Chapter 55

REDUCTION OF MINE HEAT LOADS

C.A. Rawlins	H.R. Phillips
Research Officer School of Mining Engineering University of the Witwatersrand Johannesburg, 2050	Head School of Mining Engineering University of the Witwatersrand Johannesburg, 2050

ABSTRACT

As mining operations extend ever deeper into the earth's crust, heat influx increases because of regional geothermal gradients. The research described in this paper covers the important parameters of mining depth and heat load increase, together with heat reducing mechanisms such as haulage insulation and backfill. Both heat load reduction agencies deal with the insulation of exposed rock to the ambient air. Haulage insulation reduces heat flow into that area of the mine by up to 80% depending on the material thickness and thermal conductivity of the applied material and other haulage specific conditions such as wetness, size etc.. This reduction leads to reduced refrigeration requirements and consequently a more economically viable operation. Backfill also results in a reduced heat induction (up to 53%) through the exposed rock into the stoping areas of a mine. This reduced heat load is, however, dependent initially upon the cyclic nature of slurry transportation and depth. There is initially a heat inducement during a typical 5000 m transport phase. This heat inflow could be offset by either cooling the slurry on surface, together with insulation of the pipes or a heat exchange facility over the pipe system close to the workings. Since backfill transportation could lead to additional heat being introduced into a deep underground mine, a cooling strategy for slurry type fluids need to be incorporated into the mine design to reject this heat.

KEYWORDS

Haulage; Backfill; Conductivity; Heat; Insulation; Transport; Cooling; Thermal properties

INTRODUCTION

General issues

Financial benefits for a mine and ultimately for the national economy naturally drive the decisions taken in the mining industry. For any mining operation, the maximising of profits in the 21st century can only be achieved by the use of technically sound methods and proven state of the art technology. This objective can only be achieved if mineral exploitation is undertaken with Safety and Health issues adequately catered for in a professional manner.

Heat flow into worked out areas of a deep mine and also its main intake haulages are common knowledge to mining engineers. However, the techniques associated with the reduction of this heat ingress and the technical challenges related to ultra deep level mining are fairly specialised. The research into this field reflects the challenges and problems associated with mining at the extreme depths experienced in South Africa.

Heat ingress into an underground mining operation comes from a variety of sources such as machinery (LHD's, Locomotives, Hoisting apparatus, scraping, battery loading bays etc.), lighting, fissure water inflow, blasting operations and the exposed rock. The later source (rock) is the primary heat load source in any deep level mining operation.

Mining at depths of 5000 m and beyond (as is currently being considered by the South African gold mining industry) requires special technical accomplishments. Figure 1 depicts the reality of ever increasing cooling requirements as mining operations extend deeper below the surface.

Figure 1 indicates the need for the larger refrigeration plants (more cooling) that are required to maintain the mean design reject temperature of say 28°C as indicated. Making use of rock insulation is a specific technique in controlling the influx of heat and thus decreasing refrigeration requirements This, in turn, leads to an overall reduction in both capital and operating costs.

The contribution of a mine environmental control system is currently of the order of between 10 to 15% of the cost of mining. This percentage would increase to ± 20% as a mine deepens to 5000 m, and this clearly indicates the need to reduce the heat load.

Figure 1. VRT and cooling requirements

Mine heat loads and material properties

The following is typical of the heat load distribution patterns found on most deep mines:

1. Rock heat load: 70%

2. Other: 30% (Machinery such as hoisting and mechanical equipment, fissure water, blasting, broken rock, people etc.).

Of the 70% of heat load attributed to rock exposure the distribution to haulages and production areas are:

1. Stopes: 50% and

2. Haulages: 20%.

Generally the underground heat load is countered by means of cooling the air with coil (indirect) or open (direct) type heat exchanger systems, together with backfill operations. On a large mine (say 180 ktpm of broken rock) the refrigeration capacity could be as much as 70 MW, where the mean rock breaking depth is 2700 mbc (meters below shaft collar) and the deepest production areas are at 3600 mbc.

