The main sources of dust generation in the deep mining industry are the two main operating processes, namely mineral winning and transport. Dust can also arise, or be swirled up, during the various secondary processes, such as support setting, stowing and the provision of ventilation. However, in such cases the quantities involved are generally lower and the make of dust can sometimes be easily minimised, or avoided altogether, by making appropriate choices, such as opting for hydraulic stowing or pumped packing rather than gravity stowing.
At the winning faces dust is generated when rock is cut and extracted. In the case of mechanised winning in the potash industry dust is produced by the cutting tool striking the mineral face, while in the salt industry dust is primarily caused by the activities of boom-type or full-face heading machines with their round-shank cutter picks, generally referred-to as “cutting winning”. Dust is also released and produced as a result of:
- the “cut” product falling on to the loading platform and/or floor;
- the product being loaded by gathering arms, cutting discs and/or chain scraper conveyors;
- the product (possibly) being delivered to a discharge conveyor;
- or being transferred directly to a conveying system; and
- its subsequent transport to the surface.
As the above list shows, the number of potential dust emission points is quite significant and in the aforementioned cases it is simply not possible to prevent the dust being generated at source. However, suitable measures can be put in place to reduce the dust make and manage the grain size.
The key question that must be addressed is: what kind of impact will the dust have on the workforce? Toxic particles must always be kept well away from people or at least must be maintained below certain defined limit values. Salt dust is not toxic in this particular sense and while discussions have been under way for many years as to the effect of salt dust on human health there is no need to examine this subject here in any detail.
Dust removal in conjunction with different winning methods
Any dust that is produced can be removed at the point of origin or extracted at some later point, transported to sedimentation areas, conveyed to the surface or filtered out. Containment by encapsulation or encapsulation with extraction and/or filtration also reduces the dust content of the mine air. However, all these processes are relatively costly. Filtration systems for large amounts of material can be expensive and bulky, while encapsulation creates visual obstructions and may be very expensive to repair. Furthermore, removing dust via pipelines takes up space, consumes energy for the transport operation (fan units) and requires specially designed equipment, e. g., to prevent the dust settling in transit.
Another proven dust removal method is water spraying. In this case the dust generated can be captured directly at the point of origin and then removed as required. If the material remains sufficiently wet as it passes along the conveyor route any further dust escape is effectively prevented, or at least very much reduced, downstream of the winning process. As many mines already have a supply of water “on tap” this dust suppression solution is generally not only the simplest to implement but also the most cost effective to deploy.
With mechanised winning dust generation is a constant phenomenon due to the contact action of the cutting tools. In the salt mining industry this dust tends to be regarded as relatively problem-free, subject to the aforementioned discussion about its effect on human health. This has been confirmed on the basis of operations with the V3000/3001 at Sondershausen during the early years of mechanised mining in Germany and the introduction of the URAL20KS at Bernburg up to the turn of the last century, as well as by the use of this type of equipment – which continues to this day – in the countries of the former Soviet Union and the ongoing deployment of other types of full-face cutting machines in Canada and in other parts of the world. In these cited cases dust generation has admittedly created other problems, especially poor visibility, but also the potential issue of caking and the seizing of bearings, e. g., in connection with round-shank picks. Having a clear line of sight to the roadhead is very important when it comes to directional steering and the need to stay in the salt deposit, especially as far as boom-type heading machines are concerned.
The relatively low strength of salt rock compared to ore deposits in magmatic and metamorphic rocks means that winning with boom-type and full-face cutting machines can be commercially advantageous, as long as the geology of the deposits is favourable. Severe undulation, highly alternating layer thicknesses, anhydrite outliers, gas flows (CO2) and even a subdeposit or overlay of weaker salt rock, such as carnallitite, may mean that mechanised winning either cannot be employed or will be very much restricted in scope.
