Current trends in the development of new or … · Introduction The development of any diamond...

IntroductionThe development of any diamond resourceproject inevitably requires the investigation ofa vast range of issues across engineeringdisciplines (mining, metallurgical, civil,electrical, mechanical, environmental,geological etc.). No two resources, whetherkimberlite or an alluvial deposit, are identical,hence no two process flow sheets are the same(Mackenzie and Cusworth, 2007).

Although technical viability is a strongfocus and tends to dominate the assessment ofthe development potential, the principalpurpose is to determine whether thedevelopment opportunity makes good businesssense (Mackenzie and Cusworth, 2007).

There are basic principles and areas ofimportance within a diamond beneficiationplant that require focus and thoroughunderstanding of the intended objective toensure that optimum throughput and recoveryare achieved by matching the appropriateinterrelated unit processes.

The plant is designed on the basis ofrecovering diamonds from a source (kimberlitepipe, low-grade kimberlite stockpile, kimberlitetailings, alluvial deposit, or marine deposit) ata pre-determined rate (small scale, marginal,or large scale), considering:

➤ The ROM material (nature andvariability of the feed, hardness,entrained waste, grade, and reserves)

➤ Diamond characterization (sizefrequency distribution [DSFD], revenueprofile, type etc.)

➤ Retention strategy (chasing diamonds asopposed to concentrating diamonds fromsource material)

➤ Safety protocol

Current trends in the development of new oroptimization of existing diamond processingplants, with focus on beneficiationby P. van der Westhuyzen*, W. Bouwer*, and A. Jakins*

SynopsisGone are the days of stock-standard diamond beneficiation and finalrecovery circuits that could, with minor modifications, be adapted to treatany diamond-bearing ore source. A new era has dawned, offering theopportunity to streamline existing diamond processing operations ordevelop simpler, more efficient, and economical diamond processing plants,with the focus on more efficient comminution, beneficiation, and finalrecovery. This change in scene has been brought about by:

➤ The introduction, rapid development, and maturation of multiplecomminution, sorting, and recovery technologies

➤ The need to adapt to a new standard of project approach post thecommodity super-cycle phase, where optimizing existingoperations and developing scalable, ‘fit for purpose’ new minesare fast becoming the norm in diamond processing plants in bothprimary and alluvial operations

➤ The quest for energy efficiency and lower labour costs➤ More remote and inaccessible reserves (under lakes, tops of

mountains etc.).

This paper serves to identify some technology advances anddemonstrates how these could be considered as replacements for or incombination with conventional technologies to arrive at an optimumtechno-economic solution. To name a few applications/technologies:

➤ Comminution: conventional cone crusher, modified/specializedcone crusher, and the high-pressure grinding roll (HPGR).Although significant advances have been made in recent years,this paper only briefly covers comminution within the benefi-ciation circuit design

➤ Waste sorting: NIR (near-infrared) sorting, optical (colour)sorting, XRF (X-ray fluorescence)

➤ Primary concentration by combining dense media separation(DMS) with either XRT (X-ray transmissive), pulsed X-ray, jigs,or pan plants depending on the application, scale, and economics

➤ Final recovery of diamonds with either conventional X-raytechnology, pulsed X-ray technology, or XRT.

From recent studies, it can be concluded that there is no longer astandard solution, but rather the ‘right’ or appropriate solution. Throughcombining a sound knowledge of the ore source (ore dressing studies or on-mine data gathering) and leveraging off the advances in technologies, oneis able, through trade-off studies, to arrive at the ultimate techno-economicconfiguration, the ‘right’ solution. Emphasis is placed on maximizingdiamond recoveries through appropriate technology selection andminimizing the associated costs in an effort to de-bottleneck or improveefficiencies of existing diamond processing plants, or to arrive at the idealnew diamond process plant design..

Keywordsdiamond recovery, process design, diamond concentration, optimization.

* ADP Group, Cape Town, South Africa.© The Southern African Institute of Mining and

Metallurgy, 2014. ISSN 2225-6253. This paperwas first presented at the, Physical Beneficiation2013 Conference, 19–21 November 2013, MistyHills Country Hotel and Conference CentreCradle of Humankind, Muldersdrift.

537The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 114 JULY 2014 ▲

Current trends in the development of new or optimization of existing diamond processing

➤ Product quality➤ Footprint (time to construct and supporting

infrastructure requirements)➤ Overall cost (CAPEX and OPEX)➤ Mining method➤ Geographical location, availability of services (water

and electricity/power), accessibility etc.

This paper does not focus on detailed or specific casestudies. Any aspect of the general content of this paper wouldrequire a detailed, client-approved, and specialist study. Theintention is merely to highlight the opportunities that newtechnology brings to the industry and to emphasize theimportance of following an appropriate, impartial, andobjective methodology in attempting to reach an appropriatedesign (be it for a brownfield optimization or greenfieldproject). The optimum solution is unlikely to reside in asingle technology supplier, and will inevitably consist of theright combination of technologies leading to simple, flexibledesign, supported by an appropriate and sound techno-economic study that has considered all the technologyoptions available.

