PORT DURNFORD: CLAY MINERALOGY EFFECTS IN A THICKENER 83

IntroductionThe Port Durnford mineral deposit is situated south of thecurrent Hillendale mine and extends for about 13 km southtowards the town of Mtunzini on the east coast of SouthAfrica. The Port Durnford deposit is characterized by highslimes content, typically more than 20% of the expectedrun-of-mine (ROM). Slimes are defined in this case asmaterial passing a 63 µm laboratory screen. This usuallyconsists of fine particles and clay minerals, and it is the clayminerals that cause recovery losses at the primary gravityconcentrator if not effectively removed (Marcos andGilman, 2007). Once removed, the slimes need to bedisposed of and water recovered for reuse in the process. Atypical slimes processing route in a heavy mineralsoperation is to separate the slimes from the rest of the ROMby using hydrocyclones. The slimes would then report tothe hydrocyclone overflow and further processed in athickener. Very fine particles of a few microns in diametersettle slowly by gravity alone (Wills, 2007, Hayes, 2003).Chemicals, such as flocculants, are added to aid withagglomeration and settling of these particles in largediameter thickening vessels (Wills, 2007).

When one considers the feasibility of a mining project,the technical viability of the proposed process needs to beconfirmed and relevant design information needs to begenerated. For the evaluation of the proposed Port Durnforddeposit the slimes handling process was one of the majorcost drivers of the project. For this reason a pilot thickenerstudy was under taken to assist in the technical evaluation.The aims of the pilot campaign were:

• To confirm the ability to settle the Port Durnford slimesusing available thickener technology

• To determine design information to enable the designof the thickener circuit and thus allow a detailed capitalcost estimation of the required plant

• To determine operational parameters to allowconsumption rates and thus operating cost to bedetermined.

In this paper a brief introduction of the importanttheoretical aspects that influence slimes behaviour duringthickening are discussed—especially clay mineralogy sinceit influences thickening behaviour. The approach followedfor the Port Durnford pilot thickening tests as well as resultsare discussed with the focus on how the clay mineralogy ofthe deposit influences the thickening performance. Toconclude the implications on the slimes handling system arealso discussed.

Theoretical considerations

Flocculants Flocculants are high molecular polymers, mostlypolyacrylamide monomers (PAM), which consist ofsignificantly long chain lengths. The charge of theflocculant molecule can either be positive, negative orneutral. Due to the negative surface charge of clay mineralsone would assume that cationic flocculants would showbetter results. However, the pH of the environment affectsthe surface charge of a particle. In an aqueous solution thehydroxyl group (OH-) attaches itself to the edges of the clayparticle (Svarovsky, 1981). In a solution with a low pH theacid will protonate the OH-, leading to an overall positivecharge. As the pH of the solution increases the OH-

depronates until a neutral edge charge is reached. A furtherincrease in pH will result in an overall negative charge atthe edges of the clay particle. The pH of the solutiontherefore strongly influences the flocculant charge that willbe most effective. Apart from the pH influence, differentclay mineral may also respond differently to specificflocculants.

RAMSAYWOK, P., BEUKES, J.A., and FOWLER, M. Port Durnford: clay mineralogy effects in a thickener. The 7th International Heavy MineralsConference ‘What next’, The Southern African Institute of Mining and Metallurgy, 2009.

Port Durnford: clay mineralogy effects in a thickener

P. RAMSAYWOK, J.A. BEUKES, and M. FOWLERExxaro Resources

The Port Durnford mineral deposit is situated south of the current Hillendale mine and extends forabout 13 km southwards towards the town of Mtunzini on the east coast of South Africa. PortDurnford’s run-of-mine (ROM) material typically contains more than 20% slimes on a weightbasis. As part of the feasibility study for this proposed project, thickening of the slimes wasevaluated using a pilot-scale high rate thickener. Samples representing different material typesfound in the Port Durnford deposit were tested. Results showed that the thickener solids flux ratesfor Port Durnford material are lower than that of Hillendale, which is the current Exxaro mine onthe South African east coast. Underflow densities achieved varied depending on the material typetested. The flocculant demand was also determined during the pilot campaign and the resultscompared well to actual consumption rates achieved at the Hillendale operation. For the overallslimes handling process to be successful the most important criteria for the thickening operationare the underflow properties achieved. The paper will therefore focus on the underflow propertiesachieved and show how clay mineralogy influenced the results obtained from pilot thickenertrials.