Practical example: dust extraction for heading machines and apparent results
A southern European rock salt mine has chosen to use boom-type heading machines on the basis of the benefits offered by mechanised winning technology. With mining operations being conducted at a depth of about 800 m the ambient rock temperature in combination with the output from the cutting machines and the installed power rating created a situation that required cooling systems for the motors and machine assemblies. The cutting machines were therefore fitted with water cooling equipment. The water in question is brought in from the surface and stored temporarily in open containers. It is then delivered to the machine cooling system via a system of pipes that run through the intake development roadways. This process causes the water to become heated to a temperature of as much as 60 °C. The problem posed by workplace dust make can now be solved at the same time by using this heated water for dust suppression measures, the water being ejected through spray nozzles sited close to the cutter head. This not only helps to reduce the airborne dust levels but also serves to cool the round-shank cutting picks. This kind of dust suppression is effective and very efficient. Moreover, the machine operator is able to maintain a clear line of sight to the cutting face. One beneficial side-effect is that the material being extracted is also wet and there is no further dust nuisance created when it is being transferred to the slide-box car for transport out of the mine. This particular dust control measure is therefore highly successful when it comes to the primary objective of maintaining good operator visibility. It is also a very efficient solution as it makes use of the cooling water that would be needed in any case for the machinery.
However this procedure was to result in complaints from the underground workforce. These were not directed at the dust suppression system itself but rather at the temperature levels, which were deemed too high. The company management responded and considered the possibility of increasing the airflow volume, this perhaps in combination with the construction of a new shaft for accessing future working areas or, if need be, the introduction of air cooling. The engineering firm ERCOSPLAN Ingenieurbüro für Geotechnik und Bergbau, Erfurt/Germany, was therefore commissioned, along with Freiberg University of Mining and Technology, Freiberg/Germany, to calculate the volume of airflow that would be needed to achieve an acceptable reduction in the workplace temperatures. The survey team planned to use a mine ventilation program to determine whether, and how, a sufficient flow of air could be delivered to the working faces.
Assessing the mine ventilation problems
From the team’s first underground visit, and discussions with the workforce, it soon became clear that there was essentially “only” one problem that needed resolving – and that was not the temperature per se but rather the high air humidity level. This was caused by the use of the hot cooling water for dust suppression purposes. However the mine management was of the view that it was not possible to operate without a machine cooling system and that water was needed for the dust suppression work as the dust could not be controlled in a dry state. From this the mine management drew the conclusion that the problem could be solved by increasing the airflow volume with cooler air delivered from the surface. This was a commonly held – and physically erroneous – assumption, though the management needed this fact to be demonstrated and proven.
The starting point for the investigations was to take current-state recordings of the original values and to define the relationship between heat content (which in some ways has an impact on physical comfort), temperature and air humidity. While we are basically aware of the fact that we feel uneasy and less comfortable in a muggy atmosphere where the air is warm and moist the major role that is played by atmospheric humidity, expressed as water content x kg water vapour per kg of dry air, and by relative humidity, which is expressed as the ratio of vapour pressure (water vapour) to saturation vapour pressure, is often not properly understood. To this effect measurements were first taken of the temperatures and relative humidity levels at a number of points in and around the mine. These included: the mine surface, the shaft bottom landing, some 2,200 m along the air intake roadway, near to the active working faces some 5,600 m from the downcast shaft and in the general return-air zone. These measurements were taken using LogTag Haxo-8 temperature and humidity loggers supplied by cik Solutions, which are specifically designed for the long-term logging of temperature and relative humidity levels. These Logtag devices, which are similar in size to and slightly thicker than a bank card, can be programmed to store up to 8,000 measurement values over a period of weeks or months. They can then simply be read off at any time by inserting the device into an on-site reader. The measurement interval selected in this particular case was 5 minutes. The measurement readings indicated that the temperature in the intake roadway (blue curve in Figure 1) increased as the measurement points got closer to the working face, then rose again markedly (light red curve in Figure 1) when the machine was cutting and after that hardly receded at all on the way to the upcast shaft (red curve in Figure 1).
This was about as much as could be established in this particular case. What was not known, however, was the increase in the water vapour content of the air. The relative humidity is mainly influenced by the temperature, as warmer air can potentially absorb more vapour. If the moisture content remains the same any rise in temperature will cause the relative humidity to drop. If the temperature rises and the relative humidity remains constant, or even increases, then water must undoubtedly have been absorbed by the air in the form of water vapour. However, if the relative humidity falls as the temperature rises it is generally difficult to assess whether or not moisture has actually been absorbed, as without additional means it is hard to estimate by what exact amount the relative humidity level must have fallen (Figure 2).
