Exploration Sampling

‘Exploration Sampling’, a featured article from the August 2011 issue of International Mining, is im-mining.com’s Spotlight Feature Article of the Month.The collection of reliable samples from a mineral prospect or mine is an essential basis for the production of an accurate resource estimate, and for metallurgical test work. SRK states “the strategy to be adopted must thus be carefully designed so that at each stage of the process, the chance of taking biased, unrepresentative or contaminated samples is minimised. It lists some of the key factors to be:

■ Whether the correct sample volume is being taken

■ Whether a change could be introduced which would have significant impact and be cost effective

■ Whether the drill hole or sample layout will provide the level of confidence sought for a subsequent resource estimate.

“Sampling and sample preparation protocols should be designed to produce the highest possible data quality given the nature of the target mineralisation and the ground conditions. Issues addressed in such protocols include the following:

■ Optimum sample length, volume or weight and compositing strategy in relation to the physical characteristics of the mineralisation

■ Optimum sampling methodology and collection procedure to minimise contamination, avoid loss of fines in the case of rotary percussion drilling and to produce representative unbiased samples

■ The establishment of on-site supervision and checking procedures for drilling and sampling programs

■ Monitoring of sample recovery

■ Sample homogenisation and splitting procedures

■ Duplicate sampling at each stage in the reduction and splitting process to determine the magnitude of errors at each stage

■ Quality control procedures to monitor sample preparation and analysis at the mine or commercial laboratory

■ Replicate sampling and analysis to determine precision of sample grades

■ Comparative studies between assay laboratories and determination of accuracy

■ Compatibility of assay databases produced by different sampling campaigns

■ Integration of historical data into current exploration efforts.Since 1928, Boart Longyear’s Diamond Core Drilling services division has completed drilling projects all over the world. It is now the world’s largest diamond drilling services provider, drilling on six continents, and over 40 countries. From small fly rigs, medium size underground rigs, to large deep-hole rigs; it accommodates the needs of both the surface and underground markets. In addition to its large diversified fleet it also employs the latest technologically advanced tooling to ensure safety and high productivity

“Appropriate sampling and Quality Assurance/Quality Control (QA/QC) is essential in all aspects of the mineral deposit evaluation process to ensure the best possible confidence in resultant mineral resource and reserve estimates are achieved; the quality of an estimate is dependent on the quality of the data used.”

Sample analysis 

At the beginning of the year Snowden and SGS announced a Cooperation Agreement to provide an integrated geometallurgical methodology to the minerals sector. This solution aims to deliver to mining companies a better understanding of their orebody by enhancing the technical data set through all the stages of orebody development. 

The companies said “this collaboration is grounded in the complementary services offered globally by each company and combines market leading technical expertise, operational excellence and proven delivery mechanisms.

“Snowden is a premium provider of consulting services, technology solutions and technical training to the mining and related sectors. Since inception in 1978, Snowden has grown a substantial global footprint with offices in Australia, South Africa, Canada, Brazil and the UK.

“SGS is the global leader and innovator in inspection, verification, testing and certification services. [It] is recognised as the global benchmark in quality and integrity. With over 59,000 employees, SGS operates a network of 1,000 offices and laboratories around the world.

Snowden noted that “grade is the main initial economic driver used by geologists globally to develop geological models. Grade block models have traditionally driven sample selection for flowsheet design, leading on to processing plant design. Grade is a key driver on economics but cannot be used in isolation since ore variability is not a simple function of grade, but has multiple determinants. By combining the geological, metallurgical and mineralogical data, this methodology will allow factoring of orebody variability into the flow sheet/infrastructure design and production and quality forecasts. The infrastructure for grinding and downstream plant operations is thus optimised and as a consequence the necessary capital and operating infrastructure is pro actively optimised.”

Steve Wilson Vice President – Metallurgy from SGS said: ‘I believe the combination of the SGS approach to geometallurgy with the tools and talents of the Snowden Group will bring to this area and create a truly unique and value adding offer to all of our clients.”

