history of gravity separation at Richards Bay Minerals

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    Introduction

    Gravity separation techniques have been inuse in the mineral sands industry for decades;from the Reichert cone to Wilfley tables,Wright sluices and the Kelsey Jig, allattempting that elusive perfect gravityseparation. Just as the gravity separationindustry evolved, Richards Bay Minerals alsoevolved in their use and approach to gravityseparation. This paper discusses the history ofgravity separation at Richards Bay Mineralsand briefly comments on future developments.

    History

    Primary concentration

    Richards Bay Minerals (RBM) first miningconcentrator and mineral separation plantcommenced operation in 1977 to processilmenite, rutile and zircon bearing dunaldeposits along the northern Natal coast. The

    orebody contained a greater proportion of non-valuable magnetic heavy mineral in the SGrange of 3 to 4 compared to West Australianand Florida USA deposits, making mineralseparation more difficult1.

    The deposit was mined using two dredgesfeeding a floating 2 000 tph concentrator.Concentrator feed was screened and fed to twofour-stage circuits consisting of primary,scavenger, cleaner and recleaner Reichertcones (see Figure 1). The primary units werefed at a rate of 80 tph and a density of 60%solids by weight.

    Reichert cones are high capacity flowingfilm concentrators. Slurried feed wasdistributed around the periphery of the conesurface and fed to two separation surfaces viaa mouth organ splitter. Heavy particlesseparated to the bottom of the film as theslurry flowed towards the centre and concen-trates were removed through an annular slot(tulip) and fed onto a single cone for furtherupgrading. The tailings from the double andsingle stages were fed onto the next doublestage and the process repeated (see Figure 2).Dilution water was fed to each of the singlecones at a rate of approximately 7 m3/h (28m3/h for 4DS unit). Reichert cones used incleaning duties were of the double, single,single (DSS) type allowing further upgradingof the concentrates. Variable slots were alsoemployed and these were automated and usedin grade control algorithms. The Reichert conehad low installation and operating costs due tohigh tonnage per m2 of floor area and fewmoving parts. The performance of the unitswas heavily dependent upon the efficientremoval of fine root matter in the screeningprocess. A lost or holed screen panel would

    A history of gravity separation atRichards Bay Mineralsby J.R. Walklate* and P.J. Fourie

    Synopsis

    Richards Bay Minerals has been operating gravity separationcircuits since 1977, when the first mining concentrator and mineralseparation plants were commissioned. Reichert cones wereemployed to produce a heavy mineral concentrate at the mine andwash-water spirals and shaking tables were used at the mineralseparation plant to generate a suitable rutile and zircon-richconcentrate for further processing on dry electrostatic separationmachines.

    Helical sluices, developed by Wright, provided an alternative tothe Reichert cone and all future mining concentrator plants wereequipped with these types of units. Roche (MDL) and Multotecdeveloped improved designs of helical sluice and these eventuallyreplaced the wash-water spirals and shaking tables in the mineralseparation plant, reducing water requirements and improving plantavailability.

    This paper describes the gravity separation circuits used atRichards Bay Minerals throughout its history and discusses theadvantages and disadvantages of the separation equipmentemployed. New developments in gravity separation are alsodiscussed, namely the Floatex density separator and the Kelseycentrifugal jig.

    * Richards Bay Minerals. Hatch Associates Ltd. The Southern African Institute of Mining and

    Metallurgy, 2006. SA ISSN 0038223X/3.00 +0.00. This paper was first published at the SAIMMConference, DMS and gravity concentrationoperations and technology in South Africa, 1820 July 2006.

    741The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 106 NON-REFEREED PAPER NOVEMBER 2006

  • A history of gravity separation at Richards Bay Minerals

    result in blockage of the mouth organ splitters and inserts,affecting the even distribution of slurry feed to the separationsurfaces. Cleaning the aforementioned splitters and slots wasa tedious chore not looked forward to by the operating crews.Obtaining clean dilution water for the cones was also difficultat times and blockage of the ring-mains was common.Separation surfaces could be viewed only by openinginspection hatches and peering inside with a torch.