Keywords: heavy mineral slimes, clay minerals, smectite, rheology.

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Figure 1 schematically illustrates this mechanism offlocculation. The length of the chain enables it to bepartially absorbed onto the particle surface leaving themajority of the chain length to swerve around until itattaches to an adjacent particle. This leads to the formationof multi-particle aggregates also known as ‘flocs’ whichsettle out more quickly and easily. The longer the molecularchains, the faster the bridging between two adjacentparticles can take place, leading to rapid floc formation andquicker settling (Moss and Dymond, 1978).

Once the flocculant and slimes are mixed and allowed tosettle, the process of thickening begins.

Thickening and slimes disposal

Thickening or gravity sedimentation is the most widelyapplied dewatering technique in mineral processing.Relative to other dewatering processes, thickening is cost-effective and a high capacity process (Wills, 2007). Anotheradvantage is that it involves very low shear forces, thusproviding good conditions for flocculation of fine particles.Two primary functions of a thickener are:

• Clarification of the overflow to enable reuse of water• Thickening of the underflow to a required concen-

tration (Wills, 2007).

The thickener overflow water is returned to the processwater system for reuse in the process. The thickenedunderflow is disposed of typically in a residue disposalfacility which allows for further dewatering. For the PortDurnford project, apart from the normal residue damdisposal, the reconstitution of soil using the thickenerunderflow and coarse sand was also a key considerationduring the evaluation.

During rehabilitation efforts at the current Hillendalemine it has become apparent that slimes and sand need tobe reconstituted to assure water retention of the soil andestablishment of sugar cane production (Hattingh et al.,2007). For Port Durnford it is expected that the samerehabilitation method will be required as practised currentlyat the Hillendale mine. For both the disposal of thickenedslimes in a residue disposal facility or for use to reconstitutesoil, the underflow rheology is the main performancecriteria. In the thickener pilot study for Port Durnford, thefocus was on achieving the required underflow density asrequired for the rehabilitation process based on Hillendalerheology.

Clay mineralogy

Port Durnford material was analysed to characterize its claymineralogy, since clay mineralogy can have a determiningeffect on the performance during the thickening process andsubsequent disposal. From X-ray diffraction (XRD)analysis it was determined that the Port Durnford orebodyis rich in clay minerals. In this document the term ‘clay’ isused as a mineralogical term, i.e. any of a diverse group offine-grained platy minerals, not a size fraction. Thenegative surface charge, cation exchange capabilities,swelling characteristics and small particle size of clayminerals negatively affect the settling behaviour of theslimes fraction within a thickener (Van Olphen, 1977).These factors also affects the rheology of the slurry as itinfluences the solids concentration and the manner in whichthe particles stack during settling. The latter affects therheology and consolidation behaviour of the slimes afterdeposition (Addai-Mensah, 2007).

Clay minerals belong to the phyllosilicate group—thecrystal structures of minerals in this group consist of anarrangement of octahedral and tetrahedral sheets.Tetrahedral sheets consist of two planes of oxygen atomsarranged in tetrahedral coordination around Si4+ cations andshare basal oxygen atoms between adjacent tetrahedralsheets. Octahedral sheets comprise six oxygen or hydroxylions which share octahedral edges. This type of linkingresults in a net charge of -2 which is balanced by divalentcations or trivalent cations that bond to the sheet (Horn andStrydom, 1998).

Clay minerals are defined as 1:1 or 2:1 clays. 1:1 clayminerals consist of a single octahedral sheet linked with onetetrahedral sheet. The linkage occurs through sharing of theapical oxygen between the octahedral and tetrahedralsheets. Two of the layers formed through the abovementioned bonding, link together through weakintermolecular forces (Horn and Strydom, 1998). Theenclosed ion of each tetrahedron is normally Si4+, but thiscan be replaced by Al3+ or Fe3+. Water cannot enterbetween these layers and these clays are termed non-swelling clays, which include clays such as kaolinite andserpentine.

