Mount Isa Mines Limited

2001 Occupational Health and Safety Innovation Award Application

Title of Innovation:

R67 Mine Refrigeration Plant

The Problem:

Mount Isa, situated about 400 km inside the tropics in north-west Queensland, is one of the world’s great deposits of silver, lead, zinc and copper. Initially discovered in 1923, the mining, concentrating and smelting operation continues to be one of the largest individual producers of these metals in the world and has contributed enormously to the economic development of Queensland and Australia over much of the twentieth century.

However, the original Mount Isa orebodies are now largely extracted, and substantial technical challenges exist to develop new, deeper and more complex orebodies, particularly in the light of the long-term downward trend in metal prices. In addition to technical and cost challenges, there are substantial OH&S challenges in developing some of these new orebodies.

Therefore, faced in the early 1990s with declining production from its existing mines, MIM gave approval for the development of an ambitious new mine, called the Enterprise mine, located underneath the existing operations at depths of up to 2 km.

With a production rate of 3.5 million tonnes per annum, the Enterprise mine is not only Australia’s deepest and hottest metal mine, but also one of the world’s largest underground producers of high-grade copper ore.

The most difficult technical challenge for the Enterprise operation was that of heat, which is primarily an OH&S issue. Virgin (in-situ) rock temperatures are 680C at 2 km depth due to the constant flow of heat from the earth’s core. In addition, “autocompression” – the heating of air as it drops down the intake shafts due to the conversion of its potential energy – results in a further increase in air temperature of 100C per 1000 m of depth.

These underground sources of heat, combined with the hot summer climate (+400C), result in workplace temperatures that, without the development of appropriate OH&S solutions, would be excessive.

Excessive heat stress is known to affect industrial workers in five different ways:

  • Health: excessive heat stress leads to heat illness, typically characterised by serious fatigue, headaches, dizziness/vertigo, nausea, vomiting, cramps and potentially even more severe symptoms such as syncope (fainting) and stroke.
  • Safety: any of the above symptoms will affect a worker’s ability to perform tasks safely. However, even without symptoms, excessive heat is known to affect concentration, hand-eye coordination, mental acuity, and other neurological functions and is therefore a known contributing factor to accidents.
  • Productivity: in thermally stressful environments, work must be carried out at a slower pace to avoid overheating the body. Heat stress therefore results in reduced output, poorer productivity and decreased operational competitiveness.
  • Morale: where work must be conducted day after day under significant levels of thermal stress, morale falls. Among other problems, this results in an increase in absenteeism and turnover of staff, with its problems of loss of skills, lack of care, etc. Workers are also less amenable to workplace change when they believe that one of the key issues in the workplace, the heat stress, is not being taken seriously by management. Therefore, chronic levels of heat stress frequently result in frustration and poor workforce attitudes.
  • Cost: due to the lower productivity, safety, health and morale, operating costs increase where the workforce is under significant and sustained levels of thermal stress.

For all these reasons, the Enterprise mine would not be technically viable without cooling of the air in the workplace, resulting in major economic impacts and significant loss of employment in north Queensland.

In looking for solutions to the problem of heat, MIM first examined the South African experience, since virtually all the mine refrigeration plants in the world are currently located in South Africa. However, there are major differences between the problems of heat in South Africa and Australia. These differences have important implications, but also provide interesting opportunities, in developing refrigeration solutions tailored for mining operations in Australia. In particular,

  • Most mine refrigeration in South Africa is achieved by either chilling air after it has arrived underground, or by chilling of the “service water” used extensively in the handheld drilling machines used for production in South Africa’s deep gold and platinum mines. Whilst some “bulk” air cooling on surface is conducted in South Africa, it is usually to supplement the underground cooling. Therefore, most of the refrigeration plants in South Africa produce chilled water to be sent underground. Due to the high pumping costs back to surface, water is invariably produced at 0.5 to 1.00 C, i.e. the coldest possible without freezing. Therefore as surface bulk air coolers in South Africa use the same refrigeration plants, they are designed to work with very low temperature water.
  • Power costs in South Africa are historically only about ½ of those in NW Queensland.
  • Acceptable workplace conditions in South Africa extend to 32.50 WB with no airflow, i.e. a Basic Effective Temperature of over 320 C ET or a TWL as low as 97 W/m2. These are very arduous conditions[1]. Therefore, obtaining acceptable workplace conditions in Australia would require more refrigeration than in South Africa and be more expensive.

