Borehole cores and samples constitute an invaluable asset of any exploration or mining company. These samples not only serve as an auditable record of the quality and quantity of the ore reserve but also serve as a long term geological and geotechnical reference library of the ore body. Most governments and geological surveys actually stipulate that on completion of any exploration program or mining operation that all borehole cores and geological data be delivered for the enrichment of generations to come. It is therefore of paramount importance that the integrity of these samples is maintained.
Over the years core trays have evolved from simple wood boxes manufactured locally, to corrugated iron sheets with wooden ends, various styles of galvanized steel trays, injection molded plastic trays, to the latest development, composite core trays.
Bearing in mind that they are meant to preserve the integrity of core samples under safe custody with minimal interference from environmental contamination and tamper-proof, the quality of core trays used is paramount.
In spite of new designs being continuously produced to suit contemporary demands of the exploration environment, the integrity of core trays remains the most misunderstood, if not contentious, aspect.
There are critical points that African Mining Brief learns from Grant Wilson and John Wilson, experts with extensive experience in exploration and core tray development worldwide.
First of all, while core trays serve generally similar functions, they are not always made to work under all conditions.
Core trays in general serve a multi-functional purpose; initially the trays are filled with core on the drill site, a particularly harsh environment, trays are then used as a container to safely move the core from the drill site to the processing facility, then as a tool to facilitate and assist in the recording and sampling procedures, and finally as a vessel to provide long-term preservation of the core. Nevertheless, this is determined by the type of deposit, project, company and even the region (country). This implies that products suitable for freezing cold conditions in Russia might not work as effective in South Africa.
In an environment in which commodity prices have plummeted, the need for organizations in the mineral exploration field to reduce their risks and curtail costs is paramount. Some of these risks are the hidden or indirect costs associated with core handling and core logging procedures. During this process the cores are continuously being handled or fiddled with, often as in logging or sampling the rock has to be entirely removed from the tray. Core trays with a deep profile as in the wood or plastic trays make it very difficult or in some situations almost impossible to lift the core from the tray, this is particularly relevant to large diameter cores. This has been termed the core tray efficiency index and is related to the amount of wasted time a geologist or sampler spends on handling the core.
Another ergonomical aspect of core trays is the visibility of the core, i.e. how much of the surface of the core is visible when logging the core. In some trays the cores are at the bottom deep channel and can only be viewed when position vertically above then core which makes logging very labour intensive and difficult.
Pertaining to core trays, there is no other option, apart from getting products that have been established to meet the following attributes: longevity, preservation of integrity, handling safety and ergonomics.
In terms of longevity, the core trays should ensure long term preservation of the core, in addition to being robust and durable enough to last under extreme weather conditions, all writing, labelling or marking of the core should be accomplished using UV resistant permanent paint markers, or even embossed on thin metal tags.
- Preservation of core integrity
The trays should limit the weathering and oxidation of the core lids or protective covers assist in this regard. Ensuring that the content of the core is kept as intact as possible is central to gathering accurate information. In consequence, it is critical to curb weathering or oxidation of cores, through the use of protective covers or sealed storage crates, over and above eliminating potential contamination. The shape of the channel should be such that it holds the core firmly in place and limits roll of the core.
The longer the tray the easier it is for the geologist to log, as it provides a bigger overall view of the core, small core trays actually make for difficult logging and results in the longer pieces being broken into small pieces destroying the integrity of the core.
Mining safety regulations in Africa – and indeed worldwide – have become more stringent on ensuring health, safety and environmental (HSE) issues in the mining environment, and the manual handling of excessively heavy core trays represents a potential hazard. Borehole core trays, particularly large diameter cores, are naturally heavy and as such represent a risk.
The potential risks of core trays are likely to come from the manual handling on the drill site wherein the trays are often lifted off the ground, transportation of core trays from drill site where the trays are packed into confined spaces, handling core trays at the core shed where the trays should always be handled at waist level.
A number of HSE organizations stipulate the maximum stipulated weights to be lifted at shoulder height, elbow height, knuckle height and mid lower leg height.
