Patent Application
20120181844
The present application for invention patent relates to a method of underground mining exploitation which allows for continuous ore extraction. Specifically, it relates to a mining method comprising rock pre-conditioning, as a way to prepare the rock to facilitate its response to caveability and fragmentation and then it relates to an ore material handling system whose main features are: simultaneous extraction from several draw points and haulage with stationary equipment towards main haulage systems. All theses processes are carried out continuously.
Overall mining process comprises two major stages: rock fracturing and its subsequent haulage. The aim of the first stage is to transform the solid material—which is the natural state of ore deposit—into fragmented material, and the aim of the second stage is to haul such fragments to their final destination.
In caving exploitation, ground breaking itself is a continuous process of fracturing and fragmentation that makes use of natural forces of gravity and tectonism to achieve its goal. This process occurs naturally as a consequence of the unbalance caused by the extraction of the produced fragments, i.e., each time an amount of fragmented material is drawn, a condition of instability is originated which produces more fracturing and fragmentation, thereby, more ground breaking.
However, within the conventional system of caving exploitation, material handling, which comprises extraction (loading) of ore available at points and its haulage to destination, occurs discreetly and intermittently; discreetly because the extraction is not simultaneously made from every point where ore is available, but rather from just a fraction of them; and intermittently because the extraction is made by wheel loaders working within a cycle which comprises: loading, traveling to dump, unloading and traveling back to load another bucketful. Usually, such bucketful of ore extracted discreetly and intermittently is dumped into shafts which serve as silos—where it will be loaded again at intervals into rail wagons or trucks to be hauled to the surface.
Then, the whole process is based on this discreet and intermittent hauling process, since the ground breaking depends on haulage. Therefore, in order to achieve an entirely new continuous process, a continuous material handling system is required.
The concept “Continuous Mining” comprises a stage of modifying the features of the rock mass where the ore deposit is located, the stage being called Pre-conditioning. At this stage, the extent of the rock mass fracturing is increased in situ, in order to obtain, in the following stage of caving, fragmented material in sizes which are compatible with continuous and automated material handling systems. Another main aspect of the pre-conditioning application is to guarantee that the rock breaking will occur at a constant rate and at the same rate as the extraction process.
It is well known that upon choosing an exploitation method, the location and depth of the ore deposit, deposit geometry (vein, seam, massive) and the quality of the host and mineralized rock are assessed, and based on several combinations of such elements, different solutions for each case are known. On the other hand, in the past 100 years, exploitation systems, with the exception of coal mining, have adopted the mining designs to incorporate the use of advanced construction and grund movement equipment. The development of such equipment (front loaders, trucks and others) is mainly due to the fact that within the civil work industry, the productivity is a decisive factor for business survival.
Therefore, it is clear that the proposed concept of Continuous Mining breaks both paradigms. The first one, because it is not the process which is adapted to the rock conditions but the quality of the rock in situ is modified to be adapted to an efficient process of rock breaking and extraction; and the second one, because construction industry equipment are no longer used because this method requires equipment specially designed.
Continuous Mining is conceived as a highly mechanized and automated process which permits to make the most of the resources invested in equipment and infrastructure. The idea is that the mine operates 18 to 22 hours a day, 360 days a year, at full capacity and within an environment complying with high safety and hygiene standards.
The Continuous Mining method is rather a mining process of continuous and permanent ore flow from the deposit to the treatment plant, which could be similar to a “rock factory” where at one end, in situ reserves are fed and at the other end “treated rocks” are obtained.
The method relates mainly to the continuity of the ore flow from its natural location to its final destination, which can be described as a “flow” of ore which goes through a pipe-network or means of transport without interruptions. In turn and as a consequence of the continuity of flow, there is no need to halt the process when changing shifts and thereby, Continuous Mining also means of temporal continuity in the use of mine infrastructure.
Even though the term continuous mining has been somehow used, this is mainly due to the utilization of large equipment at the working face. Basically, the equipment comprises rotary drills to weaken and fragment the rock mass but later loading equipment is used to carry the ore to the treatment plant.