Clay minerals defined as 2:1 consist of a singleoctahedral sheet which is bordered on both sides by atetrahedral sheet in which the apical oxygen point towardsthe central octahedral sheet. This forms one layer which canbe linked to a similar layer through interlayer cations. Thefact that the interlayer cations can be replaced by othercations is important as it relates directly to the ease withwhich these clay minerals can be chemically altered. Thehydration of the interlayer cations results in the physicalswelling of the clay minerals which could lead to thedisintegration of the crystallite (also known as weathering)(Horn and Strydom, 1998).

Particle-packing relationship

The packing of particles on top of each other during settlinginfluences rheology and density of the thickened underflow.The most familiar packing types are referred to as‘edge–face’ also referred to as ‘house of cards’, and ‘face—face’ also known as a ‘band structure’ packing relationship(Van Olpen, 1977).

Figure 1. Schematic illustration of flocculation mechanism (Mossand Dymond, 1978)

PORT DURNFORD: CLAY MINERALOGY EFFECTS IN A THICKENER 85

Kaolinite typically stacks into a deck of cards structure,whereas smectite forms a honeycomb structure. Figure 2illustrates this packing behaviour. The honeycomb packingretains large amounts of water which fills the voids formedby this type of arrangement (McFarlane et al., 2005a).Great effort and highly effective dewatering systems arerequired to remove the interstitial water (McFarlane et al.,2005a)

RheologyRheology is concerned with the flow and deformation ofmaterials experiencing an applied force (Boger, 2006).There are several factors that influence the rheologybehaviour of thickener underflow of which density, particlesize, stacking relationship and the clay mineralogy seems tobe the most important. The rheology of the underflow ismeasured in yield stress which refers to the stress at whichthe slurry starts to flow. Rheology is an importantcharacteristic of the underflow as it determines how theslurry behaves during transfer to the deposition site as wellas behaviour after it has been deposited. One of thedeposition requirements for tailings disposal is that theyield stress is sufficiently high to support the largestparticles which will ensure homogenous suspension wheresegregation does not occur (Boger, 2006). The non-segregating nature of the reconstituted soil for use inrehabilitation proposed for Port Durnford is the mainperformance criteria for the Port Durnford thickener design.The thickener pilot campaign thus focused on achieving therequired underflow properties as required for the twodisposal methods.

ExperimentalConsidering the theoretical background discussed, the pilotcampaign was planned to generate the required informationto assess the viability of slimes handling at Port Durnford.

Quantitative X-ray diffraction analysisFor this study all quantitative X-ray diffraction analysis wasperformed at the University of Pretoria. A PANalyticalX’Pert pro powder diffractometer with X’Celerator detectorand variable divergence and receiving slits with Fe filteredCo-Kα radiation was used. Quantification was performedusing the Rietveld method (Autoquan Program).

Yield stress measurementsYield stress (Pa) was measured using a Haake rheometerwith a built in shear program. The vane design included 4

blades. Yield stress was measured and the peak yield stresswas recorded for each measurement.

Particle size distributionsA Malvern Instruments Mastersizer was used to determineparticle size distribution. The stirrer and ultrasonic modewere utilized. The particle refractive index setting was 1.53and a 300RF lens was utilized. Water was used as adispersant and the ultrasonic stirrer and other settings werekept constant for all tests.

Sample selectionThe Port Durnford orebody is about 10 km long and around4 km wide—at its deepest point it is in excess of 60 m deep. Due to the size of the deposit care had to betaken to ensure that all geological variations are consideredwhen evaluating aspects such as thickening of the slimesfraction. As part of the geological evaluation of theorebody, extensive drilling was carried out and drillsamples analysed to determine the valuable mineralquantities in the deposit.

Based on the lithological information generated by fieldgeologists during exploration drilling, the orebody wasdivided into several sections. These sections, based onlithology, were further organized based on analyses done aspart of the exploration process to define different materialtypes found in the Port Durnford deposit. These materialtypes were distinguished from each other by features suchas typical slimes content, colour, sand sizing and responseto magnetic separation on a Carpco magnetic separator.Based on experience at the Hillendale operation, it is knownthat differences in processing behaviour can be experiencedwhen processing materials with variations in some of thesefeatures. The thickening pilot trial thus had to ensure thatall material types present in Port Durnford could besuccessfully treated.