Note also that underground refrigeration is much more expensive than surface bulk air cooling, typically at least twice as expensive, due to the need for hot and cold water storage dams on surface and underground, very high pressure shaft piping (supply and return), high-pressure shaft pumps, underground piping distribution and return systems, a network of underground pumps and sumps, underground aircoolers or spray chambers (including excavations, power supplies, etc), thermal insulation on cold dams and cold pipes, etc.

Therefore to provide acceptable workplace conditions at the Enterprise mine, with its higher power costs and requirements for cooler workplace temperatures, using standard South African practices (underground air cooling and chilled service water), would be potentially so expensive as to seriously reduce the competitiveness of the operation.

The requirement was therefore to develop an innovative solution to the requirement to refrigerate the mine, whilst keeping operating costs to an acceptable level.

The Solution:

MIM embarked on a major program to overcome the problems of heat in the workplace. This consisted of three major components:

  • Widespread adoption of air-conditioned cabins on underground equipment. The Enterprise mine made a policy decision that all new mobile plant purchased for the mine would be fitted with air-conditioned cabins, probably the first such policy in an underground mine in Australia. This included mining equipment that had never been previously air-conditioned in Australia, such as production drill rigs. However, not all underground work is conducted using mobile plant, and in any event, workers must still be able to walk out of a mine, so that temperatures ‘outside’ an air-conditioned cabin still need to be acceptable.
  • Development of new working-in-heat procedures. This occurred after a major program of research which ultimately led to the submission of two Doctoral theses and ten major technical papers, most published in leading international medical and occupational hygiene journals. This research included further development and commercialisation of the Heat Stress Meter, with over 150 HSMs now sold into mines across Australia.
  • Development of the technology for the new R67 refrigeration plant (the subject of this innovation award submission).

In regard to the R67 refrigeration plant, a basic understanding of some aspects of refrigeration theory is required to understand the nature of the innovations.

Firstly, industrial refrigeration plants use the vapour-compression process (described briefly in the Appendix). The process is fundamentally one in which a compressor is used to “pump” heat out of the evaporator, producing cold water, and then discharge this heat into a cooling tower where it is rejected into the ambient air. This process allows several kW of “coolth” (refrigeration) to be produced from each kW of electrical power. The ratio of kW of refrigeration to kW of compressor input power is called the Coefficient of Performance (COP) of the plant.

Clearly, to achieve the lowest operating costs possible, the COP must be raised to the highest possible levels.

The Ideal (or Carnot) coefficient of performance of the system (the theoretical maximum) is given by:

Carnot COP= Te/(Tc-Te)

where Te and Tc are the evaporating and condensing temperatures in Kelvin.

Carnot COPs cannot be achieved in a "real" plant; however, they give a useful yardstick against which actual plant performances can be measured. In addition, a change in plant operating conditions that affects the Carnot COP will have a similar magnitude effect on actual plant COP.

It can be seen from the above equation that the challenge was to lift the evaporating temperature to the highest possible value, and reduce the condensing temperature to the lowest possible value. This would result in the highest possible COP. By then combining this strategy with a surface bulk air cooler, the lowest capital and operating costs would be achieved.

For example, consider a refrigeration plant producing 10 C water using evaporating temperatures of -30 C and condensing at 380 C (note that evaporating temperatures must always be lower than the cold water temperature). The Carnot COP will be 6.6 [(273-3)/(38- -3)]. However, if the water temperature is lifted to 120 C and the evaporating and condensing temperatures changed to 90 C and 320 C respectively, then the Carnot COP will increase to 12.3 [(273+9)/(32-9)]. The plant will produce approximately twice the refrigeration for the same input power (or, alternately, the same refrigeration for half the input power).

