Density Separation of Solids in Ferrofluids with Magnetic Grids

Abstract

Nonmagnetic s o l i d s i n a superparamagnetic f e r r o f l u i d are s u b j e c t e d t o body f o r c e s p r o p o r t i o n a l t o the i n t e n s i t y of magnetization of t h e f l u i d and the g r a d i e n t of t h e magnetic f i e l d . the f o r c e equations, and s i n c e the apparent d e n s i t y can b e much l a r g e r than t h e t r u e d e n s i t y , i t i s p o s s i b l e t o l e v i t a t e o r f l o a t dense o b j e c t s . Mixtures of s o l i d s w i t h a d e n s i t y g r e a t e r than t h e apparent d e n s i t y sink w h i l e lower d e n s i t y s o l i d s f l o a t . An apparent d e n s i t y of the f l u i d can be d e f i n e d from I n p r a c t i c e i t i s d i f f i c u l t t o c r e a t e a uniform g r a d i e n t over a l a r g e volume and " s i n g l e gap" magnetic s e p a r a t o r s r e q u i r e very l a r g e magnets o r have a l i m i t e d throughput.problem, "multiple gap" magnetic g r i d s have been designed. Such g r i d s c o n s i s t of p l a n a r a r r a y s of p a r a l l e l b a r s of a l t e r n a t i n g p o l a r i t y , d r i v e n by permanent magnets. When immersed i n f e r r o f l u i d , magnetic g r i d s c r e a t e nonuniform f i e l d g r a d i e n t s and apparent d e n s i t i e s i n t h e f l u i d . However, both a n a l y s i s and experimental measurements show that t h e g r i d a c t s as a b a r r i e r t o p a r t i c l e s below a c r i t i c a l d e n s i t y , while p e r m i t t i n g more dense p a r t i c l e s To overcome t h a t 339 Copyright 0 1980 by Marcel Dekker, Inc. Downloaded by [Michigan State University] at 23:35 08 February 2015 340 FAY AND QUETS t o f a l l through t h e g r i d . Thus, a magnetic g r i d f i l t e r can b e used a s a h i g h throughput b i n a r y s e p a r a t o r of s o l i d s a c c o r d i n g t o their d e n s i t i e s . Such f i l t e r s can b e cascaded f o r more complex s e p a r a t i o n s . S e v e r a l magnetic g r i d f i l t e r s have been d e s i g n e d , b u i l t , and t e s t e d . Magnetic measurements q u a l i t a t i v e l y a g r e e w i t h t h e t h e o r e t i c a l p r e d i c t i o n s . Experiments w i t h s y n t h e t i c m i x t u r e s have demonstrated t h a t good b i n a r y s e p a r a t i o n s c a n b e made.

Magnetic Levitation in Chemistry, Materials Science, and Biochemistry

“…[154] Ferrofluids are one of the suspending media used in the early applications of densitybased separations of diamagnetic materials. [23,155] Although ferrofluids are optically opaque unless highly diluted, and have not been commonly used in the type of MagLev we are developing,t hey have useful characteristics to levitate diamagnetic samples (e.g., large magnetic susceptibility and commercial availability). They can be made biocompatible (one ferrofluid-based on superparamagnetic iron oxide nanoparticles-is used as ah uman therapeutic [156,157] and as aG d-free contrast agent for MRI imaging [158] ), and thus,c an be useful to study biological samples.…”