Taking a futuristic view where production would take place at a vertical rock breaking depth of 5000 mbc, the choice of environmental control systems and the cost of their operation could well determine the very existence of such a mine. The significance of haulage insulation and backfill operations cannot, therefore, be underestimated. The combination of these two highly influential heat load reduction techniques has been identified and researched in depth in this paper to determine the combined heat load reduction capabilities and the benefits attainable by their application.

Of great interest, in each case, was the choice of material used and its associated properties both during the transport phase (slurry fluid) and after actual placement.

HAULAGE HEAT LOAD REDUCTION

In the case of haulage insulation the following material properties were technically evaluated and measured:

1. Material thermal conductivity (W/mK) and

2. Material strength (Pa) (Compressive, tensile and energy absorption).

These two properties, when combined, have a direct influence on each other in that they oppose one another. When the thermal conductivity value is good (say below 0,15 W/mK) the strength property is usually reduced (say to a tensile strength of 2 MPa) and visa versa. Therefore there is a need to find the middle way, where the composite material has an acceptable thermal conductive value and an acceptable strength.

There were two options considered in providing a thermal haulage insulation effect. They were:

3. A “one pass” system and

4. A “two pass” system.

A “one pass” system is defined as the application of a material comprising of adequate thermal conductivity and strength properties. The “two pass” system can be defined as two independent materials (usually applied separately). One material consists of the thermal insulation component and the second material incorporates the strength characteristic.

The one pass system is the method of choice since a single spray application would provide both support and insulation in a logistical and cost effective manner.

In order to undertake a pre-feasibility study, a number of independent factors needed to be considered. These included the method of application of the insulation and a determination of heat load reduction and the financial benefits of undertaking this work.

The sequence of work executed can be summarised as follows:

1. Haulage heat load calculation (Un-insulated).

2. Apply insulation/support material onto the exposed rock surface.

3. Calculate the new heat load.

4. Determine the heat load reduction achievable and

5. Evaluate the financial benefits.

The points noted above (a to e) indicate the route followed to obtain the most viable material type to optimise both the insulation and the support provided by the material applied to the exposed rock surface.

Heat load calculation

The haulage heat loads were calculated using a model specifically developed for this purpose. Table 1 provides some of the inputs and Table 2 the outputs calculated:

Table 1. Parameters used in heat load calculations

No	Parameters	Value	Units
1	Haulage length	1000	m
2	Haulage dimensions	4 x 4	m
3	Air temperature (Wet/Dry bulb)	20 / 20	^oC
4	Quantity	80	m^3/s
5	Age of the airway	12	Months
ROCK PROPERTIES
6	Rock thermal conductivity	6,0	W/mK
7	Rock density	2960	Kg/m^3
8	Rock thermal capacity	850	J/kgK
9	Geothermal gradient	0,0099	^oC/100 m
10	Surface height above sea level	1600	m
11	Haulage depth below surface	5000	m
12	Surface rock temperature	20	^oC

The haulage could be one of say six main air intake airways providing air to the production sections of the mine.

Table 2. Comparative heat loads calculated

Level	Un-insulated haulage	Insulated haulage
	(W/m)	(W/m)
1000 m	146.9	70.2
2000 m	357.1	179.9
3000 m	573.7	296.0
4000 m	795.8	419.1
5000 m	1022.7	549.9
6000 m	1253.7	689.3
8100 m	1750.3	1021.5

The insulated haulage heat loads calculated above relate to an insulation material thickness of 50 mm and a thermal conductivity value of 0.15 W/mK. (Note that an insulated haulage at 8100 mL would provide the same heat load as an un-insulated haulage at 5000 mL or a 5000 mL insulated haulage relates approximately to a 3000 mL un-insulated haulage). This indicates that by insulating a 5000 mbc haulage, the haulage refrigeration requirements would equate to that of a mine at 3000 m depth.

Table 3 indicates the heat load comparison for mine wide operations (insulated and un-insulated) conditions.