Additionally, some developments directed to improve rock mass exploitation can be seen. Patent RU2186980 for example, describes a method comprising the exploitation of front faces as ore continuous fragmentation without pillars by driving drills on the work levels. However, neither the way ore is extracted from the mining zone nor whether this extraction is made continuously are mentioned. Similar solution can be found in patent publications RU2182663 and RU 2148712 which generally describes that caving itself is a continuous process, but if no continuous extraction or loading process is added, this caving processes will became intermittent and discontinuous, which is precisely the solution proposed by the present invention.
The method of the present invention comprises the design and construction of exploitation drifts or draw points arranged in such a way that the ore material extracted therefrom is driven to the ore haulage drifts. Strictly speaking, the construction of exploitation drifts takes into account that haulage drifts cross the center of two groups of exploitation drifts and subsequently through every group of exploitation drifts defined for the exploitation. Optionally, parallel to haulage drifts, service drifts should be constructed whose function is to allow personnel to reach the drift zone and service drifts when maintenance jobs and eventual failures are needed.
Within exploitation drifts, trenches or draw points are arranged where, due the effect of ore fragmentation described below, rock mass detaches and continuous ore caving is induced. Draw point should be constructed in such a way to arrange a regular layout[m1] with determined distances compatible with interactive gravitational flow. When trenches are already constructed, necessary equipment is installed for extracting the ore. Likewise, necessary means are arranged in haulage drifts so that the material extracted from trenches flow permanently through haulage drifts. For that purpose, haulage drifts have for example, belt or chain conveyors, endless and stationary, commonly called “Panzer” for its high resistance to hard works (movement of large, hard and abrasive rocks). The use of this kind of conveyors replaces typical mobile low height loaders or LHD used in conventional mining.
An optional way of constructing drifts comprises the construction of a material transferring level located one level downward regarding to the level of exploitation drifts, and consequently, with regard to the level of trenches. This layout allows receiving simultaneously ore material from more than one trench or draw point and accumulating in the duct material falling from the trench; this duct is formed between the trench and transferring levels. With this alternative, by accumulating material in the aforementioned duct, allows performing maintenance services in haulage drifts without stopping the exploitation process since the accumulated material can be unloaded afterwards.
When drifts have been built according to the previous description, the method comprises the following stages:
a) Pre-conditioning: this stage is fundamental for the method's success and comprises modifying in situ rock quality, increasing the extent of fracturing until levels which confer rocks features similar to secondary rock mass. Pre-conditioning stage can be achieved by i) hydraulic fracturing, which is a technology known in applications of oil wells exploitation, where it is used to cause fractures which facilitate oil flow from wells, and in the case of metal mining, it generates fractures which facilitate the action of the natural stresses, both for generating caving and for improving granulometry; or by ii) confined blasting which is the combined action of several firings to fracture the rock mass. Finally, both techniques can be combined.
Primary rock is a highly competent rock mass and massive pre-conditioning or pre-treatment converts it on a material which is easy to cave and fragment by caving exploitation, which could be also called process of “secondarization” for primary ore.
Nevertheless, test have shown that the best way to carry out the pre-conditioning stage is by combining Hydraulic Fracturing with Explosive Driven Dynamic Weakening, which in last case we make use of dynamic wave force collision, which is technically possible nowadays due to the electronic detonation technologies available in the market. This pre-conditioning alternative allows producing pre-stimulation of drill-holes induced by hydraulic fracturing and carrying out the electronic detonation process immediately.
Another alternative is carrying out drilling pre-stimulation induced by propellant (solid fuel-based explosive) and then applying the hydraulic fracturing technology in order to propagate the fractures, the latter is a methodology used usually in oil wells.
b) Caving: this stage is the rock mass caving operation by undercutting the base of rock mass by means of known procedures of caving method in well-fragmented rock environment; and its application does not present any innovation for this purpose. With the induced fracturing in the previous stage (a) it is expected that most of fragments can be processed by the continuous extraction and haulage system.
