AU2013100964A4 - A Method of Testing for Reactive Ground for Blasting Purposes - Google Patents

Abstract

A method of testing rock samples for exothermic reactivity with an explosive substance and a method of categorisation of the rock sample according to the results to provide a worst case categorisation which is realistic using a test that avoids introduction of testing bias due to the nature of the test.

Description

I A METHOD OF TESTING ROCK SAMPLES FOR EXOTHERMIC REACTIVITY WITH AN EXPLOSIVE SUBSTANCE TECHNICAL FIELD [0001] The present invention relates to methods of testing for the purposes of the use of explosive substances and in particular a method of testing rock samples for exothermic reactivity with an explosive substance. BACKGROUND ART [0002] The Australian Explosives Industry and Safety Group Inc. (AEISG) is an incorporated association of Australian explosive manufacturers and suppliers which publishers a code of practice which is adopted by members for the benefit of their employees, their customers and the general community. The goal of AEISG is to improve the level of safety and security throughout the industry in the manufacture, transport, storage, handling and use of explosives and related materials throughout Australia. [0003] The AEISG Code of Practice provides a scientific and objective basis for predicting the potential unwanted explosive/rock reaction and to provide practical advice on preventive mechanisms available and is the current state of the art. The current version of the Code of Practice is Version 3, revised and current as at June 2012. [0004] Section 7 of the AEISG Code of Practice states that "Reactivity sampling and testing is a crucial component of reactive ground management. It must be noted that it is rarely possible to statistically take enough rock samples tofully characterise the geology and hence the reactivity of a site. Testing, and in particular screeningfor reactivity, needs to be targeted to analyse the rocks containing sulphides in order to identify the "worst case" reactive rock samples and hence to minimise the risk of incidents. Other indicators listed in Section 4 may be used to identify the most likely locations from where rock samples should be taken. [0005] The role of geotechnologists (eg site and consulting geologists) is essentialfor the identification of high potential samples. Where possible, the expertise of site geotechnologists shall be employed and geological knowledge (targeting sulphides) shall be used to implement safety margins and to allow for the limited numbers of representative samples. The sampling method requires selection of "worst-case" samples present on the site at that point in time. [0006] Samples should be taken so that their exact location is known. Samples picked from a wall or drill core samples are the most usefulfor screening purposes. Samples picked up from the floor may have fallenfrom a much higher point or been moved from a different location and hence are less useful when it comes to identifying risk zones. Drill cuttings can result in "diluted" rock samples and allow for some oxidation to occur before testing - hence the best samples are whole rocks from known locations. [0007] In order to obtain accurate results, testing shall occur just prior to explosive supply commencing using fresh samples (ie newly exposed rock rather than old core drill samples). This is to ensure that the effects of oxidation do not "hide "potentially reactive samples." [0008] The AEISG Code of Practice further provides that before site development commences a risk assessment of the site should be carried out based on historical data, and other indicators for the presence of reactive ground which are: * the presence of sulphides (normally greater than 1%); * the presence of black sulphide bearing sediments; * sulphides within mineralised rock; e the presence of white or yellow salts on rock. This is an indication that oxidation is taking place; e acidic conditions (generally resulting from oxidation) as indicated by the colour of run off water, usually yellow-red brown in colour; * significant corrosion of rock bolts, safety meshing and fixed equipment associated with the mining operation; * the spontaneous combustion of overburden or waste rock/ore either in dumps or in the pit, especially as it is exposed to the air; * the acrid smell of sulphur dioxide caused by the naturally occurring sulphide oxidation reaction; * elevated blast hole temperatures; * elevated ground temperatures. [0009] If no indicators are present then it may be assumed that the site is non-reactive. [0010] If the risk assessment indicates the likelihood of reactive ground, the ground is to be tested for reactivity as per the Isothermal Reactive Ground Test, which is provided in Appendix 1 of the AEISG Code of Practice provides. [0011] The minimum recommended number of samples selected and collected for testing according to the AEISG Code of Practice is 12. The most suitable number is difficult to quantify but they should be a well-chosen set of samples (ie obtained with assistance from geotechnologists), following a documented risk assessment procedure conducted by the customer. [0012] Current best practice is to take samples from each region of the site and from each strata within that region that contain sulphides. If the sulphide content is unknown, then samples must be taken. [0013] The present invention relates to reactivity screening which is used to identify whether ground samples are reactive. Section 8 of the AEISG Code of Practice provides two laboratory screening tests may be used in determining the reactivity of ground samples and in determining the suitability of explosive products to be used within the ground, with the test step is outlined below with the detailed test procedures given in Appendix 1 of the AEISG Code of Practice, the two tests being: 1. Isothermal Reactive Ground Test; and 2. Temperature Ramping Test. [0014] In the interpretation of results, the Code provides that: "An exotherm that qualifies a sample as 'reactive' in this test has the following characteristics: * It is at least 2'C above the background temperature in the temperature/time trace of that particular sample. * It shows both a rise from, and a return to the background temperature when the reaction is completed. (Note: the background temperature is best measured just before and after the exotherm to exclude fluctuations caused by experimental artefacts such as physical movement of the thermocouple, changes caused by air conditioning, etc.) * Reactions may be accompanied by visible signs, such as bubbling, and/or the generation of brown nitrogen oxides. If a reaction occurs during the period that the sample is still being brought up to the isothermal set point, an exotherm is harder to distinguish. Nevertheless, a reaction would be indicated by physical signs such as bubbling and/or brown NOx fumes, and/or a temperature rise that was faster than in other (non-reacting) samples being done at the same time. In such cases, it may be advisable to re-test at a lower set-point temperature, in order to delay the onset of the exotherm -r to a time when a constant background temperature has been established. Mixtures that show visible signs of reaction but indefinite (<2'C) thermal responses are classed as marginal. Depending on the cause it may be advisable to prepare fresh samples and re-test, or to perform sulphide analyses to check the amount