    The required capacity of the mining plant was determinedby the requirements of the ilmenite smelters and the grade ofheavy mineral in the orebody. In order to satisfy the appetiteof two smelter furnaces a second mining plant was requiredand Mining Pond A was commissioned in 1982. The 1 000tph plant was a single two-stage circuit employing Wrighthelical sluices rather than Reichert cones. Metallurgical testwork conducted on site had shown these units to be more

    efficient and a two-stage sluice circuit matched or betteredfour stages of cones. There was also no requirement fordilution water. The drawback to using sluices compared tocones was the increased complexity of the feed distributionsystem with the many hundreds of starts required.

    Increased market share for RBMs products requiredfurther mine expansion. Mining Pond B was upgraded to 3 000 tph in 1986 by utilizing the scavenger cones inprimary duty and installing Roche helical sluices to replacethem and a 3 000 tph Mining Pond C was commissioned inmid 1988. The new plant consisted of two two-stage circuitsusing Roche LG5 and MG5 sluices.

    To compensate for reducing orebody grades andincreased mineral complexity, five-stage circuits weredeveloped for Mining Plants B and C in 1990. A typical flowsheet is shown in Figure 3. New sluices with improved

    742 NOVEMBER 2006 VOLUME 106 NON-REFEREED PAPER The Journal of The Southern African Institute of Mining and Metallurgy

    Figure 1Mining Pond B Cone circuit, D = double; S - single and V = variable

    Figure 2Reichert cone schematic showing the first three stages of a 4DS unit

    slurry feed

    single cone

    double conespread

    spread

    concentrate #1

    double cone

  • performance became available and high grade units wereinstalled. Further expansion took place in 1992 with MiningPond D coming on line at a rated capacity of 3 000 tph. Five-stage sluice circuits were again employed using acombination of Roche MG5, HG7 and HG8 units.

    The most recent mining expansion took place in 1999with the commissioning of the 4 500 tph Mining Plant E andthe upgrade of Mining Plant A to 1 500 tph giving RBM thecapability to process in excess of one hundred million tonnesof sand per year.

    The mineral separation plant

    The mineral separation plant commenced operation in 1977.Heavy mineral concentrates (HMC) from the mining plantswere treated on wet high intensity magnets (WHIMS) torecover ilmenite for the smelting process. The non-magneticfraction, containing rutile, zircon, quartz and other unwantedminerals was fed to a gravity circuit consisting of primaryand scavenger wash-water spirals and scavenger shakingtables. The wash-water spirals used were double-start RocheWW2A in primary duty and WW2B in scavenger duty.Concentrates were removed at every half turn of the spiral byan adjustable splitter. Wash-water issuing from a series ofspigots along the inside edge of the spiral provided repeatedwashing of the concentrate ahead of the splitter, removinglighter gangue minerals to tailings. The final recovery stagewas carried out using suspended triple-deck Deister tablesconstructed of marine plywood. A launder box providedwash-water controlled by wooden interlocking wedges. Thetable deck angles were individually adjusted. A typical feedpreparation gravity circuit is illustrated in Figure 4.

    The wash-water spirals gave few operating problems withfew moving parts to go wrong. The concentrate splitters satfirmly in their wells and were not easily moved duringcleaning of the spiral surfaces. Problems were experienced,however, with the closed wash-water hose and spigots. Slimebuild-up on the inside of the hose and fine organic matterblocked up the hose and spigots on a regular basis and

    cleaning proved difficult. The spiral distributors supplied withthe units gave uneven distribution resulting in some of theunits being over fed, with resultant lowering of metallurgicalperformance.

    The triple-deck Deister tables were very efficient from ametallurgical aspect with a high separation surface areacompared to the spiral, but they proved a nightmare tooperate in a production environment. The operator ormetallurgist had to stand on the product launders to view theseparation on the top deck and crouch down with torch inhand to see what was happening on the middle and lowerdecks. The decks and gearbox were supported by steel cables,which were prone to unequal stretching and requiredconstant adjustment by the mechanical artisans. The motionof the decks caused movement of the wooden wash-waterwedges making control of the flow rate very difficult. Thewooden decks did not appreciate the hot humid conditionsprevalent in Richards Bay and rotting of the units wasexperienced. During their time in the feed preparation circuitsof the mineral separation plant the heavy wooden decks werereplaced by fibreglass units with cast polyurethane rifflecovers. The wooden wash-water launders, with theiruncontrollable wedges, were replaced by drilled stainlesssteel piping.

    A third feed preparation circuit (FPC) was commissionedin 1988 with Roche WW6 spirals and floor-mounted single-deck Deister tables. The new tables gave excellentperformance from both a mechanical a