u r r i e s and s t i l l p r o d u c e f i n e d i s p e r s i o n s . S e v e r a l hours o r many p a s s e s t h r o u g h t h e r e c i r c u l a t i n g systems a r e r e q u i r e d to p r o c e s s a b a t c h . The A t t r i t o r r a i s e d t h e v i s c o s i t y l i m i t s o f f o r m u l a t i o n p r a c t i c e beyond t h a t o f t h e b a l l m i l l , even w i t h t h e s m a l l e r media because o f t h e a c t i o n o f the s t e e l r o d s .


Dispersion and Grinding


DOMBROWSKI AND SCHIERITZ

F i g u r e 2.

A t t r i t o r schematic d i a g r a m R e p r o d u c e d w i t h p e r m i s s i o n o f U n i o n P r o c e s s 0ο· house p u b l i c a t i o n



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T h e r e i s i n c e n t i v e t o r e d u c i n g media s i z e because t h i s p r o d u c e s more c o n t a c t p o i n t s p e r u n i t volume and i n c r e a s e s the s u r f a c e a r e a f o r s h e a r i n g a c t i o n * The v i s c o s i t y l i m i t i s approached as t h e s l u r r y l o s e s m o b i l i t y a t the c e n t r a l s h a f t and media a c t i o n i s l o s t * Hochberg and Bosse o f Ε . I» DuPont de Nemours, w i t h the "Sand G r i n d e r " p a t e n t (6) took t h e i d e a o f u s i n g s m a l l s p h e r i c a l m e d i a i n t h e form o f Ottawa s i l i c a sand w i t h an a v e r a g e d i a m e t e r o f .7 mm t o a p r a c t i c a l machine* Later g l a s s , c e r a m i c and s t e e l media m a t e r i a l s would become a v a i l a b l e and sand g r i n d e r s would e v o l v e i n t o bead m i l l s , s h o t m i l l s , a n d s m a l l media m i l l s * ( F i g u r e 3) L i k e t h e e a r l i e r m i l l s , t h e sand m i l l i s g r a v i t y dominated and the media must be a b l e t o r e s i s t f l o a t i n g i n a moving s l u r r y w i t h e v e r i n c r e a s i n g v i s c o s i t y a s m i l l i n g proceeds* L a r g e r media w i l l have more mass and t h e r e f o r e be a b l e t o work w i t h h i g h e r s l u r r y v i s c o s i t y * Since the d r y i n g r e d i e n t s i n p e s t i c i d e s a r e mixed w i t h l o w v i s c o s i t y w a t e r , v i s c o s i t y i s b u i l t up by t h e suspended s o l i d s . The o r i g i n a l sand g r i n d e r used a c e n t r a l s h a f t w i t h a e e r i e s o f a n n u a l r i n g s w h i c h s p i n between 1220 a n d 2*f00 rpm t o a c h i e v e a p e r i p h e r a l d i s c v e l o c i t y o f a b o u t 2000 f t * / m i n . or 10 m e t e r s / s e c . ( F i g u r e k) U n l i k e the b a l l m i l l o r A t t r i t o r , t h i s media i s a c c e l e r a t e d 200 - 300 G*s by t h i s c e n t r i f u g a l f o r c e a s compared t o 1 G i n t h e b a l l m i l l * The c o n t i n u o u s m i l l i n g a c t i o n o f t h e p r o d u c t i o n m i l l s depends on a r e g u l a t e d pumping s y s t e m t o push s l u r r y i n t o the b o t t o m o f the m i l l i n g chamber w i t h a c o n t r o l l e d r e s i dence time b e f o r e i t i s d i s c h a r g e d a t t h e top o f the m i l l t h r o u g h the media s e p a r a t o r . The o r i g i n a l m i l l s have c y l i n d r i c a l s c r e e n s a t t h e t o p t o r e t a i n media a n d an e x t e r n a l c o l l e c t i o n trough t o r e c e i v e the slufcry. There a r e r i n g s o r d i s c s i n t h e upper s c r e e n zone t o c r e a t e a c e n t r i f u g a l pumping a c t i o n t o a s s i e t the passage o f t h e s l u r r y through the s c r e e n . The p r e s e n c e o f t h e s c r e e n i n the u p p e r l o c a t i o n l e n g t h e n e d t h e s h a f t a s s e m b l y and t h i s i n t u r n p l a c e d a l i m i t on r o t a t i o n a l speed t o a v o i d d e structive vibration. Sand m i l l s o r bead m i l l s w i t h a v e r t i c a l s t i r r i n g s h a f t a r e c a l l e d v e r t i c a l m i l l s and a r e o l d e r t h a n t h e h o r i z o n t a l s m a l l media m i l l s w h i c h r e v e r t e d back t o a h o r i z o n t a l c y c l i n d e r l i k e t h e o l d e s t b a l l m i l l s . V e r t i c a l m i l l s have open s c r e e n s w h i c h c a n be v i s u a l l y checked d u r i n g o p e r a t i o n . There are a l s o c l o s e d or p r e s s u r i z e d v e r t i c a l m i l l s i n w h i c h i n s t r u m e n t s l i k e p r e s s u r e gages a n d thermometers must be used t o c h e c k o p eration. T h i s i s v e r y s i g n i f i c a n t t o good o p e r a t i o n s b e cause the m i l l must a c h i e v e an i n t e r n a l e q u i l i b r i u m so t h a t a s i g n i f i c a n t amount o f g r i n d i n g media w i l l be i n t h e m i l l i n g chamber and i n t e r a c t i n g w i t h t h e s u s p e n s i o n and n o t pushed up i n t o t h e s c r e e n o r s e p a r a t o r s y s t e m . The mass



