Critical Control Points – The Key To Sustaining Nameplate Flow And Realising Value Chain Optimisation

Abstract

Unlike liquids, granular (bulk) solids do not flow naturally, and design oversights are hard to correct post construction. While every installation featuring bulk solids is unique, there is one universal truth that always applies, when material flow stops so does revenue.

To sustain operation at nameplate capacity and then accommodate optimisation opportunities that arise during the plant’s operating life, bulk flow needs to an integral part of our thinking. A characteristic of flow issues is that they only tend to manifest at certain points in a process.

This article introduces the concept of Critical Control Points (CCPs), which are defined as physical locations in the value chain where bulk flow issues have the potential to damage the business (quality, productivity, optimisation); and where preventative control measures can be applied.

Adopting a CCP framework described will help ensured flow is an integral part of your plant’s design and operation without distracting from its primary processing objectives.

Why Flow Matters in Mining and Mineral Processing (M&MP)

“When  flow stops, so does revenue” is a truism that applies to all continuous value chains. For many manufacturing processes it’s the flow of granular (bulk) solids that’s the problem. Unlike liquids, the natural state of bulk solids is not to flow(1), and failure to accommodate the complexity underpinning this inconvenient leads to three major problems:

• Extended ramp-up (typically 18 months longer than a liquids-only plant)(2),
• Failure to achieve nameplate capacity (can often be as low as 60% of the target(2)),
• Life of Operation (LoO) value chain productivity issues(2),

Practically speaking, all bulk solids handling systems can be engineered to ensure flow, therefore all three problems are preventable. However, after decades of bulk flow induced failures, proactive application of bulk solids science to inform the detailed design to achieve flow seems to be the exception rather than the rule.

Something is broken in terms of our approach to design and execution as the dial is simply not moving (3,4,5) giving rise to some spectacular mega-project failures. In fact, not only is the financial underperformance situation not improving, but it’s also getting worse as processes, and their feedstocks, become more complex.

The issue is particularly bad if the process involves a heterogeneous bulk solid like ore. Given everything, we use is either grown or dug up, every process has a dependency on the mining and mineral processing (M&MP).

So, while bulk flow problems extend well beyond M&MP, this industry make a useful basis for studying the root causes and solutions.

If your business is M&MP, you are really in the business of flow. Costs (capital and operating) and productivity are linked with the ability to achieve sustainable flow through your operations’ value chain and at scale. In fact, on the revenue side the relationship is one of direct proportion: No Flow = No Revenue!

When it comes to bulk flow properties, every rock in an ore feed is unique and even the primary derivatives (concentrates, tailings…) are not liquids. In such systems a concerted design effort is required upfront to achieve reliable flow.

One of the challenges with designing for flow is that the underpinning science is a specialist discipline, with very little overlap with metallurgy or chemical engineering. In fact, the flow behaviour of bulk solids is a recognised blind spot in mainstream engineering.

While it possible to correct some types of M&MP plant underperformance during commissioning and ramp-up (with chemistry and operational tweaks), performance issues arising from baked in flow problems are not amenable.

Therefore, if you aim to get just one thing right in your M&MP design, aim to achieve controlled flow as it is the most sensitive performance factor as well as the most difficult to correct post detailed design. This sentiment is summed up in the following maxim.

A Good engineer normalises baked-in flow design errors,
A Great engineer devises workaround(s) to minimise the impact of baked-in flow errors,
A Brilliant engineer designs for flow and avoids the problems altogether!

The exciting news is that we can all be Brilliant, as the science of flow exists and is readily accessible. What’s missing is awareness of flow risks and a methodical way of identifying and then mitigating them to achieve controlled flow by design; the subject of this paper.

Designing for Reliable Bulk Solids Flow from Day One

All M&MP operations are initially justified on the strength of their financial performance with respect to their Basis of Design (BoD), the heart of which is the operation’s nameplate capacity. Designing to quickly ramp up to and then sustain operation at “nameplate” should be the primary mission of the study phase and it should be the imperative that is carried through until the end of detailed design.

According to this mindset all design considerations and decisions should be tested by the question: “What does this mean for flow?”. Working against this approach however is the fact that flow does not involve transformations (chemical and/or physical) which are the exciting aspects that most engineers are trained to deal with. This, combined with a lack of awareness on the part of the design engineers, means that no one takes accountability for ensuring flow, especially on an overall value-chain or systems level.

With no individual or discipline accountable for ensuring flow, the topic tends to slip through the cracks only to emerge in the form of intractable performance issues during ramp-up that taint all the good engineering design work in relation to the individual unit operations.

