Cleveland Potash - Boulby Mine, Cleveland, UK

Figure 1: Cleveland Potash - Boulby Mine, Loftus, UK (Courtesy of CPL Ltd.)

Introduction

Cleveland Potash Limited (CPL) mines 3.0 mtpa of ore at the Boulby underground mine in north-eastern England, producing 1.0 mtpa saleable potash by conventional flotation processes. Two tailings streams are produced as a by product of potash processing:

• 1.8 mtpa centrifuge cake consisting of coarse (+1 mm) salt particles (soluble waste).
• 0.2 mtpa filter cake comprising fine (< 50 µm) montmorillonite clay (insoluble waste), salt and calcium sulphate.

Previously all process waste was re-pulped with sea water and discharged into the North Sea. Due to the presence of trace quantities of heavy metals (mercury and cadmium) in the insoluble clay, the permitted amount of insoluble waste that CPL can discharge into the sea has been substantially reduced.

CPL initiated a study in 1996 to investigate the feasibility of disposing filter cake as backfill in the worked out areas of the mine. The research addressed the following issues:

• Backfilling of tailings composed exclusively of dissolved salts and fine clay material.
• The amount of binder addition necessary to prevent excess water bleeding from the backfill and if the resultant strength gain would be sufficient to allow the material to be used as structural backfill.
• Engineering a backfill transport system requiring a large bore vertical column with a single drop of 1,100 m and long horizontal transport distances of up to 11,000 m.
• Implications of introducing tailings water into a dry, low humidity mine environment and the consequent risks associated with dissolution of salts in underground pillars.
• Implications of the gradual transfer of overburden stress onto the backfilled tailings due to the progressive collapse of the pillar between adjacent panels.
• The practicalities of engineering and installing the surface preparation plant and underground system.

Following the positive outcome of this study, in 1998 CPL committed to a four year project involving extensive laboratory test work and the operation of a pilot-scale plant. The £3,000,000, 4-year project, which received financial support through the European Commission’s LIFE – Environment programme, was managed by CPL with the assistance of the UK’s Mineral Industry Research Organisation (Dodds-Smith 2003). The principal design consultants to the project were Paterson & Cooke Consulting Engineers.

The function of the backfill system is to re-pulp 200,000 tonnes of filter cake per year with sea water and hydraulically place it underground at as high a solids concentration as possible. The filter cake disposal is achieved through the following steps:

Table 1: Backfill system requirements

Backfill slurry properties

The results of tests with binder addition indicated that binder addition rates of between 10% and 15% were required if the backfill was to gain sufficient strength to enable it to be used for underground support. Following careful consideration by mine staff, it was decided not to add binder to the backfill for the following reasons:

• The prohibitive cost of binder.
• Logistical complications associated with transporting the binder.
• The resultant solidified backfill would be mechanically stiffer than the containing rock mass. There was concern that this could lead to subsequent localised failures.

The system design is based on the slurry properties determined from the on-site pilot plant loop tests (Fehrsen et al 2002). The properties are detailed in Table 2. Figure 2 shows backfill discharge into a placement panel.

Table 2: Backfill slurry properties

Figure 2: Backfill displacement (Paterson & Cooke Consulting Engineers (Pty) Ltd)

Underground placement

Placement panels

The principal considerations in selecting mine areas suitable for backfilling are:

• The placement panels need to be dipping generally away from the point of access in order to form a natural sump.
• The panel should be configured such that whilst undergoing gradual closure the placed backfill would not be squeezed into any other area where future access would be required.
• The physical conditions within the panel must be suitable for re-entry in order to enable installation of the placement pipework
• The placement panel conditions should be such that re-entry and observation of the placed fill material would be possible long after filling operations had ceased.

Rock mechanics implications

Calculations undertaken by the CPL’s rock mechanics department show that on completion of mining, the pillars between adjacent panels undergo consolidation and lateral deformation. As a result the cross sectional area of the panel will reduce by 36% within 4 years of mining with a further 26% reduction in the original cross section area occurring over the next decade.

An allowance is provided to accommodate the volume of backfill that could be displaced by the reduction in the panel area due to overburden load.

Ventilation

Studies by the Mine Ventilation Department confirm that the backfilling of worked out panels would not adversely affect the ventilation system due to changes in the airflow pattern or give rise to a significant increase in humidity as a result of the evaporation of moisture from the surface of the backfill.