Around June this year the ALS Group entered into an unconditional agreement to acquire Stewart Group, a global testing services provider to the minerals industry. UKbased Stewart Group is a provider of geochemical, metallurgical and inspection services to the mining and processing industries worldwide. It employs some 900 staff, operating from 26 laboratories and offices throughout North America, Africa, Asia and Europe. The Stewart Group has earned a strong reputation, built on over 40 years of service to clients in a wide range of sectors and operates three main global divisions. The Geochemical Division has operations in Ireland, Canada, Mexico, Russia, Mongolia, Kyrgyzstan, Kazakhstan, South Africa and West Africa, and provides geochemical and assay services to exploration and mining companies. The Inspection Division provides inspection and analytical services to the coal, metals and mineral trading markets through offices and laboratories in the UK, Netherlands, Belgium, Germany, Mongolia, Kyrgyzstan, and southern parts of Africa. The Metallurgical Division operates one of the largest metallurgical businesses in North America via its laboratories in Kamloops, Canada. Stewart Group will be integrated into the Minerals Division of ALS.

Metallurgical process plants that cannot achieve design production rates and/or product quality, particularly during the early stages of project life, are a contributing factor to project failures. Snowden stresses that often such problems “can be attributed to the use of inadequate and/or unsuitable test work when designing flowsheets and specifying process equipment. Inadequate test work may lead to increased project development costs, since expensive test work and engineering may have to be repeated.”

Snowden recommends larger scale test work for higher level engineering studies and this is a “requirement for feasibility level studies. This would apply even if the technology is considered mature and well proven, as each orebody may have its own unique characteristics. If these are not tested at a larger scale, this will result in significant design shortcomings, which will have to be rectified at later stages of the project, usually at substantial additional costs. Design errors based on incomplete or insufficient larger scale test work have been responsible for cost blowouts on many metallurgical projects.  When developing a process flowsheet, risk is minimised by ensuring adequate test work supports engineering and cost estimation at each phase of project development.

“The characteristics of the samples to be taken should also include the mass, particle size distribution and cost.” Sample types that can be used for metallurgical test work and their limitations are listed in the table.”

Snowden observes that the quality of sampling and quantity of metallurgical test work carried out during the various phases of engineering studies “is often insufficient to truly support a robust plant design. Good outcomes are often the result of plant design based on benchmarks of apparently similar orebodies or buoyant metal prices supporting cashflows until rectifications can be implemented.

“The quality and type of samples used for metallurgical test work are just as important as the metallurgical test work itself. The key characteristic required of any sample is that it represents some defined portion of a mineral deposit. Representivity should take into account the spatial orientation, lithology, alteration, degree of oxidation, mineralogy and competency of the ore. Statistically representative sampling procedures and an appropriate test work program should be designed to bring metallurgical understanding up to feasibility study standard and to give a basis for process plant design.”

Getting the sample Operator’s panel for the Atlas Copco Explorac 220 rig designed for reverse circulation (RC) drilling of deep exploration holes

Reverse circulation drilling has proved to be the best way in obtaining high quality of rock samples consistently, according to one of the users of the Atlas Copco Explorac 220 RC, VM Drilling in Western Australia. “The Explorac carries all its rods, sampling and all other systems on board, which in turn means quick set up and drilling can begin within minutes. It is equipped with fast hydraulics with user friendly controls and hands free handling of rods pulls. The sliding dumping mast can be adjusted to angles ranging between 45 to 90o and slips table height adjustable to 1 m which allows access to the drill hole collar. Safety handling is ensured by the hands free breakout and rod handling systems.”

Sandvik says “stationary sampling port cone splitters have proven to be the most representative and operator-friendly form of mineral drilling sample splitters available and are widely used throughout the world.  However, by rotating the cutting ports below the sampling cone, sample representivity is dramatically improved, particularly when material flow bias is present, caused when, for example: the drill rig and/or sampling system is not or cannot be levelled correctly, or a continual fines bias or a random natural flow bias is present (inherent to wet sampling).”