F i g u r e 3· Red Head s m a l l media m i l l Model 8P Photograph c o u r t e s y o f Chicago B o i l e r

Co.

Figure k. S c h e m a t i c diagram o f v e r t i c a l m i l l d e s c r i b i n g a c c e l e r a t i o n and c i r c u l a t i o n p a t h o f media. Reproduced w i t h p e r m i s s i o n o f C h i c a g o B o i l e r Co. from l i t e r a t u r e C o p y r i g h t 1965·



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o f the sand o r the s p h e r i c a l media must be s u c h t h a t a t the f l o w r a t e o f the s u s p e n s i o n i n t o the m i l l , i t w i l l be f l u i d i z e d o r s e p a r a t e d , and y e t not be f l o a t e d up and o u t by the c o m b i n a t i o n o f v i s c o s i t y and f l o w rate» In p r a c t i c e t h i s i s e a s i l y a d j u s t e d on an open v e r t i c a l m i l l by obs e r v i n g the h e i g h t o f the s c r e e n e f f l u e n t * Too much v i s c o s i t y or t o o h i g h a f l o w r a t e w i l l b r i n g the m i x t u r e o f media and s u s p e n s i o n t o the o v e r f l o w p o i n t on an open screen v e r t i c a l m i l l . I n a c l o s e d m i l l system, t h i s i s d e t e r m i n e d by s l o w l y b r i n g i n g up the f l o w r a t e and o b s e r v i n g i n t e r n a l p r e s s u r e and t e m p e r a t u r e i n the m i l l . Any c l o s e d m i l l w i l l have a r e s t r i c t e d o p e n i n g which i s the media s e p a r a t i o n s y s t e m . A h i g h e r f l o w r a t e f o r an o p e n i n g o f f i x e d a r e a w i l l r e s u l t i n a h i g h e r p r e s s u r e t o push i t t h r o u g h . E x c e s s i v e v i s c o s i t y or flow or both i n the s u s p e n s i o n w i l l c a r r y the media i n t o the d i s c h a r g e end o f the m i l l and r e s u l t i n h i g h p r e s s u r e and a l s o h i g h t e m p e r a t u r e because the p a c k e d media w i l l be s k i d d i n g a g a i n s t the d i s c s and c i r c u l a t i o n w i l l be l o s t . The v e r t i c a l open s c r e e n m i l l i s the l e a s t e x p e n s i v e m i l l t o p u r c h a s e and m a i n t a i n or o p e r a t e w i t h n o m i n a l training. A disadvantage o f v e r t i c a l open s c r e e n m i l l s i n p e s t i c i d e p r o d u c t i o n i s t h a t the h i g h s h e a r c i r c u l a t i o n i n the c y l i n d r i c a l s c r e e n p e r m i t s l a r g e amounts o f a i r to be incorporated i n thixotropic suspensions. These same s u s p e n s i o n s can be d i f f i c u l t t o handle i n the g r a v i t y c o l l e c t i o n systems which r e q u i r e t h a t they f l o w away o f t h e i r own w e i g h t i n t o the t r a y . T h i c k e n i n g i n the a b s e n c e o f s h e a r i s a v o i d e d i n the pumping p a t h o f c l o s e d m i l l s because f e e d pump p r e s s u r e i s used t o convey the s u s p e n s i o n through a c l o s e d pipe system. The most s e r i o u s d i f f i c u l t y w i t h a v e r t i c a l s m a l l media m i l l i s t h a t g r a v i t y compacts the media t i g h t l y a round the s h a f t . The a n n u l a r r i n g d i s c s w h i c h produce s h e a r a l s o c r e a t e f r i c t i o n a l r e s i s t a n c e w i t h the media when the m i l l s t o p s . The t i g h t l y compacted media a g a i n s t the s h a f t can make s t a r t i n g d i f f i c u l t . High s t a r t i n g t o r q u e motors a r e recommended and o f t e n the s h a f t must be j o g g e d w i t h s h o r t i m p u l s e s o f power t o f r e e i t up and start. T h i s makes remote c o n t r o l o p e r a t i o n much more c o m p l i c a t e d and, i n some l o c a l i t i e s w i t h l i m i t e d e l e c t r i c a l c a p a c i t y , u s i n g r e d u c e d v o l t a g e s t a r t i n g systems f o r the main d r i v e motors becomes i m p o s s i b l e . The s o l u t i o n which came from Europe i s the c l o s e d h o r i z o n t a l m i l l . ( f i g u r e 5 ) A h o r i z o n t a l s h a f t i n the h o r i z o n t a l g r i n d i n g chamber i s easy t o t u r n w i t h the g r i n d i n g media d i s t r i b u t e d a l o n g i t s l e n g t h . The v e r t i c a l sand g r i n d e r l i t e r a t u r e (7) e s t a b l i s h e d t h a t media and s l u r r y were t o m a i n t a i n e q u a l volumes i n the m i l l i n g chamber f o r optimum i n t e r a c t i o n .



