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206 I PRINCIPLES OF MINERAL PROCESSING

GRAVITY CONCENTRATION I 207

we concentrate at an overall ratio of concentration of about 1,000:1. The cassiterite is then removedfrom the other heavy minerals in a "dry" plant using magnetic, electrostatic, and gravity methods.

Ragging. Where other jigs typically remove most of the heavy particles from the bed, usually by adowneomer and darn arrangement (see Figure 6.11), the placer jig recovers most ofthe valuable material through the jig sieve and into the hutch. Accordingly, to

prevent much of the fine material from flowing directly into the hutch, ragging

must be used. The ragging consists of about three layers of particles having aspecific gravity similar to that of the heaviest particles being jigged. Hence, steel shot (p = 7.8) is used formany ores, hematite (p = 5.0) for cassitcrite, and feldspar(p = 2.56) for raw coal, in many jigging operations,the heavy constituent in the ore being jigged will quickly accumulate during operation and serve as the

ragging. However, when the material to be jigged lacks relatively coarse heavy minerals, artificial ragging isrequired.

Jig Cycles

Ilarz-type jig cycle impressed on the system by the plunger takes a sinusoidal form.

71..)mson, Laros, and Aplan (1985), who used pressure transducers in the hutch

-,- the actual situation is quite different. In the Denver, Pan.

only remotely follows the sinusoidal form

. , a d o n t h e s y , t e i n a n d d u e : , h i g h s p e e d s . T v v o p r e s s u r e t r a n s d u c e r s w e r eused in a Baum Jig, the front one below the jig bed and the rear one in the hutch water near the air cylinder.

The rear pressure trace showed a strong pressure spike of very short duration when air was admitted to thejig. The front trace was greatly muted and showed only a small pressure increase during bed fluidization.In

all of these studies, the closest approach to the sinusoidal form by a Harz, Denver Mineral, or a Pan-AmPlacer jig occurred only at excessive speed. A bed could not be maintained, water flew from the jig bed, andthe jigging process was out of control.

Just as a strong pulsion stroke favors the jigging of coarse particles, so should a strong suction strokeand consolidation trickling favor the recovery of fine, heavy particles. The use of a full-suction jig torecover fine particles illustrates the case. Another way to accomplish the same end is to use a sawtoothcycle, in which a short-duration pulsion stroke is followed by a prolonged suction phase. This cycle has

been suggested in the literature, and studies by Laros and Aplan (1991) showed its effectiveness inrecovering fine, heavy particles using either a Harz or a Baum feldspar jig. Greater details on jigs and

jigging are available elsewhere (Taggart 1945; Burt and Mills 1984; Pickett and Riley 1985; Leonard andHardinge 1991).

FLOWING FILM CONCENTRATORS, SLUICES, AND SHAKING TABLES

The processes described in thissection are used to treat intermediate- and fine-size particles in the

range of, roughly, 1/4 in. (6.4 mm) to about 15 pm and include many of the intermediate-particle-size and

nearly all of the fine-particle-size concentrating devices listed in Table 6.2. No one device is fully effective

in concentrating this entire feed size range. Furthermore, it is rarely possible to simultaneously achieve a

high recovery and a high concentrate grade, so the rougher concentrate is cleaned, often several times, on a

similar or a different device to achieve an acceptable final concentrate grade.

Flowing Film Concentration

In a flowing film concentration process, a thin layer of a slurry of fine particles in water flows down a

slight incline and is subsequently washed with a gentle flow of water. The particles on the incline will then

distribute themselves in the sequence of fine, heavy particles highest upslope, coarser heavy particles and

fine light particles in between, and coarser light particles farthest downslope. The extremely fine particles,

the slimes, are lost to the water discharge.

This process dates from antiquity, certainly several centuries BCE It is so elementary (an indinied flat

rock probably was used initially) that it was likely developed independently at several locationsaround the

globe. By the sixteenth century in Europe, Agricola in his treatise DeRe Metallica (1556) described the use of

a buddle, a form of flowing film concentrator in which the deposited heavy particles are repeatedly pushed

back uphill with a hoe to facilitate the further removal of light particles by flowing water. Subsequently,

similar devices-such as the strake-were developed. This device used materials such as canvas, hides, and

blankets to achieve a slightly roughened surface that recovered particles not readily collected on a smooth

surface. Flowing film devices, although largely made obsolete by the flotation processes (especially for

sulfides), are still used today in certain situations: to treat (1) those minerals not effectively concentratedby the flotation process; (2) large tonnages of presized material containing only a few percent of a heavy

mineral (typified by beach sands); or (3) minerals in primitive or very small-scale operations, such as those

in Southeast Asia, where the lanchut is used to remove impurities from cassiterite concentrate.

Because flowing film concentrators have historically been used to treat fines and slimes, it is neces-

sary to define these terms. Unfortunately, the terms are moving targets whose definitions have changed with

the material being treated, the devices available, and time. Fines, before the advent of flotation, werenften

defined as -10 mesh (1.7 mm), and slimes were those particles below about 150 mesh (100 ).sm). Today,

fines are considered to be particles below either -10 (165 iim) or -100 mesh (150 pm), and slimes are often

defined as about -15 pm, although these definitions may vary depending on the mineral assemblage being

treated. Flowirg film concentrators, like all gravity concentration devices, cause minerals to be recovered or

rejected on a particle-by-particle basis. The major defect of flowing film concentrators is that for fine

particles, innumerable decisions must be made to recover even a small weight of concentrate. Because the

layer of particles in the flowing film is only one to a few particles thick, the goal of equipment designers has

been to design a device with a large flowing film surface area that occupies a small floor area.

Flowing Film Conoentraticn Principles

v, p'g sin 0(20 z)),

211.

The thin film of water that flows down the incline shows a vertical, half parabolic-like flow pattern

ranging from near zero at the surface of the deck to a maximum near the top surface of the flowing film.

The size of the particles to be treated will influence the-depth of film required-the coarser the particle,

the thicker the flowing film. The push of the fluid will obviously be greater on the larger particles in the

fluid film, assuming that they are totally incorporated into the flowing film. The fluid velocity v', at any

distance in the film from the top surface, may be calculated (Gaudin 1939; Michell 1985) by(Eq. 6.9)

wherep' = the fluid density

g = the acceleration caused by gravity

a = the angle of the incline from the horizontal

= the fluid viscosity 13

= the film thickness

The feed is invariably in the form of a wet slurry, but penetration through the flowing film will be a

function of the size, shape, and density of the particles; the pulp density and viscosity; and the thickness and

velocity of the film. Based on Stokes' law (Eq. 6.2) and Eq. 6.10, the distance traveled by a spherical

particle from the top to the bottom of the film, z, is (Michell 1985)

-18p,Q

z (Eq. 6.

10) 2(p - p')d2g cos et

where Q is the flow rate per unit time and width.