This is the approach that the Enterprise mine took and it is a world-first approach.

However, there were substantial technical problems to be overcome in developing an efficient mine refrigeration plant with these truly unique operating parameters.

In particular, during the hot tropical summer periods (typical of Australia, but not of South Africa), the water returning to the refrigeration plant from the bulk air cooler (after releasing its “coolth”), especially on start-up, will be much hotter than normal (>180 C). The resulting high evaporating temperatures produce high suction pressures at the compressor and these in turn result in high back-pressures on the thrust bearings, which seriously shortens the life of the compressors. New techniques had to be developed to handle thermal pull-down and plant upset conditions, and extremities of operation.

To achieve close approach temperatures in the evaporators and condensers, a new generation of plate heat exchangers was introduced. These lowered the stainless steel plate thickness to 0.6 mm and the gap between plates to 3 mm, compared to the values used in all previous mine plants of 0.8 mm and 6 mm respectively. This lowered the cost of the plant and raised its efficiency.

The condenser cooling tower is also unique with four separate cells each with a variable speed fan controlling the water temperature leaving the cell. This allows very fine temperature control of the returning water temperature (to within 0.50 C). In turn, this allows the lowest possible condensing temperatures to be achieved without compromising the safe operation of the compressors.

The plant is the first in the world to be able to “dial up” the actual WB temperature of the air going down the intake shaft, irrespective of what the surface ambient WB temperature is or the airflow going down the shaft. The WB temperature going underground can be controlled to within 0.50 WB, even when the volume flow of air varies over a very wide range from 250 m3/s to 750 m3/s, without affecting plant performance. Previous designs controlled the plant using the cold water temperature leaving the evaporators. However, with such a wide variation in airflows, this would lead to wide variations in WB temperature, with major cost implications. For example, an “error” of 10 WB when 500 m3/s is being sent underground changes the refrigeration requirement by more than 1.5 MW(R); therefore poor control of the WB temperature going underground results in high wastage.

The resulting plant comprises the world’s largest bulk air cooler (25 MW of refrigeration, Figure 2), Australia’s largest chilled water plant (Figure 1), and the first bulk air cooler of this type in the world to be built inside the tropics. The plant has the equivalent capacity to produce 6300 tonnes of ice per day, even in the hottest Mount Isa weather. It is also the most cost-efficient mine refrigeration plant in the world.

The overall concept for the plant was developed by Mine Ventilation Australia and Townsend, van der Walt and Partners (South Africa). The plant was then completed within 12 months from award of the Design and Construct contract to Simon Engineering Australia, and within budget. The plant was wholly designed and built in Australia by SEA. Gordon Brothers Industries, Australia’s largest industrial refrigeration contractor, designed and assembled the refrigeration machines under sub-contract to SEA. The plant was thoroughly performance-tested over summer 2000/01 and shown to meet all its important design criteria.

There were significant safety issues to be considered in the plant design. This is particularly true as ammonia is the refrigerant, with between 7 and 9 tonnes being used in the system. Ammonia was chosen both on technical grounds and because it is a “greenhouse friendly” gas (one of the few refrigerants not in breach of the Kyoto protocol on CFCs and HCFCs), and does not harm the ozone layer. In terms of the safe use of ammonia, while there is a general Australian standard for refrigeration systems, this code is not designed for the situation where several hundred workers could be inside the “cold room” (the mine) with no practical means of escape in sufficient time should ammonia leak catastrophically into the intake.

There are no recognised statutory codes anywhere in the world for mine refrigeration plants, although South Africa does have an industry-developed code that is voluntarily accepted by the larger mining companies.

The lack of any statutory codes, combined with the recently introduced Queensland mining legislation with its “Duty of Care” and ALARA (“as low as reasonably achievable”) concept of risk management, put significant onus on both MIM and SEA to adopt a very formal and rigorous approach to risk assessments and HAZOP studies.

