Bulk Engineering, Modelling

Leveraging unrealised value through characterisation

Engineered-in material handling problems are responsible for significant unrealised value in mineral processing plants. Corin Holmes from Jenike & Johanson explains why a shift in the way that designers undertake projects is necessary.

A lack of awareness of bulk material handling science and how important it is, especially in early design stages is at the root of many material handling problems. Mineral processes add value to mined raw feedstock through physical and chemical transformations in unit operations, but only if the material can flow. 

To achieve design throughput requires quantification of flow properties to support the necessary customised equipment designs. Too often, assumptions of a material’s properties are used as the basis for conceptual and even detail design. This is especially true for hazardous materials like uranium ore, for which the additional precautions, costs, and time necessary for safe handling are often used to rationalise bypassing testing. 

In a study preformed over 30 years ago, the Rand Corporation reported that in 80 per cent of projects involving raw (unrefined) bulk solids typical performance was 40 to 50 per cent of design capacity without additional capital investment (Merrow, 1986). They reported that even with further investment, some solids processing facilities could not reach more than 80 per cent of design and those that did required 24 to 60 months. While it might be reasonable to expect that progress has been made over the decades and industry has improved its track-record, subsequent studies have shown that the issue persists (Merrow, 1995, 2000; McNulty, 2004; Imrie, 2006; Duinker, 2015; Nikolopoulos, 2019). Although there are some notorious examples of costly failures due to solids handling problems (on the order of $2B capital expenditure, less than 50 per cent of design throughput, sold for 15 per cent of investment), there are innumerable examples of processes that are limping along tolerating undesirably low production and whose investors are receiving lower than expected returns.

The studies conclude that the root cause of the projects’ inability to meet design capacity is problems associated with handling (storage and movement) of bulk solids. The flowsheet for a mineral process combines different unit operations, each designed to modify the product from the prior step either physically or chemically. The ore moves through the process and each progressively more valuable unique intermediate product is collected, transported, and fed into the next unit. But the carefully added value can be quickly destroyed if flow between unit operations is interrupted. In a recent article, Independent Project Analysis (IPA) (Nikolopoulos, 2019) studied over 1500 projects that involve at least one physical or chemical process step that is new in commercial use – a definition that fits most steps in a new mineral process plant. They report that 40 per cent of the projects they looked at failed to meet their respective operational and business objectives, and less than 20 per cent met their key performance metrics. As an example of the study results, a plant with only three process steps featuring a raw (unrefined) solid is likely to achieve a throughput rate of only 50 per cent of name-plate capacity. The result is idle or under-utilised capital, and ultimately reduced production and disappointing return on investment.

If technology to predict and design out handling problems were not available, it would be understandable that so many project teams seem to expect that material handling problems are inevitable in such processes. However, the technology does exist (and has existed for over a half a century) to prevent such problems. The challenge is not a technological one, but rather one that stems from lack of awareness and faulty assumptions.

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Very few engineering curricula around the world include a study of bulk solids. Many early-career engineers do not have sufficient exposure or experience to recognise and implement proper bulk material handling science in the design process. Design decisions are made without considering the handling characteristics of the ore(s), intermediate products, additives, and waste products, but rather are based on other factors, such as space constraints, cost minimisation, etc. Fortunately, or unfortunately, most experience gained regarding bulk solids is through trial and error (school of hard knocks). While such experience might be better than no experience, rarely does it provide a deep enough understanding to ensure trouble-free operations. Too often ‘standard’ or ‘library’ designs are used without giving consideration to the unique characteristics of the specific materials, or the handling conditions and problems that are engineered-in during the design phase are very difficult and costly to rectify. 

Successful projects incorporate appropriate bulk solids handling considerations early in the project, thereby avoiding costly future problems. Measuring flow properties of an ore is typically the first step in this process. 

A mineral processing project’s likelihood for success hinges on reliable feed of bulk solids into and out of the unit processes. Flowability is defined as the ability of a bulk solid to flow through a given equipment reliably. It considers the relationship of the material itself to the equipment in which it is to be handled. A poor-flowing, difficult-to-handle material may be reliably handled in properly designed equipment while an easy-flowing material may exhibit flow problems in incorrectly designed equipment (Pittenger, Carson, & Griffin, 2008). 

