Bulk Engineering, Technical articles

Understanding the material characteristics

Sean Kinder, technical convenor of the ASBSH, explains why it is so important to understand and adapt to changes in bulk material characteristics.

Sean Kinder, technical convenor of the ASBSH, explains why it is so important to understand and adapt to changes in bulk material characteristics.

Throughout its life, an operating plant will witness several changes to its raw material. These changes could fluctuate hourly or be so gradual as the material composition is altered through a deeper excavation or different mineral seam. Understanding material allows all company stakeholders to correctly identify and predict the plant’s future reliability and adapt to environment which will ultimately affect production throughput and downtime. Bulk material handling must adapt to a near limitless number of characteristics, which leads to an exciting allowance of innovation and evolution of the industry.

Common terms which are utilised within the bulk material handling industry include:

  • Sieve analysis – understanding the particle size distribution being handled by the system.
  • Moisture content – essentially, is it dry or wet?
  • Particle shape – general description of the bulk material (blasted, crushed, screened or shaped)
  • Chemical properties – corrosive, oil composition and/or flammability properties.

These features are governed by the processes installed within the operation – (crushers, screens or mills, cyclones). Generally, characteristics can be obtained or monitored from within a laboratory setting and provide the operation with an understanding of expectations to prepare for when undertaking a greenfield project or front-end engineering design (FEED) study.

Understanding these characteristics of the bulk material will aid in the selection of the correct systems required to achieve the required output – for example: belt conveyor, screw conveyor, drag-chain or pneumatic conveyor.

General arrangement drawings include useful information on the bulk material and its production capabilities including material type, proposed capacities, bulk material density and lump size. In belt conveying this information is used to estimate factors such as angle of surcharge allowing for the designer to estimate range of belt widths, speeds, and profiles to optimise the plant’s production requirements.

However, operations regularly must contend with a variety of uncontrollable challenges – water tables or weather changes. This can result in the material’s behavioural pattern changing often leading to issues. Issues may affect a combination of production, maintenance, safety or the environment depending on the situation.

Material variability due to uncontrollable conditions

Understanding the potential behavioural patterns of the bulk material will aid in the selection of accessories and components required to minimise the risk and impact of issues in the system. Some operations have strategies in place to operate under multiple seasonal conditions and schedule shutdowns and component changes in line with the wet and dry weather periods. Other operations blend multiple resources to steady the material throughput.

Cyclical patterns are the easiest to predict, generally climate related. When the material is dry, there are frequently discussions around the transfer points and dust related problems. Compared with when the material is wet, the operation generally switches their focal point to the carryback and tracking related issues concerned with poor belt cleaner performance or blockages within the transfer points.

If regular and predicable cyclical patterns are present the plant should be easily able to adopt reliability strategies to handle all conditions. For example, covering the conveyors to keep the material dry and installing lower capital conveyor accessories that can handle a range of conditions. Excess water should be removed via the belt cleaners or dewatering systems. 

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Chutes should be engineered to accommodate the material properties and correct liners utilised to support the design. Rock box ledges are terrific when the abrasive material can impact against itself providing a substantially longer wearing chute liner life, when the material is mostly free-flowing and dry, however if the material is wet and sticky, rock boxes tend to result in significant blockages. Material velocity through the plant should always be considered, as sticky material in general travels slower, resulting in blockages or output reduction.

Material testing can be completed at variety of service providers which take multiple samples of the bulk material and communicate their expected behavioural patterns in a variety of outcomes.

Issues generally emerge when the materials’ properties or plant input characteristics change its predictability. For example, a longer than anticipated or extreme climate event, uncovering a water table or uncontrolled change of plant input.

In some instances, finer material when dry is free-flowing, however when water is added, it becomes cohesive causing it to clump together resulting in larger lump. Significant hang-ups are a consequence of cohesion and upon severing from the wall can cause inordinate lump and excessive impact. In extreme instances, this lump may prove difficult to break and therefore gets caught in other downstream or bypass processes.

Considerations of particle size and shape

Crushing plants are usually designed around a steady state production rates. Due to the material sieve sizes, actual production rates tend to fluctuate (material can be presented more coarse or more fine than usual depending on its extraction. If the fluctuations are predicted and covered by the design safety factors (flooded belt calculations), this is acceptable. Unpredictable events such as relieved upstream blockages or incorrectly set/abnormal sizing parameter can temporarily increase the actual capacity. This may influence system capabilities or component life.

With an increased market demand, operators are always investing new ways to increase their production with minimal or cost-effective methods. Often the fastest way to increase throughput on a conveyor is by simply increasing its speed. However, without understanding the material several observations have been made often leading to negative consequences.

More turbulent particles, which increases the amount of fugitive dust. This can increase the airflow generated within the transfers and add more pressure to the skirting systems.

Change in discharge trajectory path. Can result in many downstream implications.

Higher impact through transfer points resulting in greater abrasion. This can increase scheduled maintenance on wear parts.

By understanding the sieve analysis and bulk density of the material handled during the design phase these consequences can be identified, recognised, and even eliminated with proper technology.

Particle shape is another key element of bulk material handling. This term is more used when discussing its break profile and crushability, when extracted from the mine pit. Jaw crushers (which have a rectangular opening) are prone to allowing ‘slabby’ material through. With control only on the depth and width of the crusher opening, it is difficult to predict the length of the lump. Therefore, even though the closed side setting on the crusher might be set at 250mm, observations of material upwards of 400mm can be allowed through and pass through the system. 

This information is important for the conveyor designer who can recommend a suitable impact bed or idler duty complementing the life and performance of the conveyor belt.

Once material is crushed, exposed edges result in the abrasive characteristics, however further crushing of materials via impactors often round the sharper edges, lessening the abrasive properties.

Other material properties of relevance

Bulk material chemical properties are an important feature of component material selection. Coal is a clear example of a flammable and corrosive bulk material requiring fire resistant and anti-static components. Agricultural bulk material properties are more difficult to distinguish. Large timber boards aren’t as heavily energised as fine sawdust particles, therefore flammability research and risk assessments need to be considered at each point in the plant’s process design process, whether fire resistant and anti-static components are utilised. Wheat is dry, whereas canola (most often conveyed on the same system) has a high oil content and the design of the conveyor system may need to be biased to the more challenging material.

Chemical corrosion is an even more challenging bulk material handling subject. A suggested simple solution is often to swap mild steel for stainless steel or polymer alternatives. However, if the material is combustible, a polymer equivalent might not be acceptable due to inability to prevent accumulation of static electricity. Chemicals such as solid sulphur change their corrosive properties when mixed with water vapour resulting in a compound capable of corroding various grades stainless steels. A recent event of a gypsum being conveyed at over 1000 tonnes per hour, contained unknown trace elements of phosphoric acid, resulting in substantial corrosion. Initially, the site utilised styrene butadiene (SBR) rubber lagged conveyor rollers, which also wore out due to the SBR’s incompatibility to handle the chemical. This was fixed by changing the rubber to a neoprene lagging following some chemical testing and research of the bulk material exposed to various lagging types.

As production techniques change and bulk material operations age, properties change and evolve. Some of these are predictable, others are not. It is important for these scenarios to be risk assessed and understood for the operation of the plant to adapt and understand the modification requirements, should any sensitive bulk material design criteria be changed. It’s important to understand your material, plan and prepare. 

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