Even after adopting the South African mine refrigeration code, the main HAZOP study led to 255 items being listed for further investigations. Numerous levels of risk assessment were conducted at the conceptual and detailed design stages in areas such as fire risks, loss of refrigerant containment, maintenance access, lightning strikes during tropical storms, and hazards to the workforce or maintenance staff from Legionella and other biological agents in the massive cooling tower and bulk air cooler water systems. These risk assessments included workforce representatives where appropriate.

The Benefits:

The benefits of this plant include the following:

  • Substantially improved health, safety, productivity and morale of workers at the Enterprise mine.
  • The ability to mine the orebody, thereby providing continuity of copper production and continuing employment in Mount Isa and Townsville in the mine, concentrator, smelter and refinery.
  • The very high efficiency that can be achieved. Typically a plant designed in this way could double the thermodynamic efficiency of previous mine refrigeration plant designs.
  • The ability to be able to closely control the WB temperature of air going underground over a wide range of airflows, reducing wastage and lowering costs.
  • The high efficiency and low wastage has substantially changed the economics of refrigerating underground mines. Consider a notional mine producing 1 Mtpa (metal) or 5 Mtpa (coal) requiring 400 m3/s of air of intake air, and a refrigeration plant that drops the WB temperature of the intake air by 40 C, with an overall coefficient of performance (see later) of 3.0 and a site power cost of 8 cents per kWh. Assume the plant is 100% capacity-utilised during the four summer months and not utilised at other times (in practice the spread over the year would be greater, but the utilisation in summer would average much less than 100% as peak capacity is only required on peak hours of peak summer days). A generous maintenance and consumables allowance of 75% of the power cost is also provided. Total refrigeration opex is $1 million per annum (metal: $1.00 per tonne; coal: $0.20 per tonne). Installed firm plant capacity would be 7 MW(R). However, there are also direct cost benefits of this 40 WB improvement to working conditions that will offset a portion of the additional opex. This refrigeration cost of $1 million per annum should also be compared to the actual cost just to move the 400 m3/s through the mine. For a total pressure drop around the mine of 2.5 kPa, and a combined fan and motor efficiency of 75%, this would be $930 000 per annum. Also note that, after refrigeration, most metal and some coal mines should be able to use less than 400 m3/s. Ventilation costs are proportional to the cube of the airflow, so that a 25% reduction in airflow requirements due to refrigeration will lower ventilation costs by 60% to $394 000. Refrigeration costs are partly proportional to airflow, so that the refrigeration costs might fall (say) 10% to $886 000 for a total V&R cost of $1 280 000, compared to $930000 without refrigeration. If the reduction in WB can improve health, safety, morale, production and operating costs by a modest $350000 per annum, then the installation of refrigeration is actually operating cost neutral. This analysis ignores capital costs, but note that refrigeration, if it makes a lower primary airflow possible, will also have substantial capital cost offsets, in that ventilation shaft and airway sizes can be reduced.
  • The successful installation of this plant therefore provides the opportunity for refrigeration to be used more widely in Australian mines, potentially improving conditions for thousands of mine workers in the Country. This is particularly true for potential new mines, which could be developed with refrigeration as an integral strategy from the outset, thereby being able to offset some of the refrigeration plant capital cost by way of reduced ventilation fan and airway diameter costs.
  • The development of a well-researched and risk-assessed safety code of practice for ammonia-based mine refrigeration plants in Australia.
  • For a variety of reasons, there are increasing restrictions on the Australian mining industry in terms of obtaining access to land. Therefore, there are some advantages in underground mines compared to surface mines as they have a much lower impact on the surface environment. The single largest technical issue restricting the depth of mining in the world today is the issue of heat. Heat radiates from the earth’s core at a continuous rate of about 0.05 W/m2. This technology will technically allow the development of the deep orebodies now being discovered with the new generation of exploration tools that can “see” further below surface than has been possible in the past.
  • The technology reinforces Australia’s’ growing reputation as a leading producer not only of mineral products, but also of mining and mineral processing technology and “knowhow” – i.e. tapping into the world’s “knowledge economy”.

The Costs: