Mohsen Yahyaei

Effect of Pulp Lifter Design on Mill Performance

Project Description

Most AG and SAG mills use a pulp lifter to remove slurry from the mill. Slurry and fine particles (but not the grinding media) pass through grate apertures at the discharge end of the mill to enter a series of radial compartments, then as the mill rotates with the slurry inside and the compartment is upended, the slurry pours out of the mill. Reducing the efficiency of discharge are: 1) flowback, which occurs when slurry pours via the apertures back into the mill before able to be discharged and 2) carryover, which occurs when some of the slurry remains within the compartment often due to the centrifugal effect.

Various pulp lifter designs are available. In addition to the commonly used radial design, various curved designs offer higher capacity. Furthermore, various chamber designs such as the turbo pulp lifter improve discharge efficiency by reducing flowback and carryover.

The pulp lifter efficiency influences the slurry level for a given throughput, and therefore the grinding performance. There is therefore a link between the lifter design and metallurgical performance of a mill. Unfortunately, models of pulp lifters are inadequate for design and more work needs to be done to understand how various aspects of pulp lifter design impact on the discharge capacity. Moreover, the link between discharge capacity and the grinding performance also needs to be quantified.

Project Objectives

Areas of possible research objectives related to the pulp lifter discharge include:

1. Quantify the relationship between mill holdup (filling) and discharge using scale experiments. Limitations of previous experimental work in this area include 1) use of spherical media/water mixtures 2) use of flat-ended designs (cone angle = 0 deg) 3) test mills lacking geometric scaling to industrial mills. 3D printing technology, for example, would enable more realistic scale models to be constructed quickly and cheaply. Data can be used for developing mathematical models of pulp lifter discharge.
2. Gain insights into pulp lifter performance including the effect of grate wear on rates of flowback and discharge using numerical methods (DEM/CFD/SPH), and investigate to what extent grate and pulp lifter design can be used to influence this. Data can be used for developing mathematical models or for comparing or developing improved lifter designs.
3. Measure SAG mill grinding performance under different slurry discharge rates. For example, a high discharge capacity will result in a lower slurry filling, but a low discharge capacity would result in a higher slurry filling and potentially a slurry pool inside the mill for a given mill filling. Its effect on grinding rates can be studied in a pilot scale mill and/or by analysis of industrial-scale mills surveyed at different times. The outcomes could be used for SAG Mill modelling and for optimising the grate relining and mill speed control strategies employed on mine sites.
4. Develop a mathematical model of the discharge capacity. This should include the sub-processes of flowback and carryover. The model would be useful for process design and optimisation.
5. Develop methods to monitor pulp lifter performance, such as through the application of sensors. The methodologies developed can be used for process optimisation and improving SAG mill control strategies.

Investigating material transport in tumbling mills

Project Description

Neither the SAG mill nor Ball Mill model contain a mechanistic sub-model of mill transport; and the empirical models used presently encompass other discharge mechanisms such as the pulp lifter and flowback in SAG mills and the flow through the trunnion in ball mills. The transport rate determines the ability of particles and slurry to flow through the mill charge and the axial diffusion rate of particles, and is dependent in particular on charge porosity / tortuosity, slurry rheology, and the mill breakage environment. This HDR project proposes to extend both the AG/SAG mill model and the ball mill model with the inclusion of a new transport sub-model based on the theoretical transport equations. A proposed route for modelling is the application of the model being developed by Prof Indresan Govender at UKZN. Govender’s modelling datasets have used bead media therefore the model needs to be tested on real ore slurries and charges containing coarse particles.

Project Objectives

The project objectives are to develop experimental test equipment and procedures to measure transport rate, while controlling slurry viscosity; experimentally and numerically validate the Govender mathematical model of mill charge transport for mineral ore slurries and charge containing balls and coarse particles; and incorporate this model into the AG/SAG and ball mill models.

Systematic evaluation of advanced grinding circuit control strategies

Project Description

A wide range of process control strategies have been developed for stabilising grinding circuits. Various degrees of control technology are applied ranging from simple PID feedback loops to advanced process control systems including expert systems, machine learning and model predictive controllers. The difference with respect to plant production performance can be substantial. However, advanced controllers are usually installed as a control system upgrade and due to their complexity, their performance can be unreliable. It is all too frequent that advanced control systems in grinding circuits are switched off to revert to conventional controllers. This PhD project aims to investigate this problem by analysing industrial case studies of advanced controllers and their effectiveness. This includes simulating the grinding circuits in Matlab/Simulink and developing a set of metrics that allow the effectiveness of the advanced control systems to be evaluated.

Project Objectives

This HDR project aims to develop a framework for assessing the effectiveness of different advanced process control strategies and tools to understand how to select process control strategies in grinding circuit applications.

Assessing digitisation opportunities for small-scale and conventional operations

Project Description

For the past two decades, large mining companies have made major investments in “digitisation” projects to integrate sensor technologies and data flows across their operations with process control and management systems using sophisticated data analytics and upgraded IT infrastructure. The explosion of new advances in this area has seen the recent availability of low-cost hardware based on open standards and high-quality open source software toolsets that can be applied digitisation projects at mine sites. The proposed PhD project will review which digitisation strategies have been successful in the minerals industry and which strategies already used in larger operations can be translated to smaller scale mining and processing operations.

Project Objectives

This HDR project is focused at mining companies that have not yet implemented large digitisation/data analytics/big data projects, nor installed centralised process control centres. The project aims to identify specific opportunities and data analytics work flow with the aim of demonstrating the value of digitisation technologies. It is intended that the study will work closely with a small-medium mining company and result in a work-flow and tool set framework for real-time data analytics and case-study implementation of ideas generated during the study.