Table 3. Heat load comparisons

LEVEL	Un-insulated		Insulated
	(kW)		(kW)
5000 mL	2045		1101
4900 mL	2998		1612
4800 mL	3906		2095
4600 mL	4655		2486
4500 mL	5450		2905
4400 mL	6200		3298
Total load	25254		13497
Mine heat load	63240 kW
Percentage load	39.9 %	21.3 %

A reduction of some 47% in heat load would be possible should this type of insulation technique be applied. A heat load reduction of ± 81% could be attained in a scenario of dry haulage conditions and a 30 and 36 % reduction respectively could be achieved under wet and medium-wet rock surface conditions. It is naturally desirable to attain the greatest reduction possible, but taking into account the conditions normally encountered in haulages, wetness always plays a role as drilling operations rely on water and there is always the chance of fissure water intersection.

Another factor to take into account is return drain water (either stope service water or chilled water returns from heat exchanger units such as spray chambers etc.). The chance that a totally dry haulage would be encountered is very small unless all return water (stoping and development service water) is pumped back through a pipe system and fissure water is piped or cementation practiced immediately it is intersected.

The “wetness factor” influence on the haulage heat load system does not in any way effect the insulation thermal conductivity as, for this example, this parameter is kept constant. The influence of water ingress on insulation thermal conductivity, especially when the insulation material could possibly absorb the water, is an important factor to take cognisance of when an insulation material is being considered for this application.

Haulage insulation cost analysis

As described earlier, the first and most important information required when considering the reduction of heat loads in a mine, especially in main intake airways, is an economical analysis before introducing new technology.

Table 4 indicates the input parameters required.

Table 4. Input parameters

Parameter		Value		Units
Refrigeration installation cost		875	$/kW
Electrical power requirement		35%	% of heat load
Electric power cost		138	$/kW/annum
Interest rate		10	%
Project life		20	Years
Insulation material thickness		0 to 100	mm
Insulation thermal conductivity		0.05 to 0,25	W/mK

One of the most influential cost figures is the refrigeration cost value (i.e. $ 875 / kW in our example). This cost would include capital expenditure and installation of the equipment on site as well as associated piping and reticulation systems.

Heat loads obtained from the analysis are provided in Table 5 and 6 below.

Table 5. Heat loads vs insulation thickness and thermal conductivity

Material Thickness (mm)	HEAT LOAD (W/m)
	K = 0.05 W/mK	K = 0.15 W/mK	K = 0.25 W/mK
0	1030.84
10	661.24	831.35	877.67
20	514.45	735.53	810.06
30	426.57	661.24	752.72
40	367.05	602.25	703.60
50	323.70	554.28	661.24
60	290.51	514.44	624.30
70	264.25	480.73	591.84
80	242.89	451.76	563.13
90	225.15	426.57	537.50
100	210.16	404.38	514.44

Figure 2 below indicates the benefits and heat reduction in relation to the thickness of insulation material thickness applied to the rock surface.

Figure 2 further indicates that for an insulation thickness of 50 mm the heat reduction would be 46% relating to a total cost benefit in the order of $ 470,000 over the 20 year life of the project.

Figure 2. Cost benefits and reduction possibilities

If the practice of insulating mine haulages became widespread, a decrease in material costs could be anticipated. At present a realistic cost figure between $ 125 and $ 188 per m³ of material applied is provided from suppliers, but estimates as low as $63/m³ could be attained for bulk use of insulation.

Applying an insulation material with a lower thermal conductivity (< 0.15 W/mK) than indicated in Figure 2 would obviously increase the benefits attained and the opposite would therefore also be true.

Haulage insulation at depth provides an alternative to conditioning the environment. The benefits attained are attractive and therefore should always be considered as a means to offset the ever increasing in-mine heat loads with depth. Full haulage insulation would be the primary target incorporating both thermal insulation aspects as well as primary support for the haulage.

BACKFILL TRANSPORT AND HEAT REDUCTION

General

The second possibility for heat load reduction, researched in tandem with haulage insulation, is that of backfill transportation.

Backfill (slurry) is transported in pipes from surface to the underground worked out areas of a production section. This slurry, when transported through the pipe network, increases in heat during its path towards the production area. The reason for the temperature increase is governed by many factors such as:

• Fluid compression with depth (Joule Thomson effect, JT).

• Friction losses.

• Non-use of energy recovery systems.

• Selection of fluid flow rate in the conveying pipes.

• Fluid properties such as thermal capacity etc.