The layout of draw point that should be used will be defined by the rock fragmentation features. For instance, in sectors with fine fragmentation carried out by caving methods, a layout with close points with distances ranging from 8 to 11 meters is required. This point closeness condition, makes it necessary the drifts must be small, in order to maintain the stability of the sector. The known and extensively applied solutions in the world are the extraction with grizzlies and shafts or scrapers, which allow extracting from multiple points and collect the extracted product in haulage drifts. On the other hand, larger layouts with spacing ranging from 13 to 17 meters are used for primary rock sectors, with thick fragmentation. In the conventional system these layouts require using very large LHD equipment and it is not possible to make parallel extraction from those points.
In the case of Continuous Mining, the sizes of the layouts that have been assessed are between 13×13 square meters and 15×15 square meters. Both layouts are quite wide and are useful to handle oversize mucks or boulders.
c) Extraction: this stage is conceived as a simultaneous operation from multiple draw points arranged on a regular layouts at certain distances which are compatible with the interactive gravitational flow. For that purpose, as it has already been mentioned, each draw point is equipped with a stationary extraction unit which feeds a collecting system that conveys the ore to the haulage drift by continuous means that leads it to its destination. The extraction and haulage equipments have an automatic command—assisted by a remote driver operated from a control room as in any modern industry. Eventually, crushers could be installed at the end of the collecting systems to produce in the mine the final feeding size for the plant. In short, wheel loaders are not used because they are replaced by continuous loading systems. By way of example, stationary “feeders” that unload continuous conveyors can be considered.
The main haulage alternative used is a metal belt conveyor (panzer) in which the preliminary assessments show lower operation costs compared to the traditional raildrift haulage system.
Applying a continuous mining system as the one that have been described has a great impact on the caved area performance which is usually expressed as “extraction rate” and is measured in tpd/m2.
In fact, in the conventional LHD-extraction system, discreet and intermittent, each loading system extracts ore from a set of draw points (generally 16 draw points per equipment) at the rate of 200 t/hour. Approximately 250 m2 influence area is associated to each extraction point so a 16 point module comprises approximately 4,000 m2, thus in a maximum operation of 15 hours a day an extraction of 3,000 t equivalent to 0.75 tpd/m2 can be achieved. On the other hand if we assume that the extraction is made regularly, less than 200 t a day is drawn from each point which is equivalent to using less than one hour daily (let us remember that LHD can draw 200 t/hour).
The historical figures for actual extraction rate are around 0.4 tpd/m2 and for effective extraction rate are around 0.5 tpd/m2, since the ore flow through the extraction points is no fast enough to saturate the production capacity of the equipments. The expression “actual extraction rate (AER)” is used to refer to the total extraction achieved in a day from a certain active area, if the points have or have not been available for extraction; and the terms “effective extraction rate (EER)” relates to the estimated extraction rate considering only the area of those points that effectively were object of extraction during that day. The difference is explained because one portion of the active area can be transiently out of service due to direct maintenance or repair of draw points, hauling or destination facilities.
Continuous Mining aims to improve these figures by increasing the use of extraction points to an average of 16 hours daily (two operation shifts and one maintenance shift) with a 40 t/hour production per stationary extractor.
Thus if we consider in an easy exercise where 8 points of 225 m2 influence (1800 m2) operating 16 hours a day, a 5,400 tpd production, a 3 tpd/m2 extraction rate (EER) and an actual extraction rate (AER) around 1.5 tpd/m2 are provided. This results in a better use of the caved area as well as a concentrated operation with its consequent resource rationalization.
For the fragmented material in such conditions, the achievable extraction rate in the caving propagation stage can reach 300 mm/day which is equivalent to approximately 0.8 tpd/m2 and theoretically there are no limitations for the gravitational extraction stage post propagation except the extraction capacity, that in the invented system could reach rates above 3 tpd/m2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 3560-2008 | Nov 2008 | CL | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB09/07556 | 11/25/2009 | WO | 00 | 3/28/2012 |