However, by using simple formulae (and measuring the air pressure) or making estimations using an h-x graph it is possible to establish the water vapour content, in other words the absolute content of the water in the form of vapour in the air. And it was shown that this particular quantity increased en route to the active working faces (Figure 3).
This would in fact be impossible in any body of salt deposits, which as is well known can only exist when no water is present. Water must therefore have been absorbed from the mine air in one way or another. As it turned out, there were a number of possible sources for this. For one thing there was water being stored temporarily in open containers placed along the mine roadways. With ambient air temperatures already fairly high even before cutting began at the winning faces these containers were certainly a source of water evaporation. Then there were the water spray jets deployed at the working faces, which were responsible for the largest rise in humidity levels. However, the moisture content of the air was already increasing well before the winning areas regardless of the siting of the water storage containers. The reason for this was quite straightforward: it was due to the wet material being transported along the intake entries. There were also wet points to be found along the development roadway. This situation was clearly caused by the location of the working faces as these tended to be sited at a greater elevation due to the slope of the deposits. After exiting the spray jets the water would run along the floor and then follow the gradient towards the intake roadway. Although this process was for the most part not visibly evident it was possible to confirm by measurement that the humidity level was increasing.
Expressed as the heat content of the air this means that for an intake-air vapour content of about 7 g/kg, a measured temperature at the working faces of 46 °C and a measured relative humidity of 25 to 32 % the water vapour content will be 15 to 18 g of water vapour per kg of dry air. This in turn equates to an enthalpy – i.e. the heat content of the air – of 85 to 95 kJ/kg moist air (all figures rounded to allow for fluctuations under actual working conditions). In such an environment there is limited scope for sweat to evaporate and the body will be unable to cool down sufficiently as a result.
This latter aspect was also borne out by findings during the in-situ measurement campaign, during the course of which various cooling solutions were trialled in the form of cooling pads that were designed to be used as personal equipment in the miner’s helmet and neck area. During the measurement trials it was found that after 5 hours of working in a warm climate the cooling pads in the neck area were completely dry. Those fitted to the helmet remained moist for a longer period of time, though in subjective terms there was no longer any noticeable cooling effect.
After an evaluation had been carried out of the measurement findings (body temperature, temperature and humidity levels inside the helmet) the conclusion that was drawn in this case was that while there was a discernible trend, there was no clearly identifiable relationship between the use of cooling pads and the experience and/or perception of miners working in hot climatic conditions.
A more expedient approach is to avoid water atomisation altogether. In this case the water vapour content of the air will remain unchanged. For an assumed even temperature of 46 °C at an intake-air water vapour content of 7 g/kg and without any absorption of additional water vapour this would result in a relative air humidity of around 13 % and a heat content of about 65 kJ/kg moist air. Under these conditions sweat can evaporate quickly and can draw heat from the human body. As a result, the body is cooled much more effectively and the person’s sense of well-being is improved. The lower the relative humidity the more water/sweat can evaporate and the body is cooled down.
Reduced water consumption at the winning faces, the fitting of covers to the storage containers and re-routing material conveying from the intake entries to the return air roads could improve the mine-climate conditions in the active working areas.
Of course it is not so easy to reduce the amount of water used for the spray jets and even less straightforward to assess whether and when such a measure is sufficient to ensure that no water or brine is able to flow in the direction of the intake air roadway. Arranging for a complete change-over in the transport circuit would also present quite a challenge.
A second and quite decisive factor as far as the project was concerned could also be established by means of the long-term measurements. During the observation period temperatures of between 13.3 and 32.3 °C were recorded above ground. However at the bottom landing, i. e. with the air only having been in transit down the shaft, the margin of temperature fluctuation was noticeably reduced and after the 2200 m point the measured temperature levels were practically constant (Figure 1).