present. Marginal results may be due to very low levels of sulphide, premature aerial oxidation of the sample or reactions other than sulphide oxidation (eg carbonates reacting with acid) etc." [0015] However, it is possible to get a 2'C exotherm from a source other than explosive ground reactivity - for example a mixture of ferrous sulfate, sand and acid generates heat but is not explosive nor would it normally lead to spontaneous combustion. [0016] Further, it is impossible to determine the difference between an exothermic reaction caused by reactive ground and another source just looking at the exotherm in a temperature time trace leading to false positives which are clearly undesirable. [0017] Still further, the exotherm height is not a good measure of how much energy is released in a reaction and an operator may not be able to differentiate a small reaction that peters out and will not lead to a sustainable explosive reaction that leads to a detonation/deflagration or spontaneous combustion leading to another false positive which are both obviously undesirable. [0018] The test as set out in the AEISG code of Practice (2012) produces an unacceptable level of false positives due to the weathering agent prescribed to be used and in fact is not a test for determining reactivity but is designed to make samples reactive that would not be reactive in nature. [0019] It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country. SUMMARY OF INVENTION [0020] The present invention is directed to a method of testing rock samples for exothermic reactivity with an explosive substance, which may at least partially overcome at least one of the abovementioned disadvantages or provide the consumer with a useful or commercial choice. [0021] With the foregoing in view, the present invention in one form, resides broadly in a method of testing rock samples for exothermic reactivity with an explosive substance including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm; and (f) Categorising the rock sample as non-reactive if any exotherm of less than 10 0 C is experienced in the run time, marginal if any exotherm of between 10 C to less than 15'C is experienced in the run time and reactive if any exotherm of greater than 15'C is experienced in the run time. [0022] In an alternative aspect, the present invention resides broadly in a method of testing rock samples for exothermic reactivity with an explosive substance including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm; and (f) Categorising the rock sample as non-reactive, marginal or reactive using a tiered categorisation system involving at least area under the exotherm as a level in the tiered categorisation system. [0023] In an alternative aspect, the present invention resides broadly in a method of testing rock samples for exothermic reactivity with an explosive substance for maximum sleep time testing including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm; (f) Categorising the rock sample as non-reactive, marginal or reactive using a tiered categorisation system involving at least area under the exotherm as a level in the tiered categorisation system; and (g) Selecting or rejecting an explosive substance based on whether the test mixture is categorised as non-reactive or reactive within a selected period representative of the sleep time of the explosive product in situ in a blasting situation. [0024] In the monitoring step above, a time-temperature trace is the preferred monitoring method. This will record data in relation to temperature in the samples over time and can normally be easily visually represented as a moving value of temperature over time. [0025] It is more preferred that the rock sample is categorised as non-reactive if any exotherm of less than 5'C is experienced in the run time, marginal if any exotherm of between 5'C to less than 10 C is experienced in the run time and reactive if any exotherm of greater than 10 C is experienced in the run time. [0026] It is most preferred that the rock sample as non-reactive if any exotherm of less than 3'C is experienced in the run time, marginal if any exotherm of between 3'C to less than 4'C is experienced in the run time and reactive if any exotherm of equal to or greater than 4'C is experienced in the run time. [0027] If in the categorisation outlined above, any test mixture is considered marginal, then further categorisation is preferably conducted. [0028] Preferably, the next level of categorisation of samples which were considered marginal under the height of the exotherm testing, is area under the exotherm. The area is basically an integration of the temperature versus time curve and is normally presented in Joules as this is the unit of energy. [0029] This approach is better than just height of exotherm as it looks at the amount of energy released to ascertain if sufficient energy is released to be a realistic reaction. For example, under the current AESIG Code, an exotherm is called reactive if it is greater than or equal to 2'C in height. Exotherms that are narrow or small time duration of, for example, an hour with small areas are classified the same as the same height exotherm that may last an extended period of time, say 18 hours. One reaction liberates a small amount of energy while the other considerably more. The amount of energy liberated is therefore a better measure of a reaction but it is more difficult to determine on a large number of samples. However, as the present invention only subjects samples which were considered marginal under the height of the exotherm testing to the area under the exotherm categorisation, the time taken is offset by the accuracy of the two tier categorisation. [0030] It is preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 3500J, marginal if the area under the exotherm is greater than or equal to 3500J but less than 6500J, and reactive if the area under the exotherm is greater than or equal to 6500J. [0031] It is more preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 2500J, marginal if the area under the exotherm is greater than or equal to 2500J and less than 3500J, and reactive if the area under the exotherm is greater than or equal to 3500J. [0032] It is most preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 2000J, marginal if the area under the exotherm is greater than 2000J and less than 5000J, and reactive if the area under the exotherm is greater than or equal to 5000J. [0033] If in the categorisation outlined above, any test mixture is still considered marginal, then further categorisation is preferably conducted. [0034] Preferably, the next level of categorisation of samples which were considered marginal under the height of the exotherm testing and marginal using the area under the exotherm testing, is sulphide analysis. As with the area under the exotherm testing, only samples that are marginal after both the earlier methods need to be analysed which means that the number tested will normally only be a small percentage. [0035] The technique used for the sulphide analysis should be low cost and accurate, preferably able to detect down to 0.0 1wt% of sulphide sulphur present in a sample. A particularly preferred technique is a gravimeteric analysis and instrumentation manufactured by Leco is preferred. [0036] It is preferred that a sample is categorised as non-reactive if there is less than 0. wt% of sulphide sulphur present, marginal is there is between 0. wt% to less than 0.2wto of sulphide