System commissioning

Preparation Plant

CPL commissioned the surface preparation plant. Before commissioning the underground distribution system, PCCE validated the operation of the surface plant with particular emphasis on the quality control loop. The surface preparation plant performs to specification and controls the backfill slurry viscosity as required. The main problems encountered during commissioning were:

• The mixing time before starting the re-circulation cycle was too short due to a shorter press discharge time than estimated during the design. A ten minute delay has been implemented between the process steps to improve the breakdown of clay lumps before starting the re-circulation pump.
• The mixing tank level parameters were incorrectly set resulting in the re-circulation pumps experiencing NPSH problems. This was been resolved by correcting the tank level parameters.
• The pump PID controllers were optimised to improve the control of the re-circulation pump speeds.

Distribution system

The installation of the underground piping and shaft column was completed while PCCE was on site. PCCE assisted CPL with the pressure testing of the shaft column and underground piping.

Following the pressure test of the shaft column and underground piping, the performance of the energy dissipaters were validated. The two smaller energy dissipaters controlled the flow to approximately 71% of the design flow rate at the duty head. This is less than the design flow rate but it will increase as the units wear, although the charging of the shaft column and filling of the underground range does take longer than estimated as a result. The larger energy dissipater unit controlled the flow rate to 91% of the design flow rate at the duty head. The first backfill placement was on 16 May 2003. Two more pours were possible before the Treatment Plant shutdown on 23 May 2003. The main problems experienced during commissioning were:

• Repair to a leaking joint in the low pressure section of the in-panel piping. The pipe end was slightly dented preventing a proper seal of the gasket. It should be noted that this section of piping was not pressure tested.
• Reconnection of two joints (each on different pours) where the pipe pulled out of the coupling. This appears to be related to the unrestricted lateral movement of the piping. Following the plant shutdown the piping was properly pinned to restrain lateral movement.
• The failure of a sleeve adaptor between the actuator and valve AV715 during a shutdown of the system. A new part needed to be machined and fitted before commissioning could continue.
• An intermittent fault on the fibre optic cable terminations led to problems with the hydraulic power unit which forced a shutdown on the first pour. This problem was finally resolved on 22 May 2003.

The measured flow rates during initial placement were slightly higher than the design flow rates. This was due to the cautious approach adopted during commissioning:- the backfill was prepared to a viscosity lower than the design value for the initial trials. The viscosity is to be gradually increased with future pours until the design flow rate is achieved.

Pressure traces were recorded on start-up and shutdown using high speed data acquisition equipment to compare the actual transient pressures with the predicted transient pressures. The maximum transient pressure step measured was approximately 2 MPa compared with 4.4 MPa predicted. This is most likely due to differences in the flow model used in the transient analysis and the non-Newtonian flow behaviour of the slurry as well as slower valve operating times.

Figure 3: Energy Dissipators (Paterson & Cooke Consulting Engineers (Pty) Ltd)

Operating Experience

The new surface preparation plant has replaced the old kibbler and mixer tanks which repulped the filter cake for disposal to sea. The new plant has proved significantly more reliable.

The backfilling operation is initiated by the operator in the mine control room and shuts down automatically. The control also monitors pressure and flow to shut the system down in the event of a failure. The backfill system has proved simple to operate and the system flow rates have been consistent over the 6 months of operation indicating the reliability of the viscosity control.

Soon after backfilling operations began the insoluble clay content of the waste stream reduced due to changes in the ore mined. This means that less waste needs to be placed underground. Suitable void space underground is limited and so to prevent unnecessary use of the void space, currently backfill is only placed once every two weeks. The system has not blocked following start up of the system after a two week shutdown with a full column indicating that there is no significant settlement of material in the pipe. For most of 2004 the clay insoluble content is expected to remain low and the system will be drained and shutdown for six to eight months.

A system of regular inspection of the shaft and horizontal delivery pipe has been implemented to monitor the pipeline wear rate.

Figure 4: Backfill 90 degree bend (Paterson & Cooke Consulting Engineers (Pty) Ltd)

Conclusions

The successful commissioning of the backfill system in May 2003 has provided the potash industry with a proven alternative method of tailings disposal to the conventional options of disposal at sea (as currently used by Boulby) or placement in a conventional surface tailings depository. The system has a number of unique features:

• Backfill quality loop to produce backfill to a set viscosity.
• The system does not require flushing for normal operation and is shut down with the shaft column full of backfill slurry.
• The valve station (comprising isolation valves and energy dissipaters) ensures that the system is started up and shut down in a controlled manner to minimise pressure transients.

Reference

Wilkins, M; Fehrsen, M; Cooke, R, (2004). Boulby Mine Backfill System