The reliability of flow is contingent upon the equipment design being matched to all of the bulk materials it will handle over the full range of conditions to which they will be exposed (Khambekar, Rulff, & Cabrejos, 2009). It is therefore very important to select/prepare representative material samples for testing and to design a purposeful test program that represents the full range of conditions being considered. For example a number of samples containing a range of expected moisture contents might be tested. Storage time at rest (the time material will be stored without any movement) should be simulated in the tests. The measured data can be used in flow modelling and conceptual design at early project stages to improve the flowsheet if required.

Figure 1 illustrates the results of cohesive strength tests simulating continuous flow conducted on a material at 25, 50, and 75 per cent of its saturation moisture content. The figure indicates the original flowability classifications as proposed by Jenike (1964, revised 1976) and demonstrates that this material changes from being on the border between easy-flowing and cohesive to being very cohesive as moisture content increases. 

The change in cohesive strength, resulting from moisture content variation alone, underlines the need to characterise the material across the full range of expected conditions to identify material flow risks and to properly design the material handling equipment. 

Flow properties and design

Flow properties are a powerful asset that can be used in an iterative process to help define/refine the flow sheet and to keep future engineering decisions aligned with achieving reliable bulk solids flow. 

Designing materials handling systems in parallel with the processes they will support imitates most engineering project execution, which permits optimisation of the overall system by allowing integral units to impact each other. Early on, designers have a great deal of influence over the project with a very favourable cost ratio to implement changes. As the project progresses, plans become more locked in, until the scope is frozen in order to finalise the detailed design. Up until this point, the impact of bad design is only on paper and has not negatively affected the project. 

Taking the design of a storage bin as an example, the iterative design process would be as described as follows. In the first step, we gather information regarding the objectives of the system and its requirements, design constraints, and operating conditions. Based on this information, we set the test program and measure the corresponding flow properties. The test results will inform the appropriate flow pattern and geometrical requirements of the bin. Design parameters include the outlet size (required to achieve the desired flow rate, prevent cohesive and interlocking arching, and rathole formation), the hopper wall slope (required to ensure the appropriate flow pattern), and the overall bin size (required to achieve the desired capacity). These in turn will inform choices regarding potential feeder and liner types and requirements.

Figure 2: Iterative system design process related to overall project timeline.
Figure 2: Iterative system design process related to overall project timeline.

Other considerations such as total system height, maintenance requirements, abrasive wear, liner attachment method and ease of replacement, robustness of design, need for further testing or modelling, plant preferences/standards, complication of design/operation, access to trained personnel, and other flow considerations e.g. multiple hopper interaction, compromises, etc. will play into the evaluation of options. If this iteration provides cost effective options that meet all requirements, the final steps of selecting the preferred option and conducting the detail design can be undertaken. This includes determination of loads, structural design, and deciding details for access doors, poke holes, instrumentation ports, and mating flanges.

If none of the options prove favourable, it may be necessary to make some concessions in the flowsheet and commence a further round in the iterative design process. For example, perhaps we can add an additional process step such as a dryer or a pelletiser to alter the bulk solids’ conditions. Or perhaps we can reduce storage time at rest in the bin by moving to a continuous process instead of batch or by changing the frequency of the product shipment to daily instead of weekly. Perhaps gravity discharge from a bin is impractical and a mechanical method will be required to move the bulk solid. Clearly, these types of decisions can have significant implications for the whole project.

Conclusion

It is critical, for successful design of material handling systems, to consider the flow properties of the bulk material(s) to be stored, under the conditions that will be present in the system. Standards exist for measuring these properties and design methods exist that provide guidelines for their use in the design of storage systems to prevent flow problems and ensure reliable discharge. If these guidelines are followed at the early stage of a new system (or modifications to an existing system), it can have a substantial positive effect on the economics of the project. 

It sounds simple, but a shift in the way that designers undertake projects is required. Designers need to view bulk material handling elements as part of the process itself instead of as an afterthought. The cost of change in the design phase is consistently less expensive than trying to rectify an issue later in the project timeline. All design considerations and decisions should be tested against the question: ‘What does this mean for flow?’ to reduce risk, start up on time, save capital, and achieve design rates. 

Do you have a bulk solids handling question? Jenike & Johanson has developed the science of bulk solids flow and specialises in applying it to solving the most challenging bulk solids handling problems. So why not put them to the test with your question? The harder, the better. 

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