Timeless principles of diamond plant design Diamonds possess an enduring value, both visually andeconomically. As the value has endured the test of time, sohave the principles that underpin the process design, whichare discussed in the paragraphs to follow. The design processis reiterative in nature and requires constant evaluation ofthese principles during the project life cycle.

BBusiness caseThe first and foremost principle is quantifying and qualifyingthe business case, since the mineral industry is founded onthe basic premise of business cases (Mackenzie andCusworth, 2007). Investors, shareholders, and owners are inthe business to realize profit commensurate with the risk.

The general factors that affect the profitability of anoperation are typically cyclical commodity prices, declininggrades, operating costs, and initiatives to upgrade and/orexpand plants in order to maintain profit margins over thelife cycle of the operation. Optimal unit process designrequires delineating the flow sheet into logical business units,taking into account the following:

➤ Mass, water, and revenue balance➤ Operating cost per unit time➤ Efficiency parameters ➤ Performance indicators, i.e. recovery, grade, upgrade

ratio etc.➤ Realized revenue indicators (after subtracting operating

costs).

Sight must never be lost of the business case during theprocess engineering design, and it should remain a keyreference point during the project life cycle.

DDiamond resource characterizationIn the formulation of the business case, understanding(geology/mineralogy) and quantifying (grade and quantity)the source (kimberlite, alluvial deposit, tailings etc.) are keyprinciples. For this purpose various drilling and/or samplingstudies are undertaken.

f fDetermination of the grade and revenue of the resource iscentral to the financial viability assessment. However, earlycharacterization of the ore is vital to the assumptions aroundfuture treatment methods. As the project progresses throughsubsequent phases, the basic understanding of the resourceis developed through more extensive drilling/samplingprogrammes, which ensure appropriate decision-making,translated into the most suitable techno-economic flow sheet.

Process design principlesProcess design is founded on the availability and accuracy ofinformation. A systematic approach is followed, and atconcept level contains the following key elements:

➤ Ore dressing studies (ODS), which go hand-in-handwith the geological resource characterization anddepend on ongoing drilling campaigns to secure oresamples. Samples are subjected to a myriad ofmetallurgical tests to develop an understanding of theore. The aim of the characterization studies (on core orbulk representative samples) is not only to elucidatematerial character, but also to highlight potentialproblem areas in terms of the four stages in thediamond winning process, viz. comminution, concen-tration, recovery, and slimes handling. Thisinformation, although not definitive owing to the lowsampling density at this stage of the project, can guidea more formal sampling programme during thesubsequent phases of the project, geared towardsdeveloping a technically appropriate flow sheet to treatthe material

➤ Development of the design criteria, which stipulate thebasis of design and take into account the characteristicsof the material to be processed and the desired product.In instances (early in the project development phase)where sparse information is available, assumptions arequantified

➤ Development of the flow sheet and associated massbalance, which typically begins with the selection of abeneficiation method. This process is driven largely bythe business case and an understanding of the resource

➤ Various trade-off studies need to be conducted for thedifferent unit processes and the combination of unitprocesses to determine the most appropriate flow sheet.It is imperative that the key findings of trade-offstudies indicate the probability that material propertieswill vary in terms of important metallurgical indicators.Trade-off studies can therefore be indicative of upfrontopportunities, which should ultimately decrease theoperational risks associated with treating the resource.The trade-off studies will influence the design phase toreduce project risks and ‘right-sizing’ of the design,thus increasing the confidence in the approved detaileddesign solution.

The process design principles, whether for an alluvialdeposit, hard rock (kimberlite), or tailings retreatment,remain unchanged in terms of the design criteria, and this,together with ore characterization test work on representativesamples, continues to be the backbone of the design.

The process engineering value proposition is developedthrough the contextualization of recommendations in order toreduce the risks associated with realizing new opportunities,

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538 JULY 2014 VOLUME 114 The Journal of The Southern African Institute of Mining and Metallurgy

fand to maximize the potential revenue generated from theseopportunities for the various flow sheets or unit processeswwithin a flow sheet.

Why a new era in diamond process design?If the principles are timeless, what has changed and why isthis a new era? The answer to this lies largely in the fact thatexceptional technologies have come to maturation over thelast decade. Although this has made the process engineer’swworkspace exciting, it has brought more complexity due tothe fact that more combinations and permutations arepossible.

Plant operating knowledge across the unit operations hasimproved significantly in the past few years. Simulations thattrack revenue across streams and incorporate the variabilityin revenue efficiency of different unit processes have aided inthis understanding. Sub-optimal operating conditions canmore easily be identified and theory and practical knowledgeare ever closer, ensuring that even small changes inoperating conditions can result in revenue efficiency changes.

The sections to follow assume that the reader has a basicunderstanding of the four main process areas (feedpreparation, concentration, final recovery, and slimeshandling) within a typical diamond process plant.

