Gravity Concentration and Magnetic Separation C.T.Castillo., I.E.Domingo., F.M.Miranda., L.W.Pile Department of Mining, Metallurgical and Materials Engineering University of the Philippines, Diliman Abstract This paper discusses two concentration methods used in mineral processing, Gravity Concentration and Magnetic Separation. Gravity Concentration processes like HandPanning is the earliest process used for separation of gold from others minerals foundriverside, it is governed by the principles of difference in specific gravities in which the denser minerals tend to settle faster compared to light materials. In the experimentconducted, 96.97% of the valuable mineral was collected and the others are reported as tails. Shaking Table method, another Gravity Concentration process performed takes into account the middling portion of the sample. Factors that affect that efficiency of the shaking table like particle size and shape of the feed, the inclination angle of the deck, and the length of the stroke was also discussed. The last experiment conducted was to determine the percent recovery of Fe using a Denver Magnetic Separator. The desiredvaluables attach to the magnet when passed through a tube with flowing water. Percentrecovery was as high as 107.18%. A very high percent recovery was attained because ofinevitable errors. 1. Introduction Concentrati on in mineral p rocessing is the sep ara tio n of the val uable min erals from the gangu e. The most importan t phys ical meth ods used to concentrate ores are those which depend on optic al prope rtie s as well as spec ific gravity differences, surface properties, electric and magnetic properties. In this experiment, two physical methods were utilized in order to concentrate the given samples namely the specific gravity difference also known as the gravity concentration and the magnetic separation of minerals. Gravity concentration methods separate minerals of different specific gravities by theirrelative movement in response to the force ofgravity and other forces such as the resistance to mot ion off ere d by a vis cous ele ment suc h as water or air. For effective separation, there should exist a marked density difference between the valua ble and the gangue. The concentration criterion given by: ( Dh - Dt ) / ( Dl - Dt ) [1] where Dh is the specific gravity of the heavy mineral, Dl is the specific gravity of the light mineral and Dt is the specific gravity of the fluid medium. When the absolute value of the ratio is 2.5, then the gravity separation is relatively easy. Hand Panning (Figure 1) is a gravity conc entra tion that is bein g prac ticed since the 15 th century until the present day. It is the oldest method of mining gold. It uses a shallow pan that has a slight indentation in the center; the oldergold pans may have been produced by machines from metal or fashioned by hand from wood oreven cow horns. Modern pans are made from metal or, more recently, from plastic. A typical pan, two to three inches deep, measures from about a foot to a foot and a half in diameter at the top and sev era l inc hes less in dia met er at the botto m. The sepa rati on t akes place with the manual action of rotating and at the same time tapping the pan.
Another gravity concentration technique usesshaking tables (Figure 2) and in the experiment,the Wilfley shaking table was used. It consists of
a large flat surface which is slightly inclinedfrom back to front and left to right. Water flow ismaintained across the table and the pulp is fedsimultaneously to the feed the box. Then, areciprocating horizontal motion causes theparticles to separate because of the relativelyslow forward motion and a very rapid return.
Figure 2. Shaking Table
Magnetic separation exploits the difference inmagnetic properties between the ore mineralsand is used to separate either valuable mineralfrom non-magnetic gangue. It is used for concentrating paramagnetic elements which areattracted along the lines of magnetic force topoints of greater field intensity whichautomatically separates them.
The objective of this experiment is for thestudents to be acquainted and be familiar withthe concepts and processes introducedbeforehand.
2. Methodology
A sample ore mainly composed of Iron andsilica was subjected to the following
concentration methods for analysis of recoveryand performance of the process itself.
2.1 Hand Panning
A 300g (270grams of Si and 30grams of Fe)
of pre-crushed and ground sample passing 100mesh was analyzed for hand panning. Thesample is then placed in a wooden pan which issmoothly indented from the center. A little water was added and the mixture of material wasmixed by hand carefully to break up lumps. Thepan was then tilted slightly and a rotating motionwas applied with constant tapping. Water wasallowed to overflow in another container and theprocess was continued with regular addition of water until the water which comes out wasalready clear. The concentrate left in the pan wasdried and weighed.
2.2 Wilfley Shaking Table
A sample of 1200g of the synthetic mixturementioned above was evaluated using theshaking table. The water and power source wereturned on and separation of the samples wasobserved. The resulting concentrate, middlingsand tails were collected and dried and the driedsamples are then weighed.