To remedy this situation a simple technique known as Flow Analysis at Critical Control Points (FA@CCP) has been devised. FA@CCP assumes no prior knowledge of bulk solid flow or even the metallurgical processes involved, and is intended for use by any of the disciplines traditionally involved in the study and detailed design phase of a M&MP operation. It also helps owners’ teams ensure that those commissioned to deliver the project do the requisite homework thus setting up for success.

At the heart of FA@CCP is the premise that a flow risk recognised during study phase is a risk that can be assigned and tracked until it has been designed out, or effectively mitigated, ahead of the detailed design process.

What is FA@CCP? A Framework for Identifying and Mitigating Flow Risks

FA@CCP is a strategic tool designed for value chains featuring bulk solids and is intended to facilitate a quick ramp up to sustainable flow at the nameplate rating. The approach is based on establishing formal flowsheet identities, and therefore accountability for critical flow points in the value chain.

It is a systematic and proactive technique, based on the extremely successful Hazard Analysis Critical Control Point (HACCP) framework used to ensure quality in food and pharmaceutical manufacturing applications.

In addition to avoiding baking-in flow problems at critical points, one of the benefits of using FA@CCP is that it ensures that Critical Control Points are seen for what they are, a network of interdependent flow nodes whose individual and collective performance is critical for overall performance. Making some simplifying assumptions we can quickly appreciate this compounding effect in relation to Critical Control Points in a value chain.

For example, there are 18 Critical Control Points in the front-end of the ore preparation circuit shown in Figure 1-Assuming the designers did their flow homework to the extent that the probability of each point performing at its instantaneous flow design rate is 95% (which historically is a gross overestimate), the time-weighted average flow of ore through this part of the value chain is just 42% of the instantaneous target rate, and this is just the front-end of the overall pit-to-port value chain often containing 100’s of CCPs!

In many cases if just one bulk flow link fails, the whole value chain is usually impacted giving rise to run erratic and out of control run charts and the Perfect Production day [Figure 2].

As an aside, the typical “solution” in this front-end of the flowsheet is to over-rate the capacity of the front end with respect to Nameplate. However, this approach can have significant financial (capital and operating cost, financial viability and construction time) impacts.

Failing to recognise Critical Control Points as part of an interdependent system and appreciating their compounded effect, is the dominant root cause failings responsible for reduced flow and therefore significant value destruction over the life of many M&MP assets.

Application of FA@CCP involves seven simple steps which should really be considered a normal activity within any professional study exercise.

1. Establish your system’s flow objective (processing rates, narrative and operational success criteria),
2. Identify CCPs within the proposed value chain,
3. Classify the possible flow issue(s) at each CCP (in terms of the impact on the business) then, for each material CCP flow deviation:
4. Establish Critical Limits around operational points in order to determine the deviation likelihood,
5. Estimate the inherent risk and if applicable the residual risk after mitigation(s),
6. Establish system(s) to verify CCP performance and identify deviations,
7. Capture all decision(s) relating to FA@CCP review in a Flow by Design database.

In the follow-up article we will unpack each of these steps and then illustrate them with a worked example.

References

1. Jenike, A., 2006. Storage and Flow of Solids. 21st ed. Utah: University of Utah, Salt Lake City, Utah. pp.10-11
2. McNulty, T. P., 2004. Minimisation of delays in plant start-ups; in E. C. Dowling and J.I. Marsden (eds.), Plant Operators Forum 2004, pp1130120.Wellwood, G. A., 2017. One perfect (production) day-a bulk solids handling perspective. Perth, AusIMM-Iron Ore 2017 Conference.
3. Yip, C, 2017. Capital Effectiveness for Mining Project Post Boom. In AusIMM Young Professional Development Webinar Series. Online Webinar, Wednesday 23 August 2017. Online: Australian Institute of Mining and Metallurgy. 1-38.
4. Merrow, E., 1986. A quantitative Assessment of R&D Requirements for Solids Processing Technologies. Available here (Accessed: 6 March 2019).
5. Merrow, E., 1988. “Problems and progress in particle processing”, Chemical Innovation, Jan. 2000 & Chemical Engineering; Oct. 1988, Vol. 95, Issue 15)
6. Wellwood, G.A., 2017. One perfect (production) day – a bulk solids handling perspective. In Iron Ore 2017. Perth, Western Australia, Jul 24, 2017. Perth, Western Australia: Australian Institute of Mining and Metallurgy (AusIMM).