DOMBROWSKI AND SCHIERITZ

Figure 5·


Dyno-Mill h o r i z o n t a l small media m i l l Type KD 15 (15 L i t e r chamber). Reproduced with permission of W. A Bachofen AG, copyright 197^ #




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E s t a b l i s h i n g e q u i l i b r i u m c o n d i t i o n s i n t h e m i l l a l s o meant t h a t i n p r a c t i c e a c e r t a i n amount o f media was a l w a y s i n the s c r e e n zone a t t h e t o p o f t h e m i l l . The h o r i z o n t a l m i l l s do n o t have e x t e r n a l s c r e e n s o r s e p a r a t o r s and s o the media c o n c e n t r a t i o n i n t h e m i l l i n g chamber c a n be maintained a t optimal l e v e l . The s h o r t e r s h a f t and d i s c a s s e m b l y i n t h e h o r i z o n t a l m i l l i s c a p a b l e o f h i g h e r speed w i t h o u t d e s t r u c t i v e v i b r a t i o n (3^00 f t . / m i n o r 17 m e t e r s / s e c ) . The h i g h s h e a r r a t e s enhance media a c t i o n f o r p a r t i c l e s i z e r e d u c t i o n ( i m p a c t a c t i o n ) and a g g l o m e r a t e s e p a r a t i o n ( s h e a r a c t i o n ) . A l l s m a l l media m i l l s o r sand m i l l s have w a t e r c o o l i n g j a c k e t s t o c a r r y away p r o c e s s h e a t . In p r a c t i c e , a 50 degree F . t e m p e r a t u r e d i f f e r e n c e i s d e s i r a b l e between c o o l a n t t e m p e r a t u r e and p r o d u c t temperature a t 10 m/sec. Minimum f l o w r a t e i s d e t e r m i n e d by s u s p e n s i o n heat susceptibility. A v a i l a b l e m e c h a n i c a l energy i s h i g h and 5 t o 7 H. P. p e r g a l l o n i s a v a i l a b l e from the power t r a i n . H i g h f l o w r a t e s o r minimum r e s i d e n c e time make power consumption r e a s o n a b l e . The o u t s t a n d i n g o p e r a t i o n a l p r o b l e m w i t h s m a l l media m i l l s i s t h a t they are q u i t e dependent on a p r e v i o u s s t a g e i n t h e p r o c e s s t o p r o v i d e them w i t h lump f r e e s l u r r y . The d i t r i b u t i o n o f the wet and d r y i n g r e d i e n t s must be s u f f i c i e n t t o p e r m i t t r o u b l e f r e e pumping w i t h o u t s e t t l i n g problems i n t h e l i n e s . T h i s s t e p c a n be a c c o m p l i s h e d w i t h a batch p r o c e s s system u s i n g a h i g h speed d i s c i m p e l l e r i n either single shaft or multishaft design. (Figure 6 ) . When e q u i p p e d w i t h a v a r i a b l e speed s h a f t d r i v e s y s tem t h e h i g h speed d i s p e r s e r i s c a p a b l e o f i n c o r p o r a t i n g dry