- Ergonomic factor
The ergonomic factor has become crucial in core trays, where there is preference for products that are idiot-proof, which can demonstrate efficiency, exposure and precision.
The efficiency can be achieved from enabling easy, less time consuming removal of cores from core trays. When more core is exposed, it makes the logging and photography processes more efficient. In terms of precision, the more core visible in a tray the easier it is to log.
Exploration and in particular drilling companies operate under extremely harsh conditions in Africa and it is for this reason that the trays must be extremely robust. Trays have then to be transported from the drill site to the repository often over rough and rugged terrain and trays that stack securely have a better chance of survival.
To arrive at the ideal product, it is critical to review the merits and demerits of core trays that are on the market from all corners of the world, with specific focus on cardboard, plastic, wood, Zinc-Alume and composite. Eventually, what emerges from this exercise is that some products are stronger in some attributes and fall woefully short on others, which puts them down the pecking order. A performance and cost effectiveness verdict on the each of the five products provides a vivid picture.
Owing to its very low tensile strength cardboard trays are generally small, lightweight and are easily lifted by one person. The disadvantage of the small size is that it results in more breaks in the core and often makes it difficult to log the cores. These trays are however easily transported, quickly assembled on site, and in most cases, it has proved to be cost effective option.
Although the cardboard is often waxed it is not completely weatherproof, creates difficulty in logging core, and has poor ergonomics, challenges in handling and its small size doubles the workload.
Plastic is widely respected for having no sharp edges, “no contamination”, UV steadiness and non-magnetic features. Corrosion resistant Nonetheless, its main disadvantages are commonly cited as: low tensile strength, the material is a pollutant, needs UV stabilisation, becomes too brittle in cold climates, and it’s difficult to log core due to low core visibility. In addition, it has no ergonomic handling locations.
Despite being produced and obtained locally, and being mostly cost effective, wood has moderate tensile strength/weight ratio, degrades with time, is vulnerable to fire risk and arson, and can be eaten by termites and other insects. In order to provide protection from vermin arsenic or old engine oil is often used, which makes it difficult to dispose of. Moreover, as it is bulky it requires additional storage space and it can if not treated regularly swell and clamp the core. Its continuous usage also speeds up deforestation, which is said to be one of the causes of global warming and climate change.
ZincAlume is today preferred over galvanize steel as it is more corrosion resistant particularly to dilute acid. This material has a high tensile strength/weight ratio which makes it preferred for roof sheeting and core trays, does not produce UV problems, it is robust and permanent, it does have a few ergonomic aspects, over and above being easily transportable. Its downsides are: the potential sharp edges, which are a safety risk; the trays are too heavy to be picked by one person; its prefabricated version cannot be easily assembled quickly and easily. Besides, it is magnetic for in situ magnetic susceptibility readings (contamination of samples from the Zinc coating is negligible and can be ignored).
Composite, combining the attributes of plastic and steel, is cost effective, can easily be assembled on site and is easily transportable. Thus far, from recent records, no disadvantages have been identified.
Going beyond the ordinary
After investing time and resources in product Research and Development (R&D), UCP Africa has created a product that has defied “convention”, which combines the attributes of Zinc-Alume and plastic.
With a thick coating (18-20 micron), composed of aluminum, zinc and silicon, ZincAlume’s high tensile strength to weight ratio of steel makes it the ideal compound for the manufacture of core trays. In particular, it shields the core tray’s steel base by providing a protective coating, protecting against acid and oxidation. It also provides a tough physical barrier between the extreme atmospheric conditions and inner core of steel and has been proven to outlast galvanised steel by two to four times longer.
The core trays designs bring about “exceptional” functional advantages to the end user. They operate within health and safety standards, has ergonomic features, more core access and visibility, negligible contamination, increased efficiencies, easily stackable, and idiot-proof and fast assembly and multiple handling points. Besides, they have comparatively decreased transport, storage and assembly costs, fit into existing racking bays, UV stabilised core tray ends which promote water drainage from the core tray.