The following tables depict some of the conventional andmore recently developed technologies that are to beconsidered within the new-generation diamond process plantdesign arena. Limited focus is given to feed preparation(Table I), more focus on concentration (Table II) and finalrecovery (Table III), and some focus on waste sortingtechnologies (Table IV). The latter could be applied as part ofeither the feed preparation process or the concentrationcircuit. Slimes handling is omitted. The technologies listed areexamples, and it is not necessarily intended to address all

favailable technologies. Views or opinions are those of ADPProjects.

Thickening and disposal are not specifically covered inthis paper. The latest technology for the optimal recovery andre-use of water is the ATA process, developed by SoaneMining and Soane Energy. This is a polymer-basedtechnology capable of eliminating the requirement for tailingsimpoundment. Furthermore, significant improvements havebeen realized in fine screening (e.g. Bivitech, Derrick etc.).

These newly matured technologies (or the case ofDebTech and Bourevestnik, only recently made commerciallyavailable to the broader diamond industry) will undoubtedlyresult in significant value being brought to bear on diamondprojects in the future. The challenge is to appreciate theresultant complexity, opportunities, and associated risks(including the cost of poor decisions) and to perform theappropriate study work that precedes the projects (no mattertheir scale).

Tailored solutionsIt is not a simple case of ‘out with the old and in with thenew’. There is a critical balance to be found between conven-tional and more recently developed technologies, especiallygiven the varying degrees of maturity of these technologies inthe diamond industry.

The authors have participated in many diamond processplant development studies/projects, and it is unquestionablethat each project is unique and is deserving of a detailed andmethodical application of knowledge, skills, understanding,and experience to arrive at the solution that will justify theexpenditure and secure the revenue, taking into accounthuman resources and skills available, as well as services(water and power availability and the cost of these).


539The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 114 JULY 2014 ▲

Table I

Some technology choices for the feed preparation circuit (focus on dis-agglomeration/clay removal)

Category Technology Principles of operationa Applications/advantages/disadvantages Some technology suppliers

Disagglomeration Log washer Abrasion-resistant paddles affixed More applicable to tough plastic-type clays. GreyStone, McLanahan,for clay removal to a horizontally mounted rotating KPI-JCI, Trio (supplied by

shaft yields an aggressive washing Diamond Equipment Group), action that breaks down clay QVM

Hydro-CleanTM High-pressure washing Advantages: Haver and Tyler (subsidiary technology, Material is – Weighs significantly less than of the Haver Boecker group)cleaned by high-pressure equivalent technologies streams of water from a – Smaller footprintwashing rotor and – Lower water consumptionspray nozzle combination – Low residence time

– Less CAPEX and OPEX

Autogenous A horizontal rotating mill. Larger Extremely effective clay disagglomeration Outotec, Metso, Polysius(AG) milling rocks constitute the grinding Advantages: FLSMidth, MechProTech

medium, and are reduced – Single stage comminution circuit resultsby impact breakage with in simpler plant design (smaller footprint)compressive grinding of – Lower OPEXfiner particles Disadvantages:

– High power consumption– Initial CAPEX high

Rotary scrubbing A horizontal rotating cylindrical Not applicable for plastic-type clays, as McLanahan, Mechprotech,drum with internal lifters that these can pelletize through the tumbling MT, FL Smidth, Bateman.continuously abrade material actionunder controlled water-to -ore ratios

aSource: vendor specifications


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Table II

Technology choices for the concentration circuit

Category Technology Principles of operation Application/advantages/disadvantages Some technology suppliers

Crushing Conventional cone Impact crushing Advantages: Sandvik, Metso, Osborn,(re-crush)a crushing – Lower capital expenditure IMS

Disadvantages:– Higher risk of diamond breakage– Requires choke-fed conditions

Modified cone Impact crushing, but with modified Advantages: IMS (Kawasaki Cybas-i crushing chamber design. A degree of inter- – Significantly improved performance, cone), Sandvik (Vibrocone(specialized particule crushing occurs capacity, and installed power, while – being explored forchamber design) retaining the reliability diamond processing)

– Excellent product shape and high percentage product passing CSS after firstpass, which is an industry benchmark withlower risk of diamond breakage

Disadvantages:– Requires slightly more comprehensive test

work to establish suitability of technology– More expensive

High pressure Inter-particle crushing Advantages: Polysius, Weir Mineralsgrinding roll – Various applications within flow sheet (KHD), FL Smidth, Polysius(Daniel and Morley – High availability (a ThyssenKrupp company),2010: Olivier et al. – Feed can be varied by varying the roller KHD (KHD Humboldt 2010) speed. Ability to produce finer PSDs Wedag AG), and Köppern

– More diamond-friendly technology (Maschinenfabrik Köppern(Daniel and Morley, 2010) GmbH &Co KG)

Disadvantages:– High CAPEX– Fine PSDs impose additional load on the

slimes handling circuit– High power requirement, resulting in

higher OPEX

Concentration Jiggingb See footnote (b) Typically employed at marginal high- Bateman, Kelsey,throughput operations Gekko, PCF Engineering Advantages:

– Low CAPEX– Semi-portable structure makes it

appropriate for alluvial deposits, whichare spread over large geographical areas

Disadvantages:– Not as efficient as competitor technologies

Rotary panb See footnote (b). Diamond-bearing Typically employed at marginal high-throughput Variousmaterial is mixed with water to create operationsa slurry (puddle) which has a density Advantages:of 1.3–1.5 g/cm³. The mix is stirried in – Lower initial CAPEXthe pan by angled rotating ‘teeth’. – Semi-portable structure makes it The concentrate settles and is pushed appropriate for alluvial deposits, which aretoward an extraction point, while spread over large geographical areaslighter waste remains suspended and Disadvantages:overflows from the centre of the pan – Not as efficient as competitor technologiesas a separate stream – Requires operational skill

Dense medium See footnote (b) Typically employed for higher LOM and EPCM designed andseparation Diamond concentration into a near- affluent operations. supplied i.e. ADP(DMS)b diamond density FeSi slurry medium Advantages: Projects, Bateman.

through use of cyclones and – Efficient diamond recoveryperipheral equipment (JKMRC, n.d.) – Lower initial CAPEX for diamond recovery

from finer size fractions compared to bulk sorting technologies

Disadvantages:– Higher initial CAPEX than jigging and

rotary pan– Higher OPEX in comparison with all

alternative technologie due to FeSiconsumption

– More labour intensive than bulk sortingtechnologies

– High yields from sources with high heavy mineral content

aFocus on re-crush function within concentration circuit only, other possibilities (i.e. VSI) not discussed as, for example, diamond damage aspects needsfurther clarification

bThese systems are based on diamonds having a much higher specific gravity (density) of approx. 3.52 g/cm³ compared to most of the gangue mineralscThe technologies listed are examples, and it is not necessarily intended to address all available technologies. Views or opinions are those of ADP Projects


The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 114 JULY 2014 541 ▲

Table II (continued)

Technology choices for the concentration circuit

Category Technology Principles of operation Application/advantages/disadvantages Some technology suppliers

Bulk diamond sorting technologies

X-ray Based on the principle that diamonds Advantages: DebTech Bourevestnik;luminescence (XRL) fluoresce, and to some degree – Lower CAPEX than XRT alternative Commodas; Flow Sort

phosphoresce, when exposed to – Lower CAPEX for diamond recovery StenertX-ray radiation (Raman and from coarser size fractions than DMSJayaraman, 1950) – Efficiently recovers high-luminescing

diamondsDisadvantages:

– Risk of loss of low-luminescing diamonds– Risk of concentrate dilution by

luminescing gangue– Higher CAPEX for diamond recovery from

finer size fractions compared with equivalentDMS technology

– Requires feed preparation to minimize thepresence of coating on diamondsmasking response to X-rays

– Bottom size process restriction

X-ray Diamonds are recovered on Advantages: Tomra (previously transmissive (XRT) principle of atomic mass (Riedel and – Efficiently recovers all diamond types CommodasUltrasort),

Dehler, 2010) – Potentially lower yields than DMS and Steinert (supplied by IMS)XRL as yields are not influenced by gangue DebTech (currently beingproperties. Can detect diamonds in tested).aggregates/conglomerates or embeddedin shell

– Lower CAPEX for diamond recovery from coarser size fractions than DMS,

– Does not require extensive feed preparationas the technology does not rely on perfectlyclean diamond surface

Disadvantages:– Higher CAPEX than alternative bulk sorting

technologies– Bottom size process restriction

t

aFocus on re-crush function within concentration circuit only, other possibilities (i.e. VSI) not discussed as, for example, diamond damage aspects needsfurther clarification

bThese systems are based on diamonds having a much higher specific gravity (density) of approx. 3.52 g/cm³ compared to most of the gangue minerals cThe technologies listed are examples, and it is not necessarily intended to address all available technologies. Views or opinions are those of ADP Projects

To illustrate this statement, some elements that relate tothe project definition and associated technology selectionconsiderations are discussed briefly (mining methods havebeen excluded from this discussion).

PProject scaleTechnology selection cannot be considered in isolation fromthe intended scale of operation, since the scale is inherentlylinked to the risk profile, or rather the adverseness to risk,wwhich in turn is linked to the funding mechanism.

➤ Small-scale operations will require cost-effective ‘off-the-shelf’ capital-sensitive solutions, possibly to thedetriment of efficiencies, to ensure that risk isminimized and steady income is generated. Oredressing studies are generally not undertaken for thisscale of operation

➤ Marginal-scale operations can bear more risk. Thesefollow the middle path of higher capital outlay againstincreased returns, but still involve a minimization ofrisk to the detriment of efficiency. Ore dressing studiesare limited to the bare necessities

➤ Large-scale operations tend to tolerate the least risk,

f fsince these are coupled to formal funding mechanismsrequiring high initial capital investment. Investorsrequire a secure understanding of the reserve andconfidence in the diamond recovery process selection toassure the desired return on capital. These typicallyrequire extensive exploration, drilling, and ore dressingstudies to facilitate the decision-making process.