ingredients

into

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to minimize

dusting. When immersion and w e t t i n g a r e c o m p l e t e , t h e d i s c speed can be b r o u g h t up t o **000 - 5000 f t . / m i n . T h i s h i g h s h e a r r a t e i s c a p a b l e o f d o i n g a c o n s i d e r a b l e amount o f d e a g g l o m e r a t i n g though p a r t i c l e s i z e r e d u c t i o n s h o u l d n o t be e x p e c t e d . Power l e v e l s a r e u s u a l l y f i g u r e d on t h e b a s i s o f 1 HP on t h e s h a f t f o r e a c h 10 g a l l o n s o f s u s p e n s i o n o r s l u r r y i n t h e premix tank a t 10,000 c p s v i s c o s i t y . I n p r a c t i c e t h e powders a r e i n c o r p o r a t e d w i t h some degree o f t u r b u l a n c e i n t h e l q u i d p h a s e . The l i q u i d volume s h o u l d be r e s t r i c t e d t o g e t t h e h i g h e s t v i s c o s i t y c o n s i s t e n t with g e t t i n g c i r c u l a t i o n t o a l l p a r t s o f the mixing v e s s e l . High p a r t i c l e c o n c e n t r a t i o n w i l l h e l p g e t goo§ i n t e r p a r t i c l e a c t i o n among the s l u r r i e d a g g l o m e r a t e s . Careful attention t o t e m p e r a t u r e r i s e w i l l e n s u r e t h a t no heat damage i s done. The d e s i r e d e f f e c t a t t h i s p o i n t i s t o g e t l a m i n a r f l o w a n d good v o r t e x i n g . Only 10 t o 20 m i n u t e s a f t e r t h e l a s t d r y i n g r e d i e n t s a r e added, r e s u l t s s h o u l d be e v i d e n t . Longer times without r e s u l t s are i n d i c a t i v e o f improper



Figure

6.

H i g h Speed D i s c d i s p e r s i o n s y s t e m s . I d e a l i z e d f l o w i n m i x i n g v e s s e l and t y p i c a l installations* Reproduced by p e r m i s s i o n o f D i a f AG from house l i t e r a t u r e . Copyright 1979



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A D V A N C E S IN P E S T I C I D E F O R M U L A T I O N