Generally backfill is transported through a so called “open” transport system and thereby incurs the Joule Thompson effect. This induced temperature increase occurs along the transport path and appears primarily at the point of discharge, i.e. in the stope.

One of the fundamental properties of any fluid relating to its capability to change temperature is its thermal capacity.

a) Thermal capacity

The larger the thermal capacity, the more the energy that is required to change the fluid temperature (either increasing or decreasing it). Water, for example, requires some 4187 Joule for every kilogram of fluid to increase its temperature by 1°C. This is not a new concept for engineers but was essential to this research with respect to the changes in fluid temperatures.

In the case of the fluid researched i.e. backfill, the thermal capacity range of the slurry is always lower than that of water, indicating that its capability to absorb heat is that much greater. The reason for this property of the slurry is the water and solid mixture combination. The solid present in the fluid can be considered the “bad” ingredient in the mixture and the reason is the thermal capacity of the solid. The solid particles used in the slurry mixture come from the metallurgical plant after the ore is processed (classified or un-classified types) and is referred to as waste material containing no mineral of economical value. The waste material is primarily quartzite particles having a thermal capacity in the ranges of 750 to 1000 J/kg°C.

The ratio of quartzite particles to water determines the fluid's thermal capacity. It varies between 1500 J/kg°C to 3000 J/kg°C depending on the density required and hence the mix ratio.

Typical slurry densities used in South African mines range between 1600 and 1750 kg/m³. The thermal capacity calculated for a 1700 kg/m³ density is 2041 J/kg°C, thus providing the fluid with the capability of increasing its temperature by some 4,81°C per vertical kilometre transported.

The range of thermal capacity change within the fluid during its transport path of 5 km vertical is minimal. The small change with temperature is due to the water component, while the thermal capacity of the solid remains constant.

This change within the dynamic fluid was introduced in the written program as the temperatures changed, therefore allowing for the system dynamics to be corrected during its simulated transportation.

b) Thermal conductivity

Tests concluded that the thermal conductivity of backfill fluid ranged between 1,3 W/m°C to 1,37 W/mK. Thermal conductivity change is directly related to slurry temperature change. The thermal conductivity of the solid particles within the slurry, however, remain constant (i.e. a property of the material). The thermal conductivity of the water changes slightly as the temperature increases and impacts directly on the total flow of material, increasing the conductivity to 1,37 W/mK.

c) Heat transfer coefficient

The heat transfer coefficient value for the slurry (i.e. at a density of 1700 kg/m³) ranged from 500 to 3200 W/m²°C within the flow range 5 to 50 m³/hour. For water in the same flow range the values are 1100 to 7300 W/m²°C.

This change in heat transfer coefficient value is influenced by the combination of the Nunner value, Prandtl number, fluid velocity, thermal capacity and Reynolds number. The boundary fluid velocity is also important in this calculation procedure.

The heat transfer coefficient value for a typical mine slurry was determined to be in the range of 2000 to 3200 W/m² °C.

Slurry heat load

The heat load ingress into a mining operation from backfilling transportation is of a cyclic nature, meaning that this occurs during the transport phase only and for a short period after placement in the stopes.

The heat loads induced into a mining operation from backfill transport and placement are best illustrated by an example. Various scenarios were considered including:

1. Fluid surface inlet temperature at 20°C

2. Fluid surface inlet temperature at. 5°C

3. Vertical distance transported (i.e. 5000 m)

4. Horizontal distance transported (i.e. 2000 m)

5. Energy recovery influences

Figure 3 indicates the flow path of the fluid.

Figure 3. Typical fluid transport path through the mine

There are 6 steps in the illustrated fluid transport path. Firstly the initial 2000 m vertical distance. Then the 2^nd leg of a 1000 m vertical distance followed by a 500 m horizontal station area which is required to reach the sub-shaft system. Two 1000 m vertical drop shaft sections to the 5000 mL (meter level) haulage complete the vertical connection. A 2000 m horizontal haulage from the 5000 mL station position completes the transport pipe section to the stoping area.

Table 6 below indicates the distance transported and the temperature changes that occur in the different scenarios considered.