Classification of the diamond resourceThe classification of the diamond-bearing resource ismentioned here for consideration. This relates to the matrixof the material from which the diamonds are to be recovered:

➤ Alluvial/marine resources require little or nocomminution, as the diamonds are generally liberated.The challenges are the possible higher levels of clayand lock-up of diamonds in shells or conglomerate.When selecting the appropriate concentrationtechnique, the presence of shells potentially createsmany challenges, one being increased FeSiconsumption due to FeSi entrapment when DMS isutilized. If bulk sorting technologies are considered, theencapsulation of diamonds in shells could render them

invisible to X-ray technology. These considerations willrequire careful CAPEX and OPEX trade-offs against thelife of the resource

➤ Hard rock (kimberlite) requires comminution to liberatediamonds. Comminution technology selection is criticalto minimize diamond damage, but diamond sizefrequency distribution (DSFD), diamond revenue persieve class, as well as CAPEX and OPEX trade-offs of

f fthe technology against the life of mine (LOM) also haveto be considered. The comminution circuit and watertreatment and slimes/slurry disposal go hand-in-hand

➤ Tailings reclamation has been receiving renewedemphasis in recent years and involves a unique clusterof conditions that are fundamentally different fromtreating hard-rock material: a finer feed distribution,potentially finer diamond distribution, and potentially


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Table III

Technology choices for final recovery (no focus on sort-house technologies)

Technology (Valbom Recovery principlesa Some technology suppliersand Dellas 2010)

Magnetic sorting This is a bulk reduction technique introduced prior to other sorting DebTech, Steinert (IMS) Erieztechniques to produce a lower volume upgraded diamond concentrate,and works on the basis of removing magnetic susceptible minerals

Conventional X-ray Diamond recovery on basis of luminescing properties DebTech Modrup Technology, luminescence Impulelo Technologies, Flow Sort

Pulsed X-ray Diamond recovery on basis of luminescing properties. Bourevestnikluminescence Increased efficiencies compared with conventional X-ray luminescence

X-ray transmissive Diamonds are recovered on the basis of atomic mass Tomra (Commodas), Steinert (IMS)Debtech (under development)

Ultraviolet sorting Can be used as scavenging technique and works on the principle of DebTech Ospreydiamonds’ response to UV light. Advantages:

– Optimum diamond scavenging with minimum gangue material at high feed rates– Spillage free– Low concentrate mass– Operator and maintenance friendly– Complete operator safety – Low operating costs – Low CAPEX– Complementary technology to X-ray sorting

Automated grease belt Can be used as scavenging or primary recovery technique. Diamond recovery based Mech Projects and Engineering (previouslyon hydrophobic properties. Grease belts are automated, more secure, more efficient, Oblique Engineering), Bateman Equipmentand more expensive (but require a larger footprint) than grease tables Technologies (BET)

Vibrating Can be used as scavenging or primary recovery technique. Diamond recovery Bateman Equipment Technologies (BET),grease tables based on hydrophobic properties. Grease tables are manually operated, less Vibramech.

secure and less expensive (but require a smaller footprint) than grease beltsbased on hydrophobic properties. Grease tables are manually operated, less

aSource: Vendor specifications

Table IV

Some technology choices for waste sorting (could be applied in either feed preparation or concentration circuits)

Technology Principles of operation Application/advantages/ disadvantages Some technology suppliers

X-Ray Utilizes well established X-Ray Fluorescence Advantages: RADOS10, Redwave,fluorescence (XRF) technology. Simultaneously measures the – Robust and able to operate in extreme Steinert (supplied by IMS)

concentration of up to four metals in the surface climatic conditionsof each particle. A matrix of these elements – Low maintenance cost(including actual quantities and/or ratios) is used – Mechanical or air ejections and very lowto classify individual particle as discard or electricity consumption concentrate fractions

Optical sorting Diamonds or diamond bearing/non-diamond Advantage: Tomra (previouslyby colour bearing particles are recovered or rejected on – Simplified sorting algorithm when CommodasUltrasort),

the basis of surface colour and/or sorting only two facies Steinert (supplied bygeometrical characteristics. Disadvantage: IMS), Redwave, OptoSort

– Sorting efficiency declining with associated colour variation of known facies

– Requires extensive feed preparation

Optical sorting Mineral recognition by individual absorption Advantage: Tomra (previouslyby NIR fingerprint in the near infra-red wavelength range. – Not subject to declining sorting efficiency CommodasUltrasort),

associated with colour variations of Steinert (supplied by IMS), known facies Redwave, OptoSort.

un-liberated diamond prevalence will require specialconsideration of alternative technologies to the primaryprocessing techniques that were employed on thesource material. Again, sight should not be lost to theCAPEX and OPEX implications against the estimatedlife of resource.

DDiamond resource considerations

Grade and volumeResource grade and volume relate directly to the LOM andrequire special emphasis on OPEX vs. CAPEX trade-offswwhen technology selections are made. For example:

➤ Short LOM prospects may not be able to bear theimplementation of capital-intensive technologies, eventhough these might offer the benefit of increaseddiamond recoveries and lower OPEX. The net effectmay not be justified over a short LOM as returns oninvestment may not be maximized

➤ Longer LOM prospects, however, present an entirelydifferent picture as the increased revenue due to moreefficient recoveries, improved liberation, and potentiallylower OPEX (XRT and HPGR combination vs. DMS andcone crushing combination) against excessive initialCAPEX will make financial sense only over anextended LOM when the net profit will be maximized.