sulphur present and reactive if there is greater than or equal to 0.2wt% of sulphide sulphur present. [0037] It is more preferred that a sample is categorised as non-reactive if there is less than 0.2wt% of sulphide sulphur present, marginal is there is between 0.35wt% to less than 0.35wt% of sulphide sulphur present and reactive if there is greater than or equal to 0.35wt/o of sulphide sulphur present. [0038] It is most preferred that a sample is categorised as non-reactive if there is less than 0.35wtof sulphide sulphur present, marginal is there is between 0.35wt% to less than 0.6 wt% of sulphide sulphur present and reactive if there is greater than or equal to 0.6wt% of sulphide sulphur present. [0039] In the highly unlikely event that all three of these categorisation techniques gives a result that the sample is marginal, then weight loss of the sample can be used although this is not a reliable technique because a portion of the sample can be ejected from the sample, masking the weight loss due to a reaction. [0040] If the extremely unlikely event that all four of these categorisation techniques gives a result that the sample is marginal, then the sample is deemed to be reactive. [0041] If an exothermic reaction is deemed to have occurred through application of the abovementioned tiered categorisation techniques, then the sample is considered to be reactive and appropriate measures must then be taken according to the appropriate regulations and the AEISG Code of Practice regarding the use of explosive material in reactive ground. [0042] In summary therefore, the preferred manner of categorising a sample based on the present invention is height of exotherm (Level 1), then any marginal results are subjected to area under the exotherm analysis (Level 2), any samples which are still marginal are subjected to sulphur analysis (Level 3) and any that are still considered marginal are subjected to weight loss of the sample (Level 4). If after all four levels, a sample is considered marginal, then according to the preferred embodiment of the present invention, the sample is deemed to be reactive. [0043] It is obvious that the order in which the categorisation can be conducted can be amended especially using area under the exotherm as Level 1 and height of the exotherm as the Level 2 etc and the invention covers these variations in order as well. [0044] Normally a predetermined amount of the powdered rock sample is combined with a weight of explosive grade, porous prilled explosive substance and a weathering agent. Preferably between 0.5wt% and 14wt% distilled water is used to form the weathering agent and is used in the test mixture. [0045] The weight of explosive grade porous prilled explosive substance used to form the test mixture will preferably be a predetermined weight. According to the AEISG Code of Practice, 18g of Ammonium nitrate technical grade pure is used, and a similar or the same amount may be used according to the present invention. [0046] A predetermined run time of 7 days or for a duration or 4 times the expected sleep time when testing known reactive samples is preferred. The minimum run time for an isothermal test should be 48 hours, even if the 'sleep time' in the field is less than this. [0047] Preferably between 0.5wt% and 14wt/o distilled water is used in the test mixture of the present invention. More preferably between lwt% and 10wt% distilled water is used. [0048] The prilled Ammonium Nitrate may undergo size reduction prior to mixture in order to increase reactive surface area which may contribute to the simulation of "worst case" conditions. In particular, the explosive grade porous prilled ammonium nitrate may be powdered (using a mortar and pestle or other suitable technique) but standard porous prilled ammonium nitrate can be used but is less preferred. [0049] Clearly the methodology of the present invention covers any test technique that has different quantities of rock to be tested (18g in the AEISG Code of Practice test) and porous prilled Ammonium Nitrate (18g od technical grade prill in the AEISG Code of Practice test) and alternative testing apparatus. [0050] The testing methodology outlined herein is for approximately 18g sample, 18g Ammonium nitrate and 4g weathering agent (3.5g of water). If the overall amount increases or decreases - that is, the total weight exceeds 40g or is less than 40g, then the amount of weathering agent will need to be adjusted. If the amount of sample to Ammonium nitrate percentage changes, then the amount of weathering agent (water) will also need to change. Normally, a pro-rata adjustment can be made, with examples as follows (sample weight/AN weight - water: * l8g/18g - current water in weathering agent is about 3.5wt% * 15g/21g - 3.5g * 12g/24g - 3.3g * 9g/27g - 3.1g * 6g/30g -2.9g * 3g/33g -2.7g [0051] It is anticipated by the inventor however that a simple pro-rata adjustment of the quantity of weathering agent used in the preparation of the test mixture may not always be the optimum calculation method. [0052] For normal temperature blasting applications at sites with either known or unknown reactivity, the background temperature to be used is preferably 55'C and the test is normally run for 7 days. The occurrence of an exothermic reaction at any time before that period (as assessed according to the categorisations outlined above) will normally result in termination of the test as a positive result for reactive ground. The test duration can be reduced down to the sleep time used in the blasting application if the sleep time is less than 7 days, but the test duration is preferably a minimum of 48 hours in length. Also, the test can be terminated before 7 days if the test is being conducted in order to identify the most reactive samples from a site or section of a site. [0053] Of course, the testing regime outlined above also makes testing at less than 55 0 C possible and produces more reliable results for testing at less than 55 0 C than the current AESIG Code test. [0054] Where elevated ground temperature is evident, the same test can be run but the test temperature is typically at least equal to the highest in-hole temperature, or at the maximum nominated temperature limit at the site. If the site changes its nominated maximum temperature then the isothermal testing should be repeated at this new temperature. [0055] Testing has shown that the use of distilled water and porous prilled Ammonium Nitrate according to the present invention instead of the methodology outlined above prescribed by the AEISG Code of Practice, produces substantially different results than the AEISG Code of Practice test or the "Code" test. [0056] A number of tests have been conducted by the inventor over a number of rock types from a number of mineral provinces using both the test of the present invention and the test provided according to the AEISG Code of Practice with considerable differences in the reactivity results produced from the same samples using the different tests. [0057] Independent testing has occurred to substantiate the finding that the AEISG Code of Practice test can indicates a sample to be reactive and yet no or insufficient sulphide sulphur (which are required for a reaction to occur) are present creating "false positive" results. [0058] The theory upon which the AEISG Code of Practice test is predicated is that for a sample to be reactive it must contain sulphide that is partially oxidised so there are sulphide