Commissioned Study on Process Control and Instrumentation in Coal Preparation

The JKTech study group surveyed personnel from coal preparation plants, equipment suppliers, consultants and coal industry policy makers. The survey found that the industry requirement for process control techniques was varied. and related to the level of sophistication, style of operation and resource/market relationships. A number of areas emerged in which further process control development can be both directly and indirectly used to achieve increased profitability for a large number of operations.  Reacting to market pressure, the industry is implementing TOM to reduce product variability. Improved process control may allow this to be achieved without undue yield loss or increases in handling costs.

The priority issues emerging from the survey are:

• Plant feed control - blending, analysis for feed forward control of separation processes, minimisation of feed biases between parallel lines.
• Dense medium process control - cut point control in parallel circuits, particularly at low density, requires further work
• Flotation circuit stabilisation and control of product ash.
• Thickener control.
• Ash determination on line - reliable and accurate sensors, calibration practice, sampling and analysis procedures.
• Product stockpile control and blending control.

Where basic PID control loops have been installed and maintained, the industry reports a qualitative benefit. This primarily derives from achieving stable inputs to separation processes and stable internal components of the process. Many operations appear to perform well at this level, with process cut points selected on the basis of resource and market information and requirements. Those operators with digital supervisory systems regarded them well, particularly when the resources were available for system maintenance.

The study revealed that on line washability monitoring was perceived by some in industry as a potential solution to supervisory process control (le. manipulation of the process cut points). The industry also regarded the various ash gauge devices as poor performers.

The separation process with most potential for yield gain through improved process control is froth flotation.

The problems of scarce, inaccurate and noisy measurements, variable feeds and unbalanced process lines, require further study of alternative control techniques. Those developed and used by the petrochemicals industries are not necessarily the best techniques for the coal preparation environment.

The technology and skills exist to achieve basic stable process control, and success is a function of dollars spent on effective design, maintenance and training. A range of problem areas, such as technical staff turnover, management support, operator skills and supplier support affect each operation differently in its endeavours to achieve improved operating efficiency.

The operators with relatively mature systems perceive significant potential for development of advanced control systems. There is a general acknowledgment of a severe lack of information regarding:

• resource characteristics and mining schedule control in real time,
• process efficiency in real time and the actual process cutpoints. Better  information will be necessary for higher level control development.

The problem of limited information and the potential for advanced control must be addressed in conjunction with the control philosophy debate. The philosophy determines which information is required for control purposes.

While the validity of the 'equal incremental ash' approach is generally accepted, there is ongoing debate in the industry about how best to implement this philosophy, and about the most appropriate place to undertake blending.

The study has also identified process continuity and availability as a critical issue. Development of the information acquisition/management capability of modern process control systems can assist in minimising process on-off events, which translates into increased yield and product quality. Maintenance based on equipment condition monitoring via data management from the process control system will benefit both plant availability and process continuity.

In summary, the study recommends support for:

• Consolidation of instrumentation and stabilising control approaches by demonstration in selected plants or alternatively by documentation of current best practice.
• Development of effective feed splitting and distribution systems.
• Development of a management system for information which embraces
• coordination of resource data, plant performance characteristics and product quality and tonnage targets.
• Equipment condition monitoring and planned maintenance
• ash monitor performance and calibration.

• A study of the potential for advanced control techniques to allow use of the quality of data realistically obtainable from the current generation of instruments.

• An investigation into the performance of on-line ash measurement devices and their associated support systems.

• A study of the relative costs of plant engineering approaches versus process control implementation to enable definition of an optimum approach to plant performance improvement (eg. feed blending versus schedule and process control).

• Preparation and maintenance of a technology transfer resource to give operators access to successful case studies and methodologies of process control implementation.

The study has identified several strategies by which the industry can effectively assimilate, apply and sustain process control techniques appropriate to the needs and constraints of coal preparation.

The strategies can be summarised as follows:

• Demonstration
• Education and technology transfer
• Integration of process knowledge, engineering practice, operating practice and the market imperative with well tried process control equipment and techniques.