Title: Classification of South African Hard Rock Mines Establishing a sound basis for classification

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1 1 First Edition 20 July 2006 Second Edition 10 September 2009 Title: Classification of South African Hard Rock Mines Establishing a sound basis for classification Author: Johannes G Auret PhD Engineering Explolabs (Pty) Ltd PO Box 467, Olifantsfontein, 1665 Tel: /3548 Fax: info@explolabs.co.za Website:

2 2 1. Introduction In recent years, South African hard rock mines have suffered increasingly from accidents caused by flammable gas explosions, and the need has arisen to take more effective preventive measures than those currently in place. The purpose of this article is to present an area classification method for hard rock (particularly gold) mines, that is consistent with accepted safety levels and with mining infrastructure. 2. The role of area classification in explosion prevention A hazardous location 1 is any three-dimensional region in or at a mine or at a works or where there could be a risk of the ignition of gas, dust, vapour or any other explosive material. NOTES 1. Certain locations in an underground coal mine are compulsory hazardous locations, see below. 2. Internationally also called area classification which is a bit of a misnomer since a threedimensional space is involved. Classification of a hazardous location means determining the probability (p g ) that (a) an explosive gas or vapour atmosphere may be present; or (b) an explosive dust cloud may be formed. This probability, which facilitates the selection of appropriately protected apparatus for use in such a location, is normally defined as a zone; zone 0, 1 and 2 for gases and vapours and zone 20, 21 and 22 for dusts. In mines, the zoning method is not generally applied although the underlying principle is the same (see clause 4). Explosion protection is one or more specialized design techniques applied to electrical or mechanical apparatus that would normally present ignition sources, in order to eliminate those ignition sources with a known degree of reliability (p i ). Explosion prevention depends on balancing the probability that an explosive gas atmosphere will be present in a particular place at a particular time with the probability that this atmosphere can be ignited. An overall explosion probability of p g x p i 10-5 is considered acceptable 2. Example 1: A Zone 0 gives a worst-case probability of gas being present of 1, therefore intrinsically safe category ia apparatus giving an ignition probability of 10-5 may be used. Example 2: Flameproof apparatus has a probability of 10-3 to ignite an explosive atmosphere and can therefore be used in a Zone 1 (explosive atmosphere present for approximately 1000 hours per year, ie probability is 10-1 to 10-2 ). NOTE Recently (2009), the IEC has introduced the risk-based approach to equipment selection as an option, although the above (statistical) approach to equipment selection will be used in most applications. 3. Overview of explosion prevention in mines Over the last century, the dangers resulting from ignitions and explosions caused by flammable materials in underground mines have been realized, studied and addressed with a fair amount of success. Flammable gas is found in hard rock mines all over the world. South Africa has flammable gas in almost all gold and platinum mines, as well as in kimberlite pipes 3.

3 3 Methane is by far the most common flammable gas (80 100% by volume) 4 ; its properties are shown in the table below. Smaller amounts of higher paraffin hydrocarbons, hydrogen (in association with carbonaceous materials in mines), hydrogen sulphide and carbon monoxide (from combustion processes) may also appear. Due to their toxicity at concentrations well below their lower explosive limits, the latter two gases are seldomly allowed to collect in explosive concentrations. To the author s knowledge, no explosions have occurred as yet where these gases played a significant role. Name Methane Chemical formula CH 4 Physical properties Colourless, odourless,, no taste Toxicity Smothering effect at concentrations above 15% by volume Relative density 0,55 Boiling point 164 C Explosive limits LEL 5%, UEL 15% by volume in air Minimum ignition temperature C Minimum ignition energy 250 μj Maximum burning velocity 0,33 0,43 m/s Maximum explosion pressure 770 kpa In gold mines in the Witwatersrand complex, the methane originates from overlying coal seams and from carbon or carbonaceous reefs. In the Bushveld platinum mines on the other hand, magmatic fluids in the earth s mantle have been postulated as the source of methane gas. The methane finds its way to areas being mined by diffusing, sometimes dissolved in water, along less dense formations such as faults. Coal mines have traditionally suffered most from this problem, as the mined product releases considerable amounts of gas and is in itself flammable. Cases of methane explosions leading to coal dust explosions are well known as well as the highly destructive nature of these combined explosions. Recent statistics show however that South African hard rock mines have experienced an unacceptable high level of accidents and concomitant damage and loss of life resulting from flammable gas explosions (see for example the table below). Year Mine Persons killed Persons injured 1999 RPM Rustenburg Section 1 0 St Helena 1 1 Beatrix Mining Co Ltd 1 0 Northam Platinum Mine, Thabazimbi 0 1 Karee Platinum Mine, Rustenburg 2 0 Oryx Mine 0 2 Impala Platinum Mining operations 1 0 Mponeng Mine 19 2 Kopanang Mine St Helena 1 0 Beatrix Mining Co Ltd 7 0 Impala Platinum Mining operations Beatrix Mining Co Ltd 13 0 Preventive measures obviously involve removing or inerting either the flammable materials or the ignition sources. The major ignition sources are flames, arcs or sparks and heat, provided either from electrical equipment, mechanical actions or chemical fires. Therefore, plugging or sealing of holes, dykes, fissures and faults in the rock, dilution of flammable gases and stone dusting of coal walls have been introduced. In areas where the formation of explosive gas atmospheres cannot be prevented, specialized, explosion protected

4 4 electrical equipment is used; light alloys, welding, brazing and smoking are not allowed; and continuous miners, cutters, shearers and road headers use lower cutting speeds and water sprays. A large degree of success has been achieved in the coal mining industry, as borne out by the reduction in the number of injuries per ignition event (see Cook), even though the number of ignitions has increased due to the increase in mining activities. The challenge to the South African mining industry is now to apply these preventive technologies to their full extent and effectively and economically in gold and other hard rock mines. In these mines, the application of explosion prevention has been a point of contention so far, mainly because no firm basis for the classification of potentially hazardous locations has been established. 4. Developing a model for the classification of hard rock mines A model is presented here for gold mines, that provides safety compatible with the safety levels achieved in surface plants and coal mines. 4.1 Surface plants In surface plants, the methodology to classify a potentially hazardous location in terms of the zoning method has been well established as follows 1 : a) Identify the sources of release of flammable materials (gases or vapours). Such sources include the seals, vents, sampling and drainage points and joints found on meters, valves, pumps, compressors and piping, as well as free liquid surfaces. b) Divide the hazardous location into zones according to the frequency with which a flammable atmosphere occurs and its duration. Briefly, three zones are allocated for potentially explosive gas or vapour atmospheres (Three more zones apply to explosive dust atmospheres). These are: Zone 0 - locations in which flammable gases or vapours are continuously present in concentrations within the lower and upper limits of flammability. Zone 1 - locations in which hazardous concentrations of flammable gases or vapours occur intermittently or periodically under normal operating conditions, or hazardous concentrations of flammable gases or vapours can occur frequently because of repair or maintenance operations or because of leakage, or breakdown or faulty operation of equipment or processes, which might release hazardous concentrations of flammable gases or vapours, might also cause simultaneous failure of electrical equipment. Zone 2 - locations in which operations concerned with flammable or explosive gases, vapours or volatile liquids are so well controlled that an explosive or ignitable concentration is only likely to occur under abnormal conditions. Natural or forced ventilation could serve to reduce the extent of a hazardous location as discussed in (c) below, or could so affect the frequency and duration of the flammable atmosphere that a reduced zone can be allocated.

5 5 c) Calculate the extent of a hazard zone by reason of the rate of release and the nature of the flammable substance, which is mainly defined by the pressure and temperature of the gas or vapour, the liquid flash point and boiling point, the density of the released vapour or gas, and the rate at which it is diluted (ventilated). d) Allocate the appropriate equipment group and temperature class (or maximum surface temperature). Where a mixture of flammable gases or vapours occurs, the worst-case values shall be specified, unless the mixture is of controlled composition and has well-defined ignition properties. 4.2 Coal mines In comparison, in an underground coal mine the following aggravating conditions have to be dealt with: The sources, rate and amount of release of flammable gas cannot be pinpointed as these vary continuously along with the mine s working faces and the local geological dynamics of the earth. Ventilation is artificial and may be interrupted, while ventilation rates also change as the faces progress. The reliability of explosion protection of apparatus is reduced and maintenance is required on a regular basis because of the mechanical and chemical aggressiveness of the mining environment: Cable damage is common, equipment is exposed to impacts by falling rock and mobile machines and to vibrations due to the mining methods, seismic activities and detonation of explosives; and the high temperatures and humidity levels together with low ground water ph causes accelerated corrosion. The primary sources of gas release in a coal mine are the coal faces being mined and the freshly mined coal. The ventilation system brings fresh air to these faces through intake airways and removes gas-contaminated air by means of return airways. As a result, the locations within 180 m from faces being mined, as well as the return airways are always considered to be hazardous (so-called compulsory hazardous locations). Evaluate now these compulsory hazardous locations according to the zone definitions for flammable gas. Gas is released into these locations as a result of normal operations, ie the sources of release are of primary grade. Without artificial ventilation, gas would soon build up and diffuse until the concentration exceeds the lower explosive limit (LEL), ie the entire area (volume) becomes explosive. (A similar situation develops in a surface plant if a normal release of gas occurs in the immediate vicinity of a poorly ventilated below-grade feature such as a sump or pit.) These locations would therefore need to be classified as Zone 0. Thanks to dilution of the gas by fresh air supplied by the ventilation system, the situation is improved to the extent that the gas concentration in the general air body is mostly kept below the LEL of the gas. When the concentration exceeds 1,4% by volume (This figure varies worldwide but approximates 20% of the LEL), electrical ignition sources are removed by turning off the power (except some essential apparatus which may have double explosion protection such as fans, methanometers, communication devices and caplamps). Although the above system prevents the Zone 0 scenario, the possibility still exists for energised electrical apparatus to be regularly exposed to explosive gas mixtures. The following analogy 5 can be drawn: Consider a closed room containing a gas cylinder, an ignition source capable of igniting the gas and a gas measuring instrument. Should the gas cylinder s valve be broken off, the ignition source could be exposed to an explosive gas atmosphere for a finite period before a dangerous collection of gas is detected by the measuring instrument. As a result, equipment used in compulsory hazardous locations in an underground coal mine needs to be explosion protected. Apparatus to be de-energised has to have a high level of protection or EPL (similar to Zone 1 apparatus on surface) and apparatus that remains under power has to have a very high level of protection or EPL (similar to Zone 0 apparatus on surface).

6 NOTE The EPLs for such equipment are respectively described as Mb and Ma according to SANS 60079-0 Edition 4.

6 6 NOTE The EPLs for such equipment are respectively described as Mb and Ma according to SANS Edition 4. Other potentially hazardous locations, such as underground coal storage bunkers, conveyor passages and transfer points, gas and fuel storage facilities, refuelling bays, battery charging bays, crushing plants, coal storage silos, coal beneficiation plants and bulk handling facilities must be classified. It has been proposed to use the zoning system in such cases. 4.3 Gold mines Classification Gold mines have similar aggravating conditions in terms of explosion prevention, although the relative contributions are different compared to coal mines. For instance, due to the enormous depth of gold mines, increased humidity and temperature will tend to accelerate corrosion, while (trailing) cable damage is not an issue. The occurrence of flammable gas is restricted to certain mines and even to certain areas in mines; it is therefore considered appropriate that each mine must be classified on merit, on a regular basis to keep track with the dynamics of the mine. However, it is possible to identify generic sources of release, as follows: Pilot holes in development ends and stopes (see photo below). These holes are specifically used to identify sources of release in the rock before those are being exposed by mining; Cover holes; Cracks, fissures, seeping/draining water, dykes, faults in the rock; Raise bore sites; Diamond drill holes, if gas is allowed to drain naturally and is not piped away; and Return airways, depending on the method used to dilute and remove gas plugs. Current experience indicates that where they occur, the above sources of release should be considered as of primary grade, but for return airways being secondary grade sources. The ventilation principles are similar to coal mines, but comprehensive ventilation monitoring and gas detection with both fixed and portable instruments in hazardous locations has only recently been introduced and is still in the development phase. The flammable gas concentration at which explosion protected apparatus must be de-energized has been set at 1,0%. It has further been proposed 6 that a source of release is considered to create a hazardous location if under normal operating conditions, there is a continuous presence of a flammable gas measured at a concentration of 0,5 % or more by volume in the general body of air.

7 Equipment Gold mines make mainly use of drills, explosives and electromechanical

The rock breaking equipment is small compared to coal mines because of the narrow,

Equipment associated with the potentially hazardous locations listed above include

switches); - lights; - caplamps; - gas sensors and meters; - radios; - telemetry

the possibility exists to relocate such equipment to through ventilation positions,

Where methane is the main flammable gas component, Group I explosion protected

7 7 Equipment Gold mines make mainly use of drills, explosives and electromechanical or hydro-mechanical hammers to break rock. The rock breaking equipment is small compared to coal mines because of the narrow, hard reefs and the variety of equipment is also smaller. Equipment associated with the potentially hazardous locations listed above include the following: - fans (with motors and switches); - winches (with motors and switches); - lights; - caplamps; - gas sensors and meters; - radios; - telemetry systems; - electrical test equipment; and - switchgear In the case of switchgear, the possibility exists to relocate such equipment to through ventilation positions, which will allow standard equipment to be used. Where methane is the main flammable gas component, Group I explosion protected apparatus should be used. Since no provision needs to be made for coal dust, Temperature Class T1 will suffice. Should hydrogen for example be the main constituent, Group IIC requirements will have to be added.

8 8 4.4 Consistency test for the above classification model A comparison of explosion protection in surface plants, coal mines and gold mines is given in the table below. EXISTING DEVELOPING ASPECT Surface Plants Coal Mines Hard Rock Mines Well-defined Not well-defined Not well-defined Fixed installations Mined coal (P) - Standard sources of Pockets, dykes, cracks Cracks, fissures, release (joints, valves, (P) seals) Coal face (P) Sources of gas release (and grade) Ventilation Classification Natural, artificial Risk analysis based Zones (0,1,2) According to relative period of explosive gas presence Zone 2 = normal Return airways (P) Artificial Dedicated intake and return airways Risk analysis based Compulsory (P) Zone m from face, return airway Classify other areas per zoning method (gas, coal dust) dykes, ground water (P) Cover holes, pilot holes, diamond drill holes if naturally drained (P) Return airways (S) Artificial Dedicated intake and return airways Risk analysis based Hazardous locations (gas stress areas) Apparatus Protection Level (EPL) Zone 1 = high Zone 0 = very high Compulsory = high, switch off >1,4% Emergency = very high P=high, switch off? Emergency = very high NOTE P = primary grade release = release that is likely to occur in normal operation. S = secondary grade release = release that is unlikely to occur in normal operation and, in any event, will be of short duration. From the table and the associated discussion, it can be concluded that: (a) (b) there is consistency in the approach followed towards explosion protection in surface plants and coal mines, and the proposed model for gold mines gives equivalent safety compared to surface plants and coal mines. 5. Conclusions To ensure the success of anti-gas explosion measures in hard rock mines, a sound technical basis is essential, starting with the key process of classifying potentially hazardous locations. The classification model presented in this article is believed to be the answer, as it is consistent with the approach followed and safety levels achieved in other industries. It is further believed to be compatible with existing infrastructure in gold mines, such as the ventilation system and the use of gas measuring instruments.

9 9 Acknowledgements This article is based on work done for the Gold Fields Technical Division and for Beatrix Mine. Information has been obtained from several publications including national and international standards, reports and courses. References Information included in this article has been obtained from several publications. Particular recognition is given to but not restricted to the following publications. 1. SANS Edition 5.1: 2005, The classification of hazardous locations and the selection of apparatus for use in such locations. 2. WF Hickes and KJ Brown, Assessment of Explosion Probability for Intrinsically Safe Apparatus, Conference on Electrical Safety in Hazardous Environments, IEE, Department of Mining Engineering, University of Pretoria, Course in Gases and Explosions in the Mining Industry, AP Cook, The occurrence, Emission and Ignition of Combustible strata gases in Witwatersrand Gold Mines and Bushveld Platinum Mines, and Means of Ameliorating related Ignition and Explosion Hazards, SIMRAC Report, October G Goodlad, Her Majesty s Principal Inspector for Health and Safety, Health and Safety Executive (now retired), private communication. 6. SANS Edition 5.2 (draft), The classification of hazardous locations and the selection of apparatus for use in such locations.

EXPLOSIVE ATMOSPHERES - CLASSIFICATION OF HAZARDOUS AREAS (ZONING) AND SELECTION OF EQUIPMENT

EXPLOSIVE ATMOSPHERES - CLASSIFICATION OF HAZARDOUS AREAS (ZONING) AND SELECTION OF EQUIPMENT OVERVIEW ASSESSING THE RISK RELATIONSHIP BETWEEN FIRES AND EXPLOSIONS CLASSIFYING HAZARDOUS AREAS INTO ZONES