The bare pipe condition indicated as 0-mm insulation material thickness clearly indicates the ambient influence on the system. Heat is exchanged through the pipe walls to and from the fluid transported depending on the fluid temperature and the air temperature at that position and point in time. The insulated pipe section relative to the bare pipe section indicates a temperature rise only because of the partially exclusion of the ambient surrounding heat flow condition.

Evaluation of results

a) Normal system, bare pipe, no recovery

This is the system currently in use on most mines. The fluid is transported through a normal steel pipe to the workings by utilising natural gravitational forces.

In the example above, some 509 kW of heat was induced into the mines atmosphere during the backfill transport phase to the workings. The difference between the ambient air temperature and the fluid temperature arriving in the stope is 7.6°C. The temperature difference would primarily impact on the stope environment from the run-off water (± 0.033 l/s on average) from the backfill bags over a period of 24 hours. The run-off water would channel primarily to the down dip strike gully together with the service water to

Table 6. Temperature changes and heat flow for different transport systems evaluated

Description	Start	Distance transported							System and heat load
	0	2000 mL	3000 mL	3000 mL	4000 mL	5000 mL	5000 H	5000 H
Heat load (kW)	0	412.24	169.22	-375.19	147.84	8.65	-4.19	155.23	513.80
Temperature (°C)	20	30.72	35.13	25.37	29.21	29.44	29.33	33.37	Normal system, fluid cooled on 3000 m level (B)
Heat load (kW)	0	412.24	169.22	-12.07	29.91	-75.38	-47.74	123.51	599.68
Temperature (°C)	20	30.72	35.13	34.81	35.59	33.63	32.39	35.60	Normal system, bare pipe, no recovery (A)
Temperature (°C)	5	8.81	10.77	11.25	13.05	13.95	14.80	19.75	Surface cooled, insulated 15-mm, recovery (D)
Heat load (kW)	0	146.27	75.38	18.72	69.16	34.48	32.83	190.09	566.93
Temperature (°C)	5	15.05	20.05	20.46	25.24	25.93	26.58	31.34	Surface cooled, insulated 15-mm, no recovery (C)
Heat load (kW)	0	386.37	192.01	15.72	183.98	26.45	25.03	182.83	1,012.39
Temperature (°C)	20	30.74	35.50	35.82	40.58	41.23	41.87	46.63	System evaluated with no ambient interference (Base case)
Heat load (kW)	0	412.78	183.06	12.38	183.06	24.72	24.64	183.17	1,023,.

the exits of the stope. The added heat load would result in an increase of ± 31 kW to a single stope heat load. A stope section (i.e. 6 stopes) heat load addition would therefore be 62 kW as the backfill placement rotates in a 3 day filling cycle.

The only heat load that would be of a real-time cyclic nature is that of the heat transfer over and within the pipe system in the shafts and haulage. Therefore additional cooling practices within the mining operation, mainly in the haulages, need to be introduced to off-set this added heat load if this type of system is utilised extensively.

b) Normal system, fluid cooled on 3000 m level

This system is not currently in use on the mines. In this system the fluid is transported through a normal steel pipe to the workings by utilising natural gravitational forces.

1. Heat introduced along pathway: 424 kW

2. Ambient air to fluid temperature difference: 5.4°C.

3. Added heat load: ± 24 kW.

Table 7 provides a summary of the different scenarios considered:

Table 7. Heat load summary

Description	Path heat load (kW)	Temperature difference	Stope added heat load (kW)
System A	510	7.6	31
System B	424	5.4	24
System C	11	3.4	18
System D	457	-8.3	-8
Base case	0	18.6	79

System D (Surface cooled, insulated 15-mm and energy recovery system) would be the obvious choice when selecting for the most environmentally friendly option although not the most economical. Surface chilled slurry could be provided by either adding chilled water or ice to the slurry (Experimentally proven to be feasible).

Insulation application is not a new invention and could easily be introduced with this kind of pipe transportation system. An energy recovery type system for this type of flow quantity is not available at present. Energy recovery design has, however, been done for large fluid flow type systems, where the flow is in the range of 600 l/s (Warman International, BCL project - 1983).

The second choice would then be the cooled surface fluid system with pipe insulation and no energy recovery applied. The heat flow from this system would correlate with current backfill heat loads at a depth of 2300 mbc.

To further illustrate the temperature changes within the transport pipe system for the different scenarios considered, Figure 4 is provided.

FINDINGS, CONCLUSIONS AND RECOMMENDATIONS

General findings

The significant findings arrived at are as follows:

1. Insulation applied to haulage rock surfaces on a mine wide basis could reduce the heat ingress into a mine by up to 50%. Therefore the 40% heat load attributed to haulage rock exposure could be reduced to an overall minimum of 21.1% of the total mine heat load contribution.

2. The combination of good thermal conductive and strength properties in a material applied in “one pass” is possible, although a conductivity value less than 0.1 W/mK would be difficult to attain.

3. A “two pass” insulation system could certainly attain the desired specifications but again the conduction property less than 0.1 W/mK would be difficult to achieve as the binder required for good placement reduces this valuable property of the material.

4. Backfill operations reduce in-mine heat loads by up to 53%. Therefore the 50% overall mine heat load from stoping operations could eventually be reduced to a 25% contribution.

5. The heat load during the fluid transportation phase is important. It is of a cyclic nature although all production areas would not apply backfill at the same day. In a large mine with typically 140 stopes, only 33% (say 46) of the stopes are filled daily, where the heat load per pipe system to the ambient environment is 510 kW, the total daily additional heat ingress would be as much as 24 MW requiring some 31 MW of refrigeration installed at a cost of US $ 27,9 million. Thereafter, running cost (electricity, maintenance etc.) would continue for the life of the project.

6. Alternatively referring to system C, a 0.5 MW heat load would be incurred, i.e. 0,7 MW refrigeration at a cost of US $ 0,6 million.

Conclusions and recommendations

Secondary benefits are attainable with haulage insulation as the reduced friction would be reflected on the main surface fan systems in terms of a pressure reduction (Smaller fan, less pressure therefore less capital required). Secondary benefits with backfill operations are increased stope face air utilisation.

It would be recommended that when no energy recovery system is used, the storage bin positions underground be ventilated independently from the main mine ventilation and that the return air from these installations be directed into the main return airway of the mine. This would ensure that this heat is not introduced into the mine and that the need for cooling is reduced.

Figure 4. Temperature changes within fluid transport phase

The need to develop an energy recovery system similar to that used with chilled water type systems would be advisable. The electric power generated could be utilised therefore reducing the mines electrical demand (cost saving related).

The application of insulation should be technically evaluated for each system design. The benefits must be determined and the most economical insulation material thickness selected.

The cooling of slurry by means of a heat exchanger over the pipe system underground could be beneficial. The pipe system, after the cooling operation, should, however, be insulated to avoid any heat transfer to the environment, thereby allowing the slurry to arrive at the stope with the least temperature rise possible, preferably at a temperature equal or below the mine's designed reject temperature.

Heat reduction overall, including haulage insulation techniques and the design of backfill operations, is definitely achievable. For a mine operating at 5000 mbc, the heat reduction could be as much as 45%, thereby indicating a heat load equal to mining at a depth of 3000mbc.

The planning of any new deep underground mine cannot avoid the application of these techniques. Otherwise a mine would incur elevated heat loads and increased refrigeration requirements, generally making it an uneconomical project.

REFERENCES

Rawlins C.A., “Insulation of Chilled Water Reticulation Systems in Underground Mines”, MSc. Dissertation: Department of Mining Engineering, University of the Witwatersrand, December 1999

Rawlins C.A., “Heat flow into underground mine excavations: Reduction of heat flow into mine excavations with special reference to insulation techniques in main intake airways and production horizons (backfill)”, PhD Thesis: School of Mining Engineering, University of the Witwatersrand, 2001

Mathews M.K., 1987. The measurement of heat flow in a backfilled stope. Journal of the Mine Ventilation Society of South Africa, November, 1987

ACKNOWLEDGEMENTS

The authors wish to acknowledge with gratitude the financial support of the Deepmine Project sponsors during this research. Much of the data contained in the paper came from Anglogold mines in South Africa and their involvement with the project is also acknowledged.

The Anglo American plc (ATD) technical division is also acknowledged for their contribution towards the manpower allocation for the project.

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REDUCTION OF MINE HEAT LOADS

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