2.3 Magnetic Separation
The iron-silica ore sample was investigated
using a low intensity wet magnetic separator.The ore sample contains 30g iron and 70g silica.The electromagnets and water supply wereturned on. Sufficient amount of sample was fedinto the separator. The 100g sample was run bybatches. The non-magnetic minerals werecollected. When all of the magnetic materials arecircling in the electromagnets, the electric supplyof the magnets was turned off. Then, themagnetic minerals were collected.Results and Discussion
The swirling circular motion in panning makesthe silica suspended with water while the ironstays at the bottom of the pan. When the panner tilts the pan as he swirls, some of the water andsediment starts to spill out. This is done untilalmost all the sediments have fallen out, leaving
behind only the heavy metal which in our case isiron. Factors that would greatly affect therecovery using this technique would be thefamiliarity of using the pan. It would take quitepractice to master the movement for goodseparation. Also, the ratio of specific gravities of valuable to waste would be a factor for separation.
Hand panning is indeed one of the simplestways to separate the heavies from the waste. It ispopular then because of its cheap cost andrelatively simple and easy process which may bethe reason why small scale miners use it untilnow. However, good recovery requires muchpractice of the motion required to fully separatethe valuables. Hand panning is not used in largeplants because of its manual operation and lowcapacity of recovery. It is also hard to separateores with minerals only slightly differing inspecific gravities using the said process. Thedistribution of particles in the pan can be easilyseen for the valuables settle and does notsuspend in the water (Figure 3)
. Figure 3. Hand Panning Process
WEIGHTTotal 1200 gTails
Middling 765 g( with plastic)Concentrate 133.4855g
Table 2: Wilfley
The shaking table utilizes longitudinalvibrations by the head of the motor using a slowforward stroke and a rapid return; this causes themineral to “creep” along the deck parallel to thedirection of the motion. And since wash water isdistributed along the feed side, minerals are thus
subjected to two forces, the longitudinal motionof the table and the force due to the flowingwater which attacks parallel to the motion.
The net effect is that the particles move in adiagonal motion across the deck (Figure 4). Andsince the effect of water is dependent on size anddensity of the particle, they will segregate in thetable. Smaller, denser ones will report to theconcentrate launder while larger, lighter ones arewashed into the tailings launder.
Figure 4. Distribution of table products
Separation using this process is controlled by anumber of operating variables, such as washwater, feed pulp density, deck slope, feed rateand feed size. Particle size is of utmostimportance because as the range of sizes in atable feed increases, the efficiency of tablingdecreases. If the feed was in that case, thecollected middling will not be only composed of those locked particles, but also with coarse dense[particles and fine light ones. In order to increaseefficiency of tabling, the use of multi-spigothydrosizer is required to produce a feed of narrow sized range, and each spigot product, of equally settling rates, are fed to separate set of shaking tables.
Vertical stratification due to table motion takesplace within the riffles on the table deck. It runsparallel with long axis of the table. Their purpose
is to stratify particles so that the finest andheaviest are settled and the coarsest and lightestare easily carried away with the water at thesurface. Usually, riffled tables normally operateon feed sizes in the range of 3mm to 100µm.Particles smaller than this will not separate
efficiently by gravity methods due to their extremely slow settling rates.
Figure 5. Vertical Stratification between
riffles
Mineral behaviour on the table is affected byspecific gravity of particles, slope of deck, sizeof particles, and shape of particles. As saidearlier size of feed should have a narrow sizerange. Specific gravity difference should beenough for the heavies to settle in the flowingfilm and the light ones to be carried away. As for particle shape, it affects the separation becauseflat particles, although light, do not roll easilyacross the deck and are carried down together with the concentrate. Also, for spherical dense
particles, they tend to move easily andsometimes reports to the tailing launder. Theslope of the deck is adjusted by hand wheels.The moderate slope, which high density particlesclimb more readily than low-density minerals,greatly improves separation, allowing muchsharper cuts to be made between concentrate,middling, and tails. It varies with feed size,normally range from a maximum of 90mm for avery heavy, coarse sand to as little as 6mm for anextremely fine feed.
The feed for shaking tables should be first
passed in a classifier, such as a hydrosizer, sothat it will have almost the same particle size andwill be efficiently separated.
Table capacity is influenced by particle size, asthe size goes down, the table capacity alsolowers. Using a 100 to 150µm feed, tablecapacity is as low as 0.5ton/hour while with afeed of 1.5mm, capacity is up to 2ton/hour. Oncoal feeds, which are tabled at sizes of up to
15mm, capacity is as high as 12.5ton/hour withhigh efficiency.
The length of stroke which makes separationusually varies within the range of 10mm to 25mor more with the speed being in a range of 240-
325 strokes/min. Stroke depends on feed size,generally, a fine feed requires a higher speed andshorter stroke. This is controlled by the headmotion, it provides a slow forward motion andfast backward using a spring. Thus, thebackward stroke produces the average maximumvelocity for the particles.
Concentration by Tabling is much simpler thanthat of Flotation. In Flotation, it requiresknowledge of such concepts as contact angles,frothers, collectors and many more to have agood separation, while in Tabling, the only thingyou add is water. It does not require addition of chemicals. But, the advantage of Flotation is thatit can treat particles below 10µm which is hardfor Tabling.
Other gravity concentration techniques whichare popular are jigging and processes usingspirals. Jigging is for concentration of coarsematerial (3-10mm), even if the specific gravitydifference range is narrow, efficient separation isachieved.it is used to separate heavy materials inclosed grinding circuits which prevent over grinding. The use of spirals, such as Humphrey`sis made with slopes of varying steepness to suitthe feed characteristics (3mm-75µm). Shallowangles are used to separate coal from shale, steepangle are normally used for normal heavymineral-silica separation. Steepest angles areapplied for separation of heavy mineral fromheavy waste material. It stratifies particles withthe combined effect of differential settling rates,interstitial trickling and centrifugal force.
The ore sample is composed of magnetic andnon-magnetic particles. The losses can beconsidered as tailings. The weight of themagnetic particles collected is greater than the
original amount of iron. This implies that themagnetic particles collected contain silica(2.1552grams). The losses which were notaccounted for can be considered as slimes.
All materials are affected in some way whenplaced in a magnetic field, although in some, theeffect is negligible. The use of magneticseparation in concentration is very mucheffective with iron ores and paramagneticelements. But it is not as flexible as the shakingtable, where diamagnetic particles and coal orescan be tabled.
Employing wet magnetic separation cangreatly benefit an operation if a low grade finaltailing can be produced, since it alleviates bothdrying and dry storage costs. Dry magneticseparators are more precise separations than wetmagnetic separators. Dry magnetic separation ismore controllable since the separation medium isair rather than water. Separating particles fromone another is naturally easier without having tofight drag forces created by water.
New developments on magnetic separators arenow used in the industry. These magnetic
separators can be also classified according to thephysical orientation of the separator: drum androll. They are more specifically called as rare-earth drum (RED) and rare-earth roll (RER). ARED is most often used to separate two or moreparamagnetic minerals into separate finishedproducts. It operates at higher unit capacity thana RER separator. A RER is more applied toresidual streams after removal of the more highlysusceptible magnetic minerals such as ilmenite,chromite and garnets. It is also used for recoveryimprovements and final cleaning of high-valueproducts (zircon) where their higher fieldstrengths are required if proper separation is tooccur. Another type of magnetic separator isinduced roll magnetic (IRM) separator. IRMseparators have historically been used in mineralsands processing. Some remain in variousmineral sands circuits but RER separators arerapidly replacing them. The RER allows mineralprocessors to get over 50% greater capacity per unit operation over IRM separators. [3]
Magnetic separation can only applicable toores with distinct magnetic characteristics. Thisis the main disadvantage of this type of concentration. On the other hand, given that thevaluable mineral you want liberate is magnetic,magnetic separation has its advantage over
gravity concentration. Gravity concentration islimited by the particle size. For ultra-fineparticle, gravity concentration (tabling) becomesinefficient. Also, magnetic separation is the sameas in settling where there are no media to replaceor dispose unlike flotation. However, thismethod requires low flow rate which in turn,increases the floor area.
The selection of magnetic separationtechnology depends on many processing factors,including particle size, and the specificassemblage of minerals and grades as well astheir corresponding magnetic susceptibility. Toachieve a high recovery in magnetic separation,it should be ensured that the intensity of themagnets can hold the magnetic materials in sucha way that all the magnetic materials will becollected. Magnetic separation is more efficientfor liberating the magnetic and non-magneticparticles of the ore.
In order to achieve high recovery of valuablesin magnetic separation, the degree of liberationshould not be as high since the particles of finesizes becomes very inefficient. Also, feeding of the sample should be done per batch so thatclogging is prevented. The intensity of themagnet should ensure collection of the valuables.
[2] Encyclopedia Britannica. Accessed 31 Jan 2010 through URLhttp://www.britannica.com/EBchecked/topic/
38
[3] Magnetic Techniques for the Treatment of Materials. Springer Netherlands. 2004. Accessed 29January 2010 through URL http://www.springerlink.com/content/kg2537830271614g/
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