The LOM is, however, highly dependent on many otherfactors (mining method, nominal throughput, waste-to-oreratio etc.) which are not covered in this paper. These arecriteria that receive considerable emphasis duringtechnology/flow sheet trade-off studies.

Waste-to-ore ratioThis section relates to kimberlitic ores and not alluvialdeposits. Diamond recovery processes have traditionallystruggled to treat feed sources with excessive waste content.The increase in waste material present in ROM feed normallycauses increased internal recycle loads, for example:

➤ An increased secondary crushing recycle load due toharder waste material and the fact that the crusherscannot maintain the normal operating closed-sidesetting

➤ An increased recovery plant recycle load due to inflatedDMS yields, placing unnecessary treatment constraintson the final diamond recovery process.

Although waste sorting techniques have been used inmany other industries, waste sorting is an emergingtechnology in the minerals processing industry, receivingrenewed attention in a variety of commodity applications. Ifapplied appropriately, it offers the benefit of bulk reduction ofthe ROM ore by selectively removing the waste component.The objective of waste sorting is to upgrade low-gradekimberlite ROM sources and, in doing so, ensure the econom-ically viable treatment of these sources by:

➤ Maximizing the revenue per hour by avoidingprocessing of a diluted ore of inferior grade

➤ Lowering the overall operating expenditure byminimizing the re-circulation and processing of highvolumes of entrained waste

➤ Improving the overall process energy efficiency.

Diamond characteristics and occurrenceDiamond type has a direct impact on the applicability ofcertain concentration technologies e.g. a high prevalence oflow-luminescing diamonds makes the selection of X-raybased technologies less attractive. Nevertheless, there arecases where the project cannot necessarily bear the excessivecapital outlay associated with alternative transmissivetechnologies.

Diamond size frequency distribution, revenue distri-bution, and extent of liberation impact on the:

➤ Top, middle, and bottom cut-off size selections. Someresources may contain a large percentage of smalldiamonds that may be of very low value, and othersmay contain a small percentage of very large diamondsthat constitute the bulk of the value. This is a criticalelement that impacts directly on technology selectionand on the project value proposition

➤ Balancing of the overall circuit with suitabletechnologies, respecting the cut-off size selections andtechnology constraints

➤ Appropriate selection and positioning of comminutiontechnologies, specifically within regards to the re-crushfunction. Cognisance needs to be taken of diamonddamage/breakage if the resource has a high prevalenceof large diamonds.

Retention strategyInvestments in treatment processes are based on the financialmodel of a company, and it is imperative to characterize the‘net worth’ and applicability of a particular unit process.Particular attention is given to the retention strategy of themineral or valuable resource, both within a unit operationand within the overall plant (i.e. revenue in circulation).Every unit process can be characterized according to thevalue-add and should contribute positively to the overallfinancial model (Petersen, n.d.)

In the diamond treatment process the valuable stonesshould be recovered as soon as practically possible post-liberation, and not retained, in order to minimize revenue incirculation. In addition, the risk of damage or losses increasesdramatically the longer the stones remain in circulation.Therefore, the technology selection within various processes,as well as the operability, must be taken into account,understood, and integrated to ensure the business case issound.

Some technology evaluation scenariosThe following examples of technology trade-off scenarios arenot client-specific, but have been generalized for the purposeof demonstrating how technology advances have influencedthe diamond process design playing field.

Primary hard rock/kimberlite

New plant design Consider a new treatment plant design for a primarykimberlite with large diamond incidence, coarse DSFD, andhigh waste-to-ore ratio. Table V outlines the technologyselections for the conventional process design versus new-generation technology choices that could be considered infinding the optimal techno-economic solution.




ffTrade-off/evaluation criteria that are to be considered inorder to maximize the revenue per hour and minimizerecirculation of revenue within unit processes (ADP Projects –confidential technical client reports):

➤ CAPEX (including equipment cost, support structurerequirements, civil requirements, footprint etc.) andOPEX (labour requirements, reagent – e.g. FeSi in thecase of DMS, power requirements of the technologyselection)

➤ CAPEX outlay and potential revenue realization (afunction of OPEX and diamond revenue) in relation toLOM

➤ Circuit balancing in terms of DSFD and revenue profileto achieve the optimal cut-off size selections, respectingtechnology restrictions while maximizing diamondrevenue (consider diamond damage and/or liberationassociated with comminution technologies and revenuelosses when performing cut-off size trade-offs in termsof concentration technology limitations)

➤ Yields of the concentration technologies (DMS, XRL,XRT) in relation to ore characteristics, impacting on thefinal recovery throughput requirement

➤ Complexity➤ Materials handling requirements➤ Plant service requirements➤ Operational challenges (including labour and skills

availability)➤ Flexibility to treat ore if resource characteristics change

over LOM (e.g. clay content, breakage characteristics,waste–to-ore ratio, diamond grades, magnetic suscepti-bility profile etc.). Impact on downstream tailingshandling requirements

➤ Diamond security challenges.