sulphur, sulphates and ferrous and ferric ions present. If there are no or low concentrations of sulphide sulphur present then it is unlikely that the sample will be reactive. The AEISG Code of Practice test introduces a weathering agent but this weathering agent is not added to blast holes in practice (the chemistry of the weathering agent will be either present in the rock or absent) so the test itself introduces a bias. Water is introduced to a blasthole either via groundwater or water applied during the drilling process to supress dust so the use of water mimics real life. [0059] Additionally, the AESIG Code of Practice test uses Technical Grade Pure Ammonium Nitrate which is not put down blast holes and therefore also introduces a potential bias or at least a misrepresentative result. Explosive grade porous prilled Ammonium Nitrate is placed down blastholes in practice so the use of this material in the test technique of the present invention is replicating what occurs in real life. [0060] There is a need for the testing performed to be "worst case" but it is also better that the test be a realistic test. There is no value in having a worst case, unrealistic test that conventional wisdom indicates is the best test technique and is used universally by the industry. The proposed new test is also a "worst case" test but is based on the actual materials used or present in a real life situation, and also uses a graduated categorisation methodology for the results in order to ascertain whether a sample is reactive or is not reactive with marginal samples subjected to the next level of analysis. The preferred materials used are as follows: 1. Uses rock samples or chips - grinding occurs during drilling so this is worst case; 2. Uses powdered porous prilled ammonium nitrate -porous prilled ammonium nitrate is placed in the hole and as emulsion or watergel explosives have fine grain size - the use of powdered porous prilled Ammonium Nitrate is therefore worst case but can occur in a blasthole; 3. Intimately mixing the powdered sample and the powdered prill is worst case but can occur in the blasthole; 4. Intimately mixing this with water is also worst case but can occur in a blasthole; and 5. The use of multiple sub samples increases the potential for the most reactive sub sample of the most reactive rock type at the mine is also worse case. [0061] The test methodology of the present invention therefore represents a worst case but realistic test. [0062] A further embodiment of the present invention involves the number of samples collected and analysed from a site. When considering the number of samples to be tested from a site, the heuristic suggests that the more samples that are tested then the higher the probability of I /_ finding a reactive one if there is reactivity for the site and vice versa. [0063] The AESIG Code of Practice prescribes a minimum of 12 samples from a mine which is generally inadequate to be able to accurately delineate reactive ground at any mine. The AESIG Code of Practice does not indicate the number of sub-samples to be tested at any test temperature and nor does it acknowledge the significant variability between individual sub samples from a sample. The higher the test temperature is, again the more chance of finding reactivity and the less of a role sub-sample variability plays (sub samples are from the same sample) [0064] According to the present invention, a method for calculating the number of samples to be tested is provided as follows: 1. Testing at greater than 55'C with less than or equal to 30 samples from a site then 2 sub samples from each of the less than or equal to 30 samples is preferred with 1 sub-sample from each of the fewer than 30 samples being less preferred 2. Testing at greater than 55'C with greater than 30 samples from the site, then 1 sub sample from each of the greater than 30 samples is preferred with 2 sub-samples from each of the greater than 30 samples being less preferred and 1 sub sample least preferred 3. Testing at 55'C with less than or equal to 30 samples from a site, then at least 3 sub samples from each of the less than or equal to 30 samples is preferred with 2 sub-samples from each of the fewer than 30 samples being less preferred and 1 sub sample least preferred 4. Testing at 55'C with greater than 30 samples then at least 1 sub-sample from each of the greater than 30 or more samples is preferred and 2 sub-samples from each of the 30 or more samples being less preferred 5. At test temperatures less than 55'C then the use of between 2 and 8 sub samples but preferably 3 or 4 sub-samples as this enables risk mitigation associated with sub sample variability at lower temperatures. [0065] Calculation of the optimum number of sub-samples according to the above takes into account the interrelationship between the number of samples, test temperature and sub-sample variability. [0066] Whilst the methodology of the present invention is particularly well suited to use of ammonium nitrate which is the main uninhibited explosive substance used in blasting, the method can also be used to test using inhibited explosive products. In both cases, the sleep time testing can go well beyond the 30 days which is provided by the AESIG Code of Practice. In particular, the sleep time testing can extend to 56 days and beyond by the addition of suitable amounts of distilled water. [0067] The present invention is somewhat based on the creation of a test mixture by ombining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution. As indicated above, the weathering agent can be produced in a number of ways and distilled water can be used. Alternatively, a liquid which more accurately reflects the in situ use situation is preferred. [0068] According to a particularly preferred embodiment, each mine should provide an analysis of its mine water that preferably has the concentration of preferably both ferrous and ferric ions or at least total iron, along with pH and sulphate as part of the analysis. If the pH is greater than or equal to 8, the agent solution added to the test should preferably be 2g to 4g of distilled water or less preferably from 0.5g to 8g of the distilled water or the agent solution as detailed below. [0069] If the pH is less than 9, the agent solution should be made according to the ferrous and ferric ion concentrations in the mine water. [0070] If pH is greater than or equal to 9, the agent solution is distilled water [0071] If the individual concentrations are unavailable, the total iron concentration may be used. The agent solution in the test should be made up so that the concentration of each of the ferrous and ferric ions is I to 2000 times of the concentration in the mine water but preferably in the region of 5 to 1500 times and more preferably (100 to 1000) times . [0072] If only the total iron concentration is available, then the ratio of ferrous and ferric ions should be greater than 2 but less than or equal to 3 or preferably greater than 1 but less than or equal to 2 or more preferably or greater than 0.1 but less than or equal to 1 by mole fraction. [0073] After the agent solution has been prepared, the pH should be tested. If the pH of the agent solution is greater than the mine water pH - 1 as a minimum, it should be adjusted to reduce the pH to at least pH - 1 lower than the mine water but preferably in the range of pH- 1 to pH-2 and less preferably pH-2 to pH-4. This adjustment should be performed using a