This led to two conclusions:
- The envisioned solution of cooling the intake air would have had no impact, as even when the outside temperatures were low the temperature of the mine air would have reached a constant level after just 2200 m of roadway. This in fact would roughly correspond to the ambient rock temperature, no matter how warm or cold the air was flowing in from the surface.
- If the volume of airflow is increased the point at which a constant air temperature is reached tends to move further in towards the active winning faces. However, as a constant temperature level is established at no more than 2200 m from the shaft, and the distance from here to the workings is another 1.5 times as much, it would never be possible to use this strategy to reduce the temperatures in and around the working areas.
These findings have also been established by measurements taken elsewhere, e. g., in the Freiberg mines (1960, 1997) and in the course of the project “Air conditioning at Freiberg hospital” (1999), and were also confirmed by experience acquired at the Zielitz potash plant.
The two conclusions that have been drawn in regards to the latter aspect therefore show that the actual task at hand – namely to apply an increase in airflow volume in order to reduce the temperatures at the workplaces – does not constitute a workable approach. The potential alternatives are:
- a) in-situ cooling, with all its associated technical challenges, or
- b) reducing the air humidity.
While the first solution is feasible as far as individual extraction chambers are concerned, the heat emission factor from such local systems would have to be carefully planned so as not to create even more negative effects.
Conveying chilled air through ducting and right up to the working faces would prevent any contact between the air and the warm rock or any other sources of heat and humidity. However, on reaching the newly exposed winning face the chilled air would very quickly lose its cooling capacity as it mixes with the warm and moist air that is present in this area.
Even though direct in-situ cooling represents the most effective of a number of different cooling options, this method would not be the preferred solution in this case due to the many problems that would arise during installation and the fact that from a mineworker’s point of view it would have a fairly minimal impact on the mine climate conditions.
As stated above, the use of cooling pads is not a particularly effective way to help the workforce deal with climate problems of this kind. A much more efficient solution would in fact be to provide cooling at permanent workstations such as the operator’s cab of the heading machine.
The two cooling options available are as follows:
- Operator’s cabs:
It is now standard practice to have enclosed cabs on new heading machines. Machines already in service below ground could essentially be retrofitted, though the design of the hydraulic controls and the problem of heat emission could pose problems in this respect.
- Cooling jets:
This solution would require the installation of a small air-conditioning unit that would be used to direct a stream of cooling air towards the machine operator. Here the temperature difference between the cooling stream and the surrounding air should not be more than a few degrees.
In addition to these cooling measures the other alternative – namely reducing the air humidity – is a possibility provided that cooling water for the machine is not atomised, or better still not used in the first place. When winning machines are being replaced, or newly procured, it may be possible to opt for a model that operates without the need for water cooling or where the water from the cooling system is not released but is recycled and re-cooled. While there will inevitably be some increase in mine-air temperature due to waste heat from the machinery (heading machines and transport cars), nevertheless if no water is vaporised the additional heat content of this water (after evaporation as water vapour) cannot be absorbed by the air and the resulting total heat content of the air will therefore be less than when atomisation has taken place. This will help create a much more tolerable working climate.
This would in turn require the introduction of additional dust control measures. And even the spraying of cold rather than warm water would do relatively little to alter the workplace climate, especially as here the term “cold” would still mean a storage-tank temperature approaching that of the surrounding rock of over 30 °C. This then leaves dust removal as the only viable alternative, e. g., using a targeted airflow with aspiration/extraction being provided as close as possible to the machine’s cutting head or loading platform. The dust-laden air could then be transferred to a dust collector or be transported to abandoned areas of the mine for deposition.
As this example shows, the real problem in this case was caused not by the temperature at the active winning faces but rather by the dust suppression measures in place. Without the use of water sprays the climate conditions would have been within an acceptable range. This also means that the dust control factor cannot be the sole focus from an operational and cost point of view. The type of dust suppression method used can have wider implications in other areas – here particularly for the mine climate and for the various problems and costs associated with it – and these have to be properly included in the analysis. While there are all manner of alternative solutions available, these are not always technically practical and may vary in their effectiveness when it comes to bringing about real improvements in the mine-climate situation.