Existing plant optimizationConsider an existing plant treating an ore with finer DSFDand increasing incidence of waste with a typical flow sheetconfiguration consisting of primary crushing, scrubbing,screening, and closed-loop secondary crushing feeding

f fmultiple split fraction DMS modules (i.e. fines DMS modulestreating the 1–12 mm size fraction and coarse DMS modulestreating the 12–32 mm size fraction) (JKMRC, n.d.).

When tasked to find a solution to handle increasedthroughput requirements due to increasing waste dilutionwhile minimizing the capital investment and maximizingrecoveries, the following options could be considered and thecapital investment phased to accommodate cash flowconstraints (ADP Projects, confidential technical clientreports).

➤ Inclusion of waste bulk sorting (NIR, XRF, or coloursorting) in the feed preparation as well as in theconcentration re-crush/tertiary circuits to eliminateunnecessary load on the concentration and finalrecovery circuits. The net effect will be an overallincreased ROM throughput capability, more efficientrecovery of diamonds in downstream processes, andimproved overall economics

➤ Converting all DMS circuits into fines DMS modules(with minor modifications), and bulk sortingtechniques (XRL or XRT) to recover diamonds from thecoarser size fraction

➤ Combining the abovementioned technologies with otherchanges to an existing plant can create additional headfeed capacity, reduce unit costs, and optimize theoperation as a whole.

Trade-off/evaluation criteria that are to be consideredinclude (ADP Projects, confidential technical client reports).

➤ CAPEX (including equipment cost, support structurerequirements, civil requirements, footprint, expansionrequirements to downstream processes – slimeshandling, final recovery etc.), and OPEX (labourrequirements, reagent e.g. FeSi in the case of DMS,power requirements of the technology selection,downstream process expansions to accommodateadditional throughput, slimes handling)

➤ Revenue realization in relation to LOM➤ Circuit balancing, leveraging the benefits of technology

selection while respecting limitations

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Table V

Conventional versus new generation technology considerations

Process area Conventional technology choices New-generation technology considerationsa

Feed preparation • Primary jaw crusher followed by scrubbing and screening • Waste sorting technologies (NIR, XRF or colour sorting) to removein a closed loop with conventional cone crushing waste prior to concentrationfor the secondary duty • Alternative crushing technologies (HPGR or modified cone crushers)

for secondary crusher duty for the protection of large diamonds

Concentration • Treatment of the secondary crusher product by DMS • Bulk sorting technologies (pulsed X-ray or XRT) for the recovery of (split or single-fraction approach would typically be diamonds down to the MCOS, paired with DMS for the recovery considered, depending on the DSFD and revenue of diamonds down to the BCOSprofile) into an upgraded concentrate • Alternative crushing technologies – HPGR (Daniel and Morley, 2010)

• Inclusion of a conventional cone crusher in a re-crush or modified cone crushers in a re-crush duty for the protection ofduty for the further liberation of diamonds large diamonds

Final recovery • Treatment of DMS concentrates through • Magnetic separation (if high incidence of heavy minerals inflates conventional X-ray technologies and/or grease recovery DMS yields)unit processes, to yield diamonds ready for export • Pairing of bulk sorting and final recovery sorting technique is critical.

This principle is to be adhered to in final recovery trade-off studies.• Inclusion of a scavenging technology (grease belt,

table or UV sorter).

aConfidential client reports by ADP Projects

➤ fYields of the concentration technologies (DMS, XRL,XRT) in relation to ore characteristics, impacting on thefinal recovery throughput constraints

➤ Complexity➤ Materials handling requirements and incorporation of

these into an existing plant➤ Plant service requirements in relation to availability

(power, water etc.)➤ Operational challenges➤ Flexibility to treat ore if resource characteristics change

over the LOM (e.g. clay content, breakage character-istics, waste-to-ore ratio, magnetic susceptibilityprofile)

➤ Impact on downstream tailings handling constraintsand final recovery constraints

➤ Expandability of existing plant infrastructure (e.g. finalrecovery building)

➤ Opportunity to incorporate bulk sorting technologieswithin final recovery building

➤ Impact on security philosophy➤ Compatibility with downstream processing techniques

(i.e. sorting technologies that are considered for theconcentration circuit in relation to the existing finalrecovery process).

AAlluvial examplesConsider the following alluvial type plants:

➤ Small-scale high-grade gravel (typically remote Africanoperations):– The typical circuit would comprise a scrubbing and

screening module followed by DMS and wet X-rayprocessing as the final recovery technique

– The new-generation approach would considercombining DMS with bulk sorting technologies(XRT, XRL) or simply replacing the entire DMSmodule with bulk sorting.

➤ High-capacity low-grade gravel with low clay content(typically Orange River, South Africa)– The typical circuit will consider dry screening

followed by rotary pans, feeding pan concentratesinto DMS, followed by diamond recovery by wetXRL

– The new-generation approach would consider bulksorting (XRT/XRL) instead of, or combined with,DMS.