For the Port Durnford thickening pilot campaign,dedicated samples representing each of these material typeswere obtained using Wallis Air Core drilling at nominatedpositions in the deposit. Several drill samples of eachmaterial type were combined to obtain bulk samples. Waterfrom the Hillendale plant was used as the proposed PortDurnford mine would use the same water source.

Sample preparationAs with Hillendale, the Port Durnford ore is shear sensitive.Figure 3 illustrates the shear sensitive nature of PortDurnford slime.

Figure 2. (A) Honeycomb packing relationship (B) Deck of card packing relationship

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As energy is transferred to the slurry, more fine particlesare generated which in turn influences the settlingbehaviour of the slimes. It was important to ensure thatrepresentative feed was prepared for the thickenercampaigns. The mining process proposed for Port Durnfordwould include slurrying of the ore and pumping itsignificant distances before sand and slimes are separated incyclones. Before feeding slimes to the pilot thickener, theslimes had to be exposed to a similar amount of shear tothat expected during the actual mining process. To preparethe slimes for the thickener trials, a sump and cyclone set-up in closed circuit was used to ensure adequate andconsistent shear to the ROM. The required time in the set-up to simulate the mining process was determined based onprevious thickener campaigns conducted at Exxaro’sHillendale mine. The assumption made for the PortDurnford pilot trial was that the mining process at PortDurnford would be similar to that of the current Hillendalemine.

Thickener test set-up

Figure 4 is a photograph of the pilot test thickener duringinstallation. The thickener consisted of a 190 mm diameterPerspex vessel standing about 2 metres high with a feedwell and rake mechanism.

The sheared and diluted feed slurry was introduced to thethickener unit at a controlled rate using a peristaltic pump.The feed density was kept constant for all tests at 1.012 kg/l. Flocculant was made up to a concentration of0.1% by mass and then further diluted to 0.01% by massprior to dosing to the thickener. Dosing was controlled bytwo peristaltic pumps which allowed two-stage dosing. Thethickener feed lines had some inserted bends to aid mixingof the flocculant and feed. The thickener underflow wasextracted using a peristaltic pump and discharged into aholding bucket for further testing and analysis.

Thickener test program

During the sample selection process six distinct materialtypes were identified. For the purpose of this article onlythe two main types representing around 80% of the PortDurnford orebody are discussed. The two material types arereferred to as type 2 and type 6. A sample from each of thematerial types of Port Durnford were subjected to theprocedure as shown in Figure 5.

Flocculant screeningPrior to the thickener pilot study flocculant screening trialswere conducted on a laboratory scale by flocculant supplycompanies. Static jar settling tests were conducted andsettling rates achieved were recorded. The best performingflocculants based on the settling rates were shortlisted forfurther evaluation during the thickener pilot trial. The pilotthickener was run at fixed settings to evaluate the differentcandidate flocculants based on static jar tests. The flux rate,

Figure 3. Shearing of slime in the cyclone test rig showing howthe number of particles of less than 1µm increase in time as

analysed using a Malvern particle size analyser

Figure 4. Pilot thickener rig

Figure 5. Thickener experimental plan

PORT DURNFORD: CLAY MINERALOGY EFFECTS IN A THICKENER 87

flocculant dosage and bed heights were kept constant foreach flocculant trial and the underflow density wasmeasured. The flocculant providing the best underflowdensity was selected.

Flocculant demandTests on the pilot thickener were run for each material typeto determine the flocculant demand (optimum dosage rate).The bed height was built to 40 cm and the flocculant dosagerate (g/t) systematically increased. As the flocculant dosageincreased the underflow was sampled and underflowdensity determined. The flocculant demand was taken as therate at which any further increase in flocculant dosage ratehad no further beneficial effect on the underflow. Theflocculant demand tests were all run at a constant flux rate.