Features of Sandvik’s RotaPort Cone Splitter include:

■ For continuous wet & dry drill sampling

■ Material blockage, bridging, hang-up & cross contamination is minimised

■ In wet drilling, centrifugal forces push water out of the rotating sample bag, retaining coarse & fine particulate, removing costly bulk collection & time-consuming evaporation methods post drilling to retrieve samples

■ A fraction of the height of any other comparable splitter on the world market

■ A lubricated eccentric vibrator improves gravity-fed material flow

■ Multiple samples can be taken simultaneously (primary, duplicate, logging, bulk) via radial cutters with options to change split percentages by simply changing cutting blades

■ Sample bags are loaded into buckets that are enclosed within the splitter and rotate inside during operation, ensuring total protection from: outside elements during sampling, tearing of bags, sample loss, etc.  Bags can be loaded into buckets on or away from the machine

■ As the sampling cone is stationary and only the cutting ports rotate, feed material flows off the cone without spiralling, ensuring a true 360º rotational sample cut per revolution. This enables the cutting ports to rotate faster, providing greater precision and representivity

■ Rotation is achieved by variable speed hydraulic motor fitted with antifriction conical thrust bearings to absorb uneven side loads

■ A “deflector cone” shrouding the sampling cone ensures all feed material for sampling is directed into the cutting ports, preventing delimitation bias

■ Cutter ports are fitted with blades designed in accordance with sampling theory guidelines to prevent extraction bias

■ Fines and flow bias through the splitter captured per revolution of the cutters.

In the laboratory

In the sample preparation laboratory considerable attention must be devoted to ensure that the subsamples processed through the laboratories are representative of the samples received from the client. Laboratory cleanliness and quality control systems in the sample preparation areas should ensure that reliable results are generated in the mineralogical, mineral processing and analytical laboratories.

Liberation of the valuable mineral components of ores is the paramount requirement and this, we know, requires comminution. Retsch points out that “for chemical and physical analytical methods such as AAS, NIR, ICP or XRF it is essential that the specimen is perfectly homogenised to an adequate degree of analytical fineness. A reliable and accurate analysis can only be guaranteed by reproducible sample preparation.”

The choice of grinding tools and accessories must ensure contamination-free and reliable sample preparation prior to laboratory analysis.  Retsch offers a comprehensive range of the most modern mills and crushers for coarse, fine and ultrafine size reduction of almost any material.

Retsch points out that to “generate size reduction effect, the comminution principle of the mill should be matched to the breaking behaviour of the particular material. Hardbrittle materials are best pulverised with impact, pressure and friction whereas soft and elastic substances require cutting and shearing effects to be successfully comminuted. Size reduction machines for large particle sizes above 40 mm are known as crushers or shredders while particle sizes below this are processed with mills.

“Usually, various size reduction principles are combined in one mill, such as pressure and friction in mortar grinders or shearing and impact in rotor mills.

“When selecting a suitable instrument and before beginning the preparation process, a thorough evaluation of the sample material is essential. Properties such as density, hardness, consistency [and] residual moisture have to be examined.”

Photograph courtesy of BGSIn all cases, the requirements of the subsequent analysis have to be taken into account when carrying out a particular grinding task.

Retsch jaw crushers are used for the rapid, gentle crushing and pre-crushing of mediumhard, hard, brittle and tough materials. The company says “no grinder can beat the speed of a Vibratory Disc Mill when it comes to preparing samples for spectral analyses.  The powerful drive of the RS 200 allows for the extremely rapid formation of the movement pattern of the grinding discs inside the jar which leads to analytical fineness after a very short grinding time.  Moreover, the mill provides grinding results with excellent reproducibility. This mill can be used for a wide range of sample materials. With its robust design, it has proven to be very suitable for use in geology, mineralogy [and] metallurgy.”

Mixer mills are also very useful for ores and Retsch’s Mixer Mill MM 400 is a laboratory ‘all-rounder’. With a frequency of 30 Hz, it achieves grind sizes down to the low micron range. It features two grinding stations and allows for the simultaneous preparation of up to 20 samples. A classic field of application is reproducible dry grinding, e.g. for sample preparation to XRF analysis. Planetary ball mills are used wherever the highest degree of fineness is required.

Sepor began business in 1953 with the introduction of the Sepor ‘Microsplitter’, a Jones-type riffle splitter, developed by geologist Oreste ‘Ernie’ Alessio for his own use in the lab. Sepor grew over the next several decades to offer a complete line of mineral analysis tools, as well as pilot plant equipment for scaled operations. Today it is a manufacturer and supplier of several thousand items of equipment for the mineral processing and related fields.

