Bulk Engineering, Technical articles

TUNRA’s tips for developing a materials testing program that fits your project

Shaun Reid and Priscilla Freire, engineers with TUNRA Bulk Solids, discuss typical project phases and testing requirements.

Shaun Reid and Priscilla Freire, engineers with TUNRA Bulk Solids, discuss typical project phases and testing requirements, including considerations on sampling. 

TUNRA’s engineers often get asked about what types of tests are recommended for a particular project.

As with everything else in bulk solids handling, there is no easy, off the shelf answer. 

Designing effective equipment for new plant or remediation relies on the correct application of materials handling inputs. Materials testing underpins the design of a handling chain that has been optimised to address the many competing criteria involved in the system. However, the nature and extent of testing needs to be carefully considered with respect to the project or study phase and the availability of test samples as a project progresses through conceptual, feasibility and definitive study milestones. 

Flow properties testing has a multitude of applications within materials handling, from informing design parameters for coarse ore plant and crushing, through processing equipment and finally for the storage and transport of both product and waste fractions. These tests include mechanical tests of bulk material strength which inform chute and hopper design, particle and density characterisation, and operational tests such as those required to inform moisture limits. Table 1 shows a non-exhaustive summary of testing and applications.

Designing a testing scope without firstly understanding the handling systems can result in unnecessary expense in budget and time. For example, high-consolidation flow properties testing may not be necessary if the data is only intended for application to low consolidation equipment such as transfer chutes. On the other hand, failing to include tests that are essential for the design of certain pieces of equipment can lead to inadequate designs prone to operational difficulties. 

It is also important to assess the nature of the bulk material samples that are available to test at the given project stage. For example, if only indicative samples are available that do not closely meet the expected characteristics of the final handling fractions, then a limited testing regime may be developed to provide general guidance, with an eye to developing more detailed test data once more refined pilot samples become available.

Sampling considerations

Flow properties testing is typically conducted on the ‘fines’ fraction, which stems from Andrew Jenike’s pioneering work in the 60s, where it became clear that the cohesion of bulk solids, which determines the material behaviour from a flow point of view, is dictated by the ‘fines’. It is worth noting that different testing providers may have varying definitions of ‘fines’. For reference, TUNRA Bulk Solids conducts most flow properties testing in the -4 mm size fraction. 

In practice, when an engineering company wishes to undertake flow properties testing to assist in either remediation of brownfield plants or in the design of greenfield plants, the first point to consider is the purpose and application of the testing results. Considerations such as: ‘what are the characteristics of the material that is handled?’ and ‘what handling conditions are developed?’ are a good starting point in understanding the need for testing and scoping out which tests are recommended. 

In terms of ‘what material is handled’, understanding the type of ore, concentrate or bulk material is not sufficient. It is also important to understand the material’s particle size distribution (PSD), how it changes throughout the plant, and the expected range of moisture contents. It is important to note that information on top-size only is insufficient, as different materials with the same top-size may behave very differently depending on the percentage of fines they contain. 

A question that often arises from the particle size distribution discussion is: ‘what percentage of fines is significant?’. The answer is not straightforward, nor can it be generalised. Factors like moisture content and the material’s ability to hold moisture are at play, as well as mineralogy. As a rule of thumb, 20 percent of -4 mm particles in the material is likely going to demonstrate sufficient cohesion such that flow properties testing is required. This percentage may be lower for materials that are naturally more cohesive and/or are handled at higher moistures. For particularly low fines content materials, many of the traditional flow properties tests may not provide value and alternate testing approaches may need to be considered.

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For brownfield projects, obtaining an appropriate sample requires consideration of time variance in material properties and whether a specific material type should be targeted for testing, i.e a known troublesome ore. Sample collection opportunities within the plant must also be considered. For instance, sampling off the base of a stockpile is often inappropriate due to segregation or moisture migration. Where possible, it is recommended that sampling is undertaken from a belt or transfer with a cross-cut sampler (or similar device). Composite sample generation from multiple collections across a period of time is able to address variance in material conditions.

For greenfield projects it becomes significantly more difficult to obtain a representative sample given that the material has not yet been mined or produced. The following section includes some considerations for designing a materials handling testing scope in greenfield projects.

Flow property testing for greenfield projects

There are several nomenclatures for project phases according to different project management methodologies. However, a project delivery may be broadly categorised by conceptual, pre feasibility, feasibility, and definitive type studies. 

Scoping or conceptual studies are typically the earliest phase where basic samples may become available and preliminary materials handling testing programs can be designed. Assumptions for the plant design at this stage are based on very limited metallurgical testwork and prior knowledge of similar materials from the same geological setting. Consequently, and unsurprisingly, the level of accuracy is fairly low at this stage. Given the basic nature of available test samples, perhaps from core drilling programs, highly detailed testing may not be suitable at this point. 

Pre-feasibility studies, on the other hand, are usually considerably more comprehensive, and metallurgical testwork is conducted on core samples taken from several ore types in the deposit, with the objective of aiding in the development of preliminary engineering and the flowsheet. It is also during the pre-feasibility study phase that the definition of processing parameters for equipment selection commences, and only after these initial process definitions does the handling plant start to take shape. 

With a preliminary plant design available, bulk samples will ideally undergo a specifically designed pilot processing campaign to generate test samples that are good representations of the fractions being handled in practice and upon which detailed testing and design parameters may be developed. It is also during the pre-feasibility stage that the overall project viability is assessed, with capital and operating cost estimates being derived from a combination of preliminary testwork, assumptions, and vendor quotes. 

From a materials handling perspective, once the preliminary flowsheet has been defined and the main processing equipment selected, details such as throughputs and size fractions can be determined. An overall idea of plant dimensions also becomes available, so that parameters for the design of belt conveyors can be defined, such as lengths, inclination angles and widths. The need for stockpiles or bins for storage or surge capacity is also defined. With the progression of the flow sheet comes the need for more detailed definition of design parameters such as critical opening dimensions and hopper half-angles. That is when detailed handling testwork should come into play, given that such design parameters are required to inform preliminary mechanical general arrangement drawings, which will be further detailed in the feasibility stage.  

Final remarks

The design of effective materials handling systems requires a thorough characterisation of the bulk solid being handled and how its properties vary throughout processing. Flow properties testing techniques have been developed specifically to inform the design of critical materials handling equipment. However, the testing program should be designed with careful consideration of the project phase and sample availability. 

When designing flow properties test programs for a given application, TUNRA takes the time to first review the purpose of the testing program and how it fits into the larger project picture, with its engineering team involved from project scoping and throughout delivery to ensure that materials handling needs are met. 

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