Flux rate The thickener solids flux rate is measured in the units oftonnes/hour metre2. For a fixed thickener set-up as used forthe pilot trial the flux rate will be directly proportional tothe solids feed rate. To determine the optimum flux rate foreach material type the thickener was operated at a bedheight of 40 cm and the underflow density determined for aset of flux rates. At low flux rates the underflow density isindependent of the flux rate. As the flux rate increases apoint is reached where further increases in the flux rateyields reduced underflow density. The optimum flux rate isdefined as the point where the underflow density becomes afunction of the flux rate. The flux rate is used to size thethickeners.

Underflow densityThe final design parameter determined was the underflowdensity achievable. As the bed height in the thickener isincreased the underflow density increases due to highercompression. The bed height was increased for each of thematerial types and the thickener underflow densitymeasured at different bed heights. The terminal density isthe density achieved where any further increase in bedheight has no further impact in increasing the density. Theterminal density that could be achieved would give anindication of densities that could be expected from anoperational thickener. Apart from underflow density, theyield stress of the underflow was determined as it wouldinfluence the design of the raking mechanism. This wasdone by using the Haake rheometer. Both the rheology ofthe underflow as removed from the thickener as well as therheology of the underflow following shearing wasdetermined. This was done as underflow rheology changeswith shearing. Generally the underflow yield stress willreduce following shearing. Figure 6 shows typical thickenerunderflow produced during the pilot campaign.

Results and discussionThe required design information for the thickener wasgenerated based on the tests described. Flux rates weredetermined and these would serve as input to specify thethickener area required. The relative required thickeningarea for Port Durnford material was found to be larger thanfor material from the Hillendale deposit. Flocculant dosingrates were determined and compared well to those currentlyexperienced at the Hillendale mine. The main focus of thispaper is, however, the key design criteria for the thickeningoperation, which are the underflow properties from thethickener.

Figure 7 shows the relation between the underflow solidsconcentration in percentage and the bed height from thethickener trials for type 2 and type 6 materials respectively.On comparing underflow solids concentration as bed heightis increased for the two material types, it can be seen thattype 2 yielded higher solids concentrations than type 6. Therate of increase of solids concentration as bed heightincreases is also lower for type 6 material than type 2material. This indicates that type 6 material does notdewater efficiently under the compressive conditions in athickener bed.

In parallel to the pilot thickener campaign, claymineralogy was investigated on the same samples togenerate information to assist in the understanding of thematerial’s behaviour in processing. XRD analysis was usedto define the clay minerals present in the different materialtypes. Type 6 material was found to have high amounts(>35%) of the clay mineral smectite [Na0.7(Al3Mg0.7).Si8O20(OH)4.nH2O]. Smectite is a swelling clay and absorbsvarious amounts of water thereby increasing volume andthus decreasing bulk density. As shown in Figure 2 thesmectite tends to form honeycomb like structures that retainwater (McFarlane et al., 2005a). The main clay mineralpresent for type 2 material was kaolinite (Al2Si4O5(OH)4),which typically has a low swelling capacity and is also themain clay mineral in the current Hillendale mine slimesfraction. The difference in clay mineralogy thus explainsthe difference in observed behaviour as shown in Figure 7.

Figure 6. Underflow discharged into holding bucket for furthertests

Figure 7. Pilot thickener underflow solids percentage as afunction of mud bed height

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The yield stress of the thickener underflow produced wasalso determined. Figure 8 gives the yield stress for bothmaterial types at different underflow solid percentages.

From Figure 8 it can be seen that the yield stress as afunction of the underflow solids percentage has a similarrelation with type 6 marginally higher at similar underflow% solids. A larger difference was, however, expected basedon the differences as observed in Figure 7. The underflowtested and represented in Figure 8 was, however, tested asremoved from the test vessel and the influence of theflocculant on the yield stress is expected to still besignificant.

In practice the thickener underflow is removed from thethickener by pumps and pumped to the final disposal sites.The pumping operation shears the underflow leading to aloss in yield stress (for a shear thinning application). Figure 9 shows the yield stress and underflow percentagesolids’ relation for the material tested following a shearingsimulation. The rheometer had a built-in shear simulationprogram; after measuring the yield stress the vane wouldspeed up for 10 seconds to shear the slurry after which theyield stress was remeasured.