TECHNOLOGY

i n g r e d i e n t b a l a n c e and can even l e a d t o heat p r o b l e m s . P r a c t i c a l f o r m u l a t i o n d a t a i n an e a r l y d e f i n i t i v e work was p u b l i s h e d by F r e d e r i c k D a n i e l (8) which i s e s p e c i a l l y u s e f u l f o r o i l based systems* A p r o c e s s c o n c e p t w h i c h may h e l p t o f i x the t e c h n i q u e i n mind i s t o c o n s i d e r the h i g h speed d i s c to be deagg l o m e r a t i n g a t a h i g h v i s c o s i t y , f o l l o w e d by a l e t d o w n s t e p o r r e d u c t i o n i n v i s c o s i t y f o r the s m a l l media m i l l * The i n i t i a l h i g h v i s c o s i t y produced good i n t e r p a r t i c l e a c t i o n i n the s l u r r y * T h i s i s c o n t i n u e d i n the s m a l l media m i l l but a t r e d u c e d v i s c o s i t y i n the p r e s e n c e o f permanent a g g l o m e r a t e s c a l l e d g r i n d i n g media* Because no more than 1 p e r c e n t o f the p a r t i c l e s a r e p r o b a b l y i n c o n t a c t w i t h the d i s c s i n the m i l l , good c i r c u l a t i o n and r e p e a t e d c o n t a c t s a r e e s s e n t i a l t o the s t a t i s t i c s o f d e a g g l o m e r a t i n g the b i l l i o n s o f p a r t i c l e s i n the the s u s p e n s i o n . E x c e s s i v e v i s c o s i t y impedes the a c t i o n and i n c r e a s e s heat i n the m i l l . The m i l l s w i l l work o v e r a wide range o f v i s c o s i t i e s but p r o d u c t i o n e f f i c i e n c y w i l l s u f f e r i f the mix i s wrong* Appropriate l a b o r a t o r y t e s t s made w i t h s i m i l a r equipment s h o u l d be made and c e r t a i n l y when making i n g r e d i e n t s u b s t i t u t i o n s * Excessive v i s c o s i t y w i l l r e s u l t i n incomplete d i s p e r s i o n , h i g h powerdraw and damaging t e m p e r a t u r e s * Very low v i s c o s i t y c a u s e s h i g h m i l l wear, h i g h media wear, and sometimes unwanted p a r t i c l e s i z e r e d u c t i o n s * Some p r o d u c t s w i l l not g r i n d e a s i l y and may have s t r o n g c r y s t a l s up t o 1 mm i n s i z e o r a t l e a s t s i g n i f i c a n t numb e r s beyond 150 m i c r o n s . S l u r r y p a r t i c l e s as l a r g e as t h i s w i l l cause v e r y low p r o d u c t i o n r a t e s i n the s m a l l media m i l l o r n e c e s s i t a t e r e p e a t e d p a s s a g e s t h r o u g h the m i l l * The 1 to 1,5 mm media d i a m e t e r s f r e q u e n t l y used f o r m i l l i n g must have a 10 to 1 s i z e advantage o v e r the c r y s t a l s f o r effective milling. L a r g e p a r t i c l e might r e q u i r e a 2 s t a g e s y s t e m w i t h l a r g e 2 o r 2*5 mm media i n the f i r s t m i l l f o l l o w e d by a s e c o n d m i l l w i t h s m a l l e r media. T h i s d u p l i c a t i o n can be e x p e n s i v e and i n E u r o p e , c h e a p e r r o t o r s t a t o r m i l l s a r e used o f t e n t o a c h i e v e the g r o s s r e d u c t i o n . (£) These i n c l u d e t o o t h c o l l o i d m i l l s , T r i g o n a l t o o t h r i n g m i l l s , and a b r a s i v e d i s c m i l l s ( s t o n e mills). H i g h s h e a r l o a d i n g i s a c h i e v e d by c o n t r o l l i n g the gap (0.03 mm t o 2 mm) between the r o t a t i n g p a r t s . The s l u r r y i s pumped t h r o u g h the c e n t e r o r outward between the r o t o r and s t a t o r s u r f a c e s . These m i l l s a r e not a s common i n the USA owing to the a v a i l a b i l i t y o f micrènized d r y ingredients* A c t u a l l y e x c e s s i v e p a r t i c l e s i z e r e d u c t i o n o f the a c t i v e i n g r e d i e n t w i l l i n c r e a s e s u r f a c e a r e a and t h e r e f o r e s u r f a c t a n t demand* T h i s i s not easy to measure w i t h i n s t r u m e n t s but soon a p p a r a n t i n p r a c t i c e *



5.

DOMBROWSKI A N D SCHIERITZ


73

The latest combination of high speed disc dispersere and small media m i l l s offer the pesticide manufacturer versatile high capacity tools which require minimum plant floor space and i n s t a l l a t i o n problems. The combination of shear and impact action i s s u f f i cient to break the t y p i c a l l y small crystals of active ingredients to desired size and the high shear w i l l activate suspension agents adequately for desired s t a b i l i t y . As compared to the older b a l l m i l l or A t t r i t o r s , their action has more impact and pulverization effects than a small media mill. With proper selection of raw materials which do not require pulverization, i t i s possible to formulate suspensions for high production with minimum process time. Premixed ingredients with p a r t i c l e sizes in the 30 to 60 micron range can be e f f i c i e n t l y processed with small media m i l l s at profitable flow rates. This i s an achievable condition for a wide range of products. LITERATURE CITED 1. Harrison, T. "Pesticidal Dispersion and Method for Preparing the same" U.S. Patent 3 157 486, 1964 2. Carr, W. Journal O i l Chemists CA, 1982, 65, 373 3. P a r f i t t , G. D. XIVth FATIPEC Congress Budapest 1978 4. J a r , B a l l and Pebble M i l l i n g , May 1980. Publication of Norton Chemical Products Co. 5· Szegvari, A. "Ball Containing Apparatus for Finely Grinding l i q u i d suspended p a r t i c l e s . U.S. Patent 3 008 657. 1961 6. Hochberg, S. & Bosse D. "Agitator B a l l M i l l - Sand Grinding Process. U.S. Patent 2 855 156 Nov. 1958 7· Bosse, D. "Pigmented Systems and Application of the Sand Grinder" Purdue Univ. Short Course March 1959. 8. Daniel, F . "Determination of M i l l Base Composition for High Speed Dispersers" Jour. of Paint Technology 1966 V o l . 38. No. 500 9. Giersieden & Schwedes, "Experiences with machines used for wet reduction in the Chemical Industry" Chemie Ingenieur Technik 1982 RECEIVED February 9, 1984


Dispersion and Grinding of Pesticides - ACS Publications - American

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