➤ Conglomerate, clay, and shell-rich gravel (typicallySouth Africa West Coast):– The typical circuit configuration would include

scrubbing/milling and screening, followed by acombination of jigs and DMS to produce concen-trates that would be treated by wet XRL in the finalrecovery circuit

– The new-generation approach would consider bulksorting technologies (XRT or XRL) eitherexclusively or in combination with conventionaltechniques for the concentration circuit.

General trade-off/evaluation criteria that are to beconsidered:

➤ Requirement for mobile modular configuration

➤ CAPEX (including equipment cost, support structurerequirements, civil requirements, footprint etc.) andOPEX (labour requirements, reagents e.g. FeSi in thecase of DMS, power requirements of the technologyselection)

➤ Cognisance to be taken of the fact that bulk sortingtechniques are to be paired with final recoverytechniques. If XRT is employed, diamonds that arelocked up in conglomerates or shells have a fair chanceof being detected and will report to concentrate. If thefinal recovery circuit makes use of XRL, these willreport to tailings since the technology relies on a cleandiamond surface to ensure detection

➤ Downstream tailings handling requirement➤ Risk of diamond loss (technology limitations with

regard to bottom size that can be treated).

ConclusionAlthough new technologies have brought the opportunity forimprovements in diamond recovery, they have also increasedthe probability of poor decision-making. The level ofcomplexity has increased, requiring a systematic factualapproach versus one that is riddled with traditions andinappropriate assumptions. New technologies, no matter howattractive in principle, are not always applicable orappropriate. This reality has been borne from experience, butmust not deter the inexorable improvement to plant designthat technology always can, and mostly does, bring.

The selection of the appropriate unit processes and/or thecombination of these is reduced to the analysis of the trade-off between efficiency considerations and capital/operatingcosts. Various simple quantifications of the potentialefficiency improvements over the life of mine versus thecapital savings/increases must be included to aid in decision-making. In addition to the financial aspect, considerationmust be given to practical operating, maintenance, andsecurity aspects. These include the existing operating andmaintenance culture. Unit processes should not be introducedto act as a safety net for inappropriate operating andmaintenance culture, since culture within an operation issomething that can be re-established or changed.

It is contingent on all process design engineers to applythemselves fully to understanding the technologies to theextent necessary to be able to make objective decisions ontheir potential applicability on projects. That, in turn, requireskeeping an open mind, being impartial, and engagingmeaningfully with and listening carefully to the technologysuppliers at all times. Most importantly, it requires doing the’hard yards’ on the study work, never pre-judging theoptimal flow sheet solution at the outset, and allowing thefacts to speak for themselves whenever possible throughthorough front-end engineering design work.

References

DANIEL, M.J. and MORLEY, C. 2010. Can diamonds go all the way with HPGRs?

Colloquium: Diamonds – Source to Use, Gaborone International

Convention Centre, Botswana, 1–3 March 2010. Southern African Institute

of Mining and Metallurgy, Johannesburg.




FICKLING, R.S. 2011. An introduction to the RADOS XRF ore sorter. 6thSouthern African Base Metals Conference, Phalaborwa, South Africa,

18–20 July 2011. Southern African Institute of Mining and Metallurgy,

Johannesburg. pp. 99–110.

JJKMRC. Not dated. DMS Workbook (Samancor). Indooroopilly, Queensland,

Australia.

MACKENZIE, W. and CUSWORTH, N. 2007. The use and abuse of feasibility studies.

Project Evaluation Conference, Melbourne, Victoria, 19-20 June 2007.

Australasian Institute of Mining and Metallurgy, Carlton, Victoria.

OLIVIER, D., BORNMAN, F., ROODE, L., and ACKER, A. 2010. Finsch Mine treatment

plant upgrade project. Colloquium: Diamonds – Source to Use, Gaborone

International Convention Centre, Botswana, 1–3 March 2010. Southern

African Institute of Mining and Metallurgy, Johannesburg. pp. 253–266.

PETERSEN, K.R. Personal communication.

RAMANRR , C.V. and JAYARAMAN, A. 1950. The luminescence of diamond and its

relation to crystal structure. Proceedings of the Indian Academy ofSciences, vol. A32.

RIEDELRR , F. and DEHLER f, M. 2010. Recovery of unliberated diamonds by X-ray

transmission sorting. Colloquium: Diamonds – Source to Use, Gaborone

International Convention Centre, Botswana, 1–3 March 2010. Southern

African Institute of Mining and Metallurgy, Johannesburg. pp. 193–200.

VALBOMVV , D.M.C. and DELLAS, G. 2010. State of the art recovery plant design.

Colloquium: Diamonds – Source to Use, Gaborone International Convention

Centre, Botswana, 1–3 March 2010. Southern African Institute of Mining

and Metallurgy, Johannesburg. pp. 243–252.

DisclaimerThe SAIMM recognizes that the list of technologies andsuppliers described within this paper are not comprehensiveand as such there will be other technologies and suppliersavailable and it is incumbent on the reader to research theiravailability in their specific geographic location. The papertalks rather to principles that should be considered whendesigning Diamond processing plants. ◆

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