From Figure 9 it can be seen that after shearing type 6material has higher yield stress at similar underflow solidspercentages compared to type 2 material. Figure 10represents the pre-shear and post-shear yield stress for eachtest for the two material types.

Figure 10 shows that type 2 material exhibits a higherdegree of shear thinning compared to type 6 material. Thiscan be attributed to the high smectite content in type 6material. In the swelled state, smectite occupies greaterpulp volume, forming a high yield stress, water retaininggel structure (McFarlane et al., 2005a). McFarlane et al.(2005a) also concluded that the formation of honeycombnetwork structures that retain water also results in higheryield stress and opposes gravitational and shear inducedcompaction. McFarlane et al. (2005b) described howkaolinite dispersions flocculated with polyacrylamides(PAM) based flocculants deform when subjected tocompression. The inter-flocculant porosity is also reduced,forming a higher pulp density following shear.

It can thus be seen that for type 2 material higherunderflow solids concentrations can be achieved withcorresponding high yield stress. Following shearing, theyield stress of type 2 material will, however, be reduced.For type 6 material the thickener will not be able to producehigh underflow solids concentrations. Subsequent yieldstress will typically also be lower than the high solid

concentration underflow when treating type 2 material.Type 6 material will, however, maintain its as thickenedyield stress following pumping operations yielding a higheryield stress at the point of disposal than for type 2 materialof similar solids’ concentration.

The objective for the thickener tests was to achieve highunderflow densities that would allow maximum flexibilityfor downstream disposal processes. The proposed PortDurnford project would dispose of slimes on a sub-aerialresidue dam as well as a bulk mix with sand for dunecoating. For the bulk mixing process, the requirements toachieve high densities and yield stress in the thickenerunderflow are more stringent than for the residue damdisposal. The residue dam’s pumping system incorporatesdilution facilities which enable it to handle high thickenerunderflow densities and yield stress. The successfuldisposal dam as well as bulk mixing operation is, however,dependent on the thickened slimes rheology more than onthe underflow density. There are significant differencesbetween the behaviour of type 2 and type 6 materials asdetermined from the pilot thickener campaign. The higheryield stresses achieved for type 6 following shear comparedto that of type 2 at similar densities can, however, possiblyoffset the lower thickener underflow solid concentrationsachievable with type 6 material compared to type 2material. This would thus possibly lead to both materialtypes, even though behaving differently in the thickenerprocess, being successfully disposed of via the two disposalroutes.

Figure 8. Yield stress as a function of underflow solids percentagemeasured as discharged from the thickener (unsheared)

Figure 9. Yield stress as a function of underflow solids percentagemeasured following shearing

Figure 10. Comparison of yield stress achieved before and aftershearing for each of the material types

PORT DURNFORD: CLAY MINERALOGY EFFECTS IN A THICKENER 89

ConclusionsFrom the pilot thickener campaign the required designinformation was generated to confirm the suitability of highrate thickening for treating Port Durnford material. Theinvestigations revealed marked differences in thickeningbehaviour between the material types tested for PortDurnford. It was shown that these differences are primarilydue to differences in clay mineralogy. Since the rheology ofthe thickened slimes will determine the downstreamdisposal behaviour, follow-on studies will have to be doneto confirm the performance of the material types in disposalprocesses.

AcknowledgementsThe authors would like to acknowledge Schalk Bekker andChris Meintjies from Outotec for providing equipment andvaluable assistance in executing the test programme. Theauthors would also like to thank Dr Thys Vermaak andHennie Burger from Exxaro for guidance and valuablefeedback during the compilation of the paper.

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P. RamsaywokChief Scientist, Physical Beneficiation, Exxaro Resources

Exxaro KZN Sands (6 years of experience), Exxaro R&D (3 Years of experience) – started atExxaro as a technician, later appointed as a technologist and recently as Chief Scientist. Premeethwas involved in the setup and commissioning of the quality control laboratory at KZN Sands.Presently Premeeth manages heavy mineral and occasionally coal projects. Premeeth is alsocurrently studying towards a BSc Honours degree in Technology Management at the University ofPretoria.

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