Bico offers crushers, pulverisers, ball mills, centrifuges, and electric furnace and analytical equipment for the laboratory. 

Endecotts is well known for its sieves and sample collection, preparation and analysis equipment. Its associate company Glen Creston makes a range of laboratory mills, and crushing machine’s for sample preparation (for analysis purposes).

Deister offers No.15 Lab Tables, which provide laboratory performance characteristics that can be scaled up to determine commercial-size Table performance. They feature:

■ Two interchangeable decks, one riffled for sand and one riffled for slime

■ Supplied as a package with mounting bench, launders and water manifold

■ Adjustable speed, end elevation and side tilt

■ Produces final concentrates in gold operations.

Now part of ALS, the laboratory at G&T Metallurgical Services has the ability to apply a wide range of techniques to solve metallurgical challenges:

■ One, two and three-product flotation flow sheets (rougher, cleaner and cycle) and reagent schemes

■ Mild steel, inert and stirred mill grinding applications

■ Gravity determinations (including Knelson and Falcon)

■ Leaching applications (including cyanide and acid)

■ Galvanox® copper leaching

■ Thickening, filtering and magnetic separation techniques.

In practice

Xstrata Process Support’s (XPS) Dr. Norm Lotter, CIM Distinguished Lecturer 2010/2011, and Jorge Oliveira, presented an initial short course on the representative sampling of ore for flotation testing to the 43rd national meeting of the Canadian Mineral Processors (CMP) in Ottawa early this year. No matter how carefully the flotation tests are performed, the results are meaningless unless the ore sample is representative.

The four hour short course, which is now available to XPS clients, explains the minimum sample mass models of Pierre Gy, and how to attest to the fundamental variance of the representative sample, be it taken from SAG mill feed, crushed ball mill feed, or drill core.  Additionally, the sampling of drill core using the distribution method for cases where the drill core is in short supply was presented.

At the request of a majority of the attendees, the course is being expanded to include flotation circuits, and will be presented again at the next national meeting of the CMP in Ottawa in January 2012.

Bateman Engineering is another group very active in metallurgical test work. The first copper cathode ever was produced from Russia’s Udokan deposit ore recently in laboratory test work in Perth, by Bateman Engineering in Australia. This confirmed in principle, the Bateman process flowsheet developed for the project.

Photograph courtesy of SRKThis test work was carried out as part of further work awarded to Bateman by Baikal Mining Co (a subsidiary of the Metalloinvest Group), following Bateman’s successful completion of the prefeasibility study for the project. The test work has also allowed all required information to be obtained for development of a definitive bankable feasibility study for Udokan. This deposit located in the Kalarski district of the Chita region, is Russia’s largest known untapped copper resource.

Bateman is carrying out the role of general designer for this next stage of the work. The contract also includes coordinating a number of Russian and international subcontractors, on a series of projects leading up to commencement the feasibility study for a 36 Mt/y copper processing complex, inclusive of its support infrastructure.

The sub-projects include a trade-off study of different types of main technology to be used in processing the complex copper ore types at Udokan, and piloting the Udokan ore through a proposed technology process. This includes input from world recognised specialists and Russian and international piloting facilities. Other sub-projects include determination of a robust process and reliable flowsheet to take into account the variability and nuances of the oretypes; preparation of a geo-metallurgical model for setting a baseline of production, and operation for the deposit; preparation of a piloting facility close to site, according to the selected processing flowsheet; and preparation of various work packages that will facilitate commencement of the feasibility study. These range from tailings disposal tradeoffs, through to site selection.

It is anticipated that Udokan’s 36 Mt/y copper processing complex will be in production by 2016, producing 474,000 t/y of copper cathode and also recovering gold and silver doré. Udokan is a large, complex project in a remote, environmentally sensitive area.

The process plant will include crushing, grinding, sulphide and oxide flotation, pressure leaching of the sulphide concentrates and tank leaching of oxide concentrates. The plant will be fully integrated so that the sulphuric acid produced from the sulphide ore pressure leaching is consumed and neutralised in leaching the oxide ores.

The leach solutions will be fed to and SX/EW circuit, to produce LME grade A copper cathode for sale on world markets. Gold and silver will be recovered from copper ore residues to produce a precious metal doré. IM