sulfuric acid.For Sleep time testing, the most reactive sample should be used to make a slurry of least preferably - less than 20wt% reactive sample or greater than 40wt% reacyive sample but less than 70wt% reacivec sample with the rest being distilled water or most preferably greater than or 1 -r equal to 20wt% to less than or equal to 40wt% reactive sample with the rest being distilled water and left until all sediment has fallen out of suspension. The resultant solution is then preferably used as the agent solution added to the sleep time test, preferably from 2g to 4g or less preferably from 0.5g to 8g. [0074] Whilst the abovementioned holds true, it is clear that the test mixture can be used for both maximum sleep time testing (which tends to be product related) and for particular reactivity testing in an ongoing manner (which tends to be for reactivity testing of a particular site). Therefore, any weathering solution can be used for maximum sleep time testing with the preferred weathering solutions being those that are used for reactivity screening of ground samples. [0075] Reactivity screening of ground samples to determine the suitability of explosive products to be used within the ground whereas maximum sleep time testing is generally undertaken once a sample has been classified as either reactive ground or not, in order to ensure that any blasting product used also has an appropriate sleep time in that particular ground. Generally, sleep time testing involves testing reactive ground with inhibited product using the "Isothermal Reactive Ground Test" for a period equivalent to four times the required product sleep time, up to a maximum test period of one month. Therefore, the reactivity testing regime outlined is typically used to determine firstly whether a ground sample is reactive or not using the testing and classification methodology outlined above but a similar testing methodology can also be used with maximum sleep time testing albeit without the categorisation methodology. [0076] Normally, the step of selecting or rejecting an explosive substance based on whether the test mixture is categorised as non-reactive or reactive within a selected period representative of the sleep time of the explosive product in situ in a blasting situation will use a selected period equivalent to four times the required product sleep time, up to a maximum test period of one month. Should the product be required to sleep for greater than one week (ie laboratory test for one month) then the test period may be increased accordingly. [0077] For existing reactive ground sites, the maximum recommended sleep time in reactive ground is one week (ie laboratory test for one month). Beyond this period, a risk assessment involving the explosives provider and the customer should be carried out. There may be a requirement to advise the outcome of the risk assessment to the appropriate regulator. [0078] Any of the features described herein can be combined in any combination with any one or more of the other features described herein within the scope of the invention.

[0079] The reference to any prior art in this specification is not, and should not be taken as an acknowledgement or any form of suggestion that the prior art forms part of the common general knowledge. DESCRIPTION OF EMBODIMENTS [0080] According to a particularly preferred embodiment of the present invention, a method of testing and categorising rock samples for exothermic reactivity with an explosive substance is provided. [0081] With the foregoing in view, the present invention in one form, resides broadly in a method of testing rock samples for exothermic reactivity with an explosive substance including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; and (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm. [0082] This test is used to assess rock samples for exothermic reactivity with ammonium nitrate. The test conditions are intentionally designed to mimic a 'worst likely case' of rock particle size, temperature, exposure time and the presence of natural by-products from oxidative weathering of the rock but in a realistic manner and not to introduce factors into the test environment that will increase the likelihood of a sample being found to be reactive. This is partly to counter the fact that it is never known whether the most reactive ground on the site has been sampled and tested. [0083] The test mixture of the preferred embodiment includes finely powdered rock, finely powdered explosive ammonium nitrate and a weathering agent which is based on distilled water rather than a solution of ferrous and ferric sulphates which is prescribed for use according to the current AEISG Code of Practice test. [0084] The term 'isothermal' is used to indicate that rock is being tested in an environment with a constant and pre-determined background temperature. Exothermic events are signified by an experimentally significant increase in the temperature of the sample mixture beyond that of the background.

I %J [0085] For normal temperature blasting applications at sites with either known or unknown reactivity, the background temperature to be used is preferably 55 0 C and the test is normally run for 7 days. The current testing regime also allows for the samples to be tested and their reactivity categorised, at temperatures below 55 0 C. [0086] The occurrence of an exothermic reaction at any time before that period will normally result in termination of the test as a positive result for reactive ground. The test duration can be reduced down to the sleep time used in the blasting application if the sleep time is less than 7 days, but the test duration is preferably a minimum of 48 hours in length. Also, the test can be terminated before 7 days if the test is being conducted in order to identify the most reactive samples from a site or section of a site. [0087] Where elevated ground temperature is evident, the same test can be run but the test temperature is typically at least equal to the highest in-hole temperature, or at the maximum nominated temperature limit at the site. If the site changes its nominated maximum temperature then the isothermal testing should be repeated at this new temperature. Equipment Required. * Aluminium blocks - these are metal blocks with holes drilled to accommodate the glass tubes and distribute heat to the tubes evenly. In a preferred embodiment, the holes are 75mm deep with a 15mm radius rounded bottom, 30mm diameter. If there is more than 1 hole per block, the holes should be separated by a minimum of 10mm between the walls of the holes. The blocks may also feature a shallow depression on the top to capture any overflow from vigorous reactions. Some variations in dimensions may occur, however in all cases the tubes shall be sized to ensure that samples sit below the top level of the heater block. * Ammonium nitrate - explosive grade porous prilled and preferably powdered. * Data logger - any data logger able to log temperatures at a rate of at least one data point per 30 seconds. It should be accompanied by a computer system to process the data. * Dry block heater - an electrically heated tray able to heat the aluminium blocks uniformly at a heating rate of 0.6 0 C min-i or slower, and keep them at a pre-set temperature in the range 10 - 200'C and to maintain that temperature to +1 0 C. * Jaw crusher - Any crusher allowing fist-sized rocks to be broken into pieces of suitable size for the pulverising mill.

I / * Pan balance - any electronic pan balance may be used provided it can weigh at least 300g to an accuracy of 0.2g or better. * Protective clothing and equipment - safety glasses, rubber gloves, ear muffs or plugs, disposable dust mask and face visor. * Pulverising mill - A mill with a capacity of 600 - 1000cc and capable of milling samples from the jaw crusher down to a sieve size of -250 micron within 60 seconds. A commercially available mill that meets these requirements is the Labtechnics LM1-P. * Reaction vessels - glass centrifuge tubes of approximately 50ml capacity, 26mm ID neck and wall thickness of 2mm. * Rubber stoppers - the stoppers fit into the glass tube have two 2mm diameter holes, one for a thermocouple probe, the other for pressure release. * Sample bags - thermally sealable plastic vacuum bags able to hold at least 200g of powdered rock. * Sieve - a stainless steel sieve with aperture size of 250 micron. The sieve should be inspected prior to use to ensure the mesh is not damaged. * Vacuum bag sealer - commercially available household vacuum bag sealers are suitable for this test. Test method Step Key Points Safety/Quality Rock Crushing Check that the rock sample Wear safety glasses, can fit into the jaw hearing protection and full crusher. If required, use a face visor. hammer to break the whole rock into smaller pieces first. Ensure the collection tray for the crushed rock is clean, dry and in place. Feed rock into the crusher. The sample should be Activate the crusher and milled and bagged within allow enough time for all 15 minutes of crushing to sample to be consumed. guard against premature Collect a minimum of aerial oxidation of 200g of crushed rock. sulphides if present in the sample. Rock Milling Ensure the internal surfaces of the mill are 10 clean and dry. Load the mill with 200g of The short milling time will freshly crushed rock and help to minimise aerial mill for not more than 1 oxidation of sulphides if minute. present in the sample. Mill for a minimum time to achieve 250um. This is usually 30s or less. Sieving Remove the powdered rock from the mill. Sieve it using a 250 micron sieve and collect the material that passes through in a heat sealable plastic vacuum bag. Sieving need only be performed once for a set of samples to ensure that the sample has been milled to 250um. This sieved sample must be discarded as it will have been exposed to oxidation. Milled sample storage Immediately vacuum seal The integrity of the the bags for storage. vacuum and seal should be The bagged samples verified by visual should also be frozen for inspection of the bags. further protection against aerial oxidation. Immediately vacuum seal the sample after milling. Retesting Samples may be required If sample is inert, then a to be tested to confirm a fresh sample must be result or for sleep time milled. testing. The sample should be stored frozen and vacuum sealed. Samples should be retested within 60 days of milling if stored correctly. If tested after this time for sleep time, the test must include a reactivity test with AN to confirm that the sample is still active. Preparation of reactivity In a 100ml glass beaker, Re-vacuum seal the bag test mixture combine 18g of milled immediately after taking rock sample, 18g of the sample. ammonium nitrate and the I j/ between 0.5wt% to 14wt% of distilled water. Mix the contents with a The test steps following spatula and immediately this must be done without transfer the mixture to the delay since reaction may glass reaction vessel. start at any time. Mixing can occur in the vessel if it is large enough. Insert a 2-holed rubber stopper into the glass vessel. Mixtures should be This will reduced the rock prepared in series rather exposure to aerial than in parallel eg oxidation completed weighing and mixing one sample before starting the next sample. Temperature probe set-up Place the reaction vessel The Dry Block Heater is to containing the test mixture be set up in a well into an aluminium block ventilated fume hood. on the Dry Block Heater. The aluminium block shall be at ambient temperature at this stage of the test. Insert a thermocouple through the rubber stopper such that the tip of the thermocouple sits in the test mixture approximately 1 cm from the bottom of the tube. Data logger set up If milling occurs on site Refer to the operating the data logger should be instructions of the data set up prior to milling. logger. Configure the data logger to log one data point every 30 seconds. Testing the rock sample Set the temperature of the For regular blasting Dry Block Heater to the applications, use 55 0 C. For appropriate background testing related to ground at temperature. elevated temperatures, the test should be run at a temperature equal to or greater than the highest temperature actually measured at the site concerned Allow the sample to reach The minimum run time for the pre-set background the isothermal test is 48 temperature and continue hours, even if the 'sleep the test by monitoring the time' in the field is less sample temperature for a than this. run time of 7 days or for a The test duration can be duration of 4 times the reduced to the sleep time expected sleep time when used in the blasting testing known reactive application if less than 7 samples. days. Also, the test can be The test is terminated if an terminated before 7 days if exothermic reaction occurs the test is being conducted or the run time is reached in order to identify the without reactions most reactive samples occurring. from a mine [0088] According to the preferred embodiment of the present invention, the optimum number of samples to be tested is provided as follows: 1. Testing at greater than 55'C with less than or equal to 30 samples from a site then 2 sub-samples from each of the less than or equal to 30 samples is preferred with 1 sub sample from each of the fewer than 30 samples being less preferred 2. Testing at greater than 55'C with greater than 30 samples from the site, then 1 sub sample from each of the greater than 30 samples is preferred with 2 sub-samples from each of the greater than 30 samples being less preferred 3. Testing at 55'C with less than or equal to 30 samples from a site, then at least 3 sub samples from each of the less than or equal to 30 samples is preferred with 2 sub samples from each of the fewer than 30 samples being less preferred 4. Testing at 55'C with greater than 30 samples then at least 1 sub-sample from each of the greater than 30 or more samples is preferred and 2 sub-samples from each of the 30 or more samples being less preferred 5. At test temperatures less than 55'C then the use of between 2 and 8 sub samples but preferably 3 or 4 sub-samples as this enables risk mitigation associated with sub sample variability at lower temperatures. [0089] Calculation of the optimum number of sub-samples according to the above takes into account the interrelationship between the number of samples, test temperature and sub-sample variability. Calibration of Equipment [0090] Heater blocks can give an overrun when reaching the set test temperature. This can be mistakenly interpreted as a reaction or a marginal sample. Reducing the heating rate of the /I I dry-block heater to 0.6 0 C/minute or less usually eliminates heater overrun. However, all blocks should be calibrated to establish if overrun does occur for each position in the heater block. The calibration trace can then be used as a guide to interpret the result of the samples. As an alternative sample blanks consisting of sand (rather than a rock sample) and porous prilled ammonium nitrate and weathering agent may be preferred to the above calibration curves so there is a reference that is in all tests (1 sample blank per 3 to 5 samples being tested). [0091] To calibrate, substitute sand for the rock sample and mix with the ammonium nitrate and weathering by-products and run the test. The resulting time-temperature trace will be used as the calibration curve for that position in the heater block. [0092] Calibration curves must be stored indefinitely. Calibration curves are generally valid for 2 years, unless the testing facility has been instructed to complete calibrations more frequently as a result of either an internal or external audit of the test equipment. [0093] According to a particularly preferred embodiment, each mine should provide an analysis of its mine water that preferably has the concentration of preferably both ferrous ions and ferric ions or at least total iron, along with pH as part of the analysis. According to the preferred embodiment, if the pH of the mine water sample is greater than or equal to 8, the weathering agent solution added to the test should preferably be 2g to 4g of distilled water or less preferably from 0.5g to 8g of the distilled water or the agent solution as detailed below. [0094] If the pH of the mine water sample is less than 9, the weathering agent solution should be made according to the ferrous and ferric ion concentrations in the mine water. [0095] If the individual concentrations are unavailable, the total iron concentration may be used. The agent solution in the test should be made up so that the concentration of each of the ferrous and ferric ions is I to 2000 times of the concentration in the mine water but preferably in the region of 5 to 1500 times and more preferably (100 to 1000) times . [0096] If only the total iron concentration is available, then the ratio of ferrous and ferric ions should be greater than 2 but less than or equal to 3 or preferably or greater than 1 but less than or equal to 2 more preferably or greater than 0.1 but less than or equal to 1 by mole fraction most preferably. [0097] After the agent solution has been prepared, the pH should be tested. If the pH of the agent solution is greater than the mine water pH - 1 as a minimum, it should be adjusted to reduce the pH to at least pH - 1 lower than the mine water but preferably in the range of pH- 1 to pH-2 and less preferably pH-2 to pH-4. This adjustment should be performed using a sulfuric acid. [0098] For Sleep time testing, the most reactive sample should be used to make a slurry of least preferably - less than 20wt% reactive sample or greater than 40wt% reacyive sample but less than 70wt/o reacivec sample with the rest being distilled water or most preferably greater than or equal to 20wt% to less than or equal to 40wt/o reactive sample with the rest being distilled water and left until all sediment has fallen out of suspension. The resultant solution is then preferably used as the agent solution added to the sleep time test, preferably from 2g to 4g or less preferably from 0.5g to 8g. Interpretation of Results [0099] The sample will normally be categorised as non-reactive or reactive in order that the blasting operators or mine planners can design an appropriate blasting programme. The categorisation of the sample will preferably be according to a tiered categorisation system in which the sample is firstly categorised as non-reactive, reactive or marginal according to exotherm height, then if marginal, categorised as non-reactive, reactive or marginal according to area under the exotherm, and if marginal according to that test, categorised as non-reactive, reactive or marginal according to sulphide analysis. If after the three levels of categorisation, the sample is still categorised as marginal, the sample is categorised according to weight loss of the sample. If after all four levels, a sample is considered marginal, then according to the preferred embodiment of the present invention, the sample is deemed to be reactive. [00100] Additionally, there are preferred ranges within each categorisation test in which the samples will be categorised as non-reactive, reactive or marginal. [00101] In the height of the exotherm category, the rock sample will preferably be categorised as non-reactive if any exotherm of less than 10 0 C is experienced in the run time, marginal if any exotherm of between 10 C to less than 15'C is experienced in the run time and reactive if any exotherm of greater than 15'C is experienced in the run time. [00102] It is more preferred that the rock sample is categorised as non-reactive if any exotherm of less than 5'C is experienced in the run time, marginal if any exotherm of between 5'C to less than 10 C is experienced in the run time and reactive if any exotherm of greater than 10 C is experienced in the run time. [00103] It is most preferred that the rock sample as non-reactive if any exotherm of less than 3'C is experienced in the run time, marginal if any exotherm of between 3'C to less than 4'C is experienced in the run time and reactive if any exotherm of equal to or greater than 4'C is experienced in the run time. [00104] If in the categorisation outlined above, any test mixture is considered marginal, then further categorisation is preferably conducted. [00105] Preferably, the next level of categorisation of samples which were considered marginal under the height of the exotherm testing, is area under the exotherm. The area is basically an integration of the temperature versus time curve and is normally presented in Joules as this is the unit of energy. [00106] This approach is better than just height of exotherm as it looks at the amount of energy released to ascertain if sufficient energy is released to be a realistic reaction. For example, under the current AESIG Code, an exotherm is called reactive if it is greater than or equal to 2'C in height. Exotherms that are narrow or small time duration of, for example, an hour with small areas are classified the same as the same height exotherm that may last an extended period of time, say 18 hours. One reaction liberates a small amount of energy while the other considerably more. The amount of energy liberated is therefore a better measure of a reaction but it is more difficult to determine on a large number of samples. However, as the present invention only subjects samples which were considered marginal under the height of the exotherm testing to the area under the exotherm categorisation, the time taken is offset by the accuracy of the two tier categorisation. [00107] It is preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 3500J, marginal if the area under the exotherm is greater than or equal to 3500J but less than 6500J, and reactive if the area under the exotherm is greater than or equal to 6500J. [00108] It is more preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 2500J, marginal if the area under the exotherm is greater than or equal to 2500J and less than 3500J, and reactive if the area under the exotherm is greater than or equal to 3500J. [00109] It is more preferred that a sample is categorised as non-reactive if the area under the exotherm is less than 2000J, marginal if the area under the exotherm is greater than 2000J and less than 5000J, and reactive if the area under the exotherm is greater than or equal to 5000J.

[00110] If in the categorisation outlined above, any test mixture is still considered marginal, then further categorisation is preferably conducted. [00111] Preferably, the next level of categorisation of samples which were considered marginal under the height of the exotherm testing and marginal using the area under the exotherm testing, is sulphide analysis. As with the area under the exotherm testing, only samples that are marginal after both the earlier methods need to be analysed which means that the number tested will normally only be a small percentage. [00112] The technique used for the sulphide analysis should be low cost and accurate, preferably able to detect down to 0.01wt% of sulphide sulphur is present in a sample. A particularly preferred technique is a gravimeteric analysis and instrumentation manufactured by Leco is preferred. [00113] It is preferred that a sample is categorised as non-reactive if there is less than 0. wt% of sulphide sulphur present, marginal is there is between 0. wt% to less than 0.2wto of sulphide sulphur present and reactive if there is greater than or equal to 0.2wt% of sulphidesulphur present. [00114] It is more preferred that a sample is categorised as non-reactive if there is less than 0.2wt% of sulphide sulphur present, marginal is there is between 0.35wt% to less than 0.35wt% of sulphide sulphur present and reactive if there is greater than or equal to 0.35wto of sulphide sulphur present. [00115] It is most preferred that a sample is categorised as non-reactive if there is less than 0.35wtof sulphide sulphur present, marginal is there is between 0.35wt% to less than 0.6 wt% of sulphide sulphur present and reactive if there is greater than or equal to 0.6wt% of sulphide sulphur present. [00116] In the highly unlikely event that all three of these categorisation techniques gives a result that the sample is marginal, then weight loss of the sample can be used although this is not a reliable technique because a portion of the sample can be ejected from the sample, masking the weight loss due to a reaction. [00117] If the extremely unlikely event that all four of these categorisation techniques gives a result that the sample is marginal, then the sample is deemed to be reactive. [00118] If an exothermic reaction is deemed to have occurred through application of the abovementioned tiered categorisation techniques, then the sample is considered to be reactive and appropriate measures must then be taken according to the appropriate regulations regarding the use of explosive material in reactive ground. [00119] In the present specification and claims (if any), the word 'comprising' and its derivatives including 'comprises' and 'comprise' include each of the stated integers but does not exclude the inclusion of one or more further integers. [00120] Reference throughout this specification to 'one embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations. [00121] In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.

Claims (5)

1. A method of testing rock samples for exothermic reactivity with an explosive substance including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm; and (f) Categorising the rock sample as non-reactive if any exotherm of less than 10'C is experienced in the run time, marginal if any exotherm of between 10 C to less than 15'C is experienced in the run time and reactive if any exotherm of greater than 15'C is experienced in the run time.

2. A method of testing rock samples for exothermic reactivity with an explosive substance including the steps of: (a) Size reduction of a rock sample to be tested to produce a powdered rock sample; (b) Combining a predetermined amount of the powdered rock sample and a weight of explosive substance and a weathering agent solution to form a test mixture; (c) Mixing the test mixture; (d) Adjusting the temperature of the test mixture to an appropriate testing temperature; (e) Monitoring the test mixture temperature for a predetermined run time for any exotherm; and (f) Categorising the rock sample as non-reactive or reactive using a tiered categorisation system involving at least area under the exotherm as a level in the tiered categorisation system.

3. A method of testing rock samples for exothermic reactivity with an explosive substance as claimed in either claim 1 or claim 2 further including a step of selecting or rejecting an explosive substance for use based on whether the test mixture is categorised as non-reactive or reactive within a selected period representative of the sleep time of the explosive product in situ in a blasting situation.

4. A method of testing rock samples for exothermic reactivity with an explosive substance as claimed in any one of the preceding claims wherein the step of categorising the rock sample LI includes categorisation according to height of exotherm, area under the exotherm analysis, sulphur analysis and weight loss of the sample, in any order with samples considered marginal subjected to one or more other categorisations and if marginal after all, then deeming the sample as reactive.

5. A method of testing rock samples for exothermic reactivity with an explosive substance as claimed in any one of the preceding claims wherein the appropriate testing temperature is either above or below 55'C.