After
preliminary studies it is also necessary to combine the different
loading and transport techniques in order to obtain the system which
presents the best compromise " price - operating constraints" and in particular the environmental constraints. This choice is defined on the basis of the following parameters:
- Nature and hardness of the materials to be extracted
- Production to be ensured
- Transport distance
- Need to crush the materials before their recovery for further processing
- Opportunity to create stocks (marketable materials, ores , topsoil) or fill (waste)
Although
there are many combinations that can optimize the rolling load function
we will describe as an example, the most common and especially those
that use the most frequently encountered materials:
a) Hard materials slaughtered at explosive
Distances less than 200 m. - Low or medium flow
The loading machine, also carries the transport. In this configuration a suitably calculated wheel loader provides these two functions.
This
solution is widely used in quarry aggregates when it is equipped with a
mobile crusher or it is to build stocks of aggregates all coming.
The wheel loader has the advantage of its mobility and its fast speed of travel which can reach 25 km/ h under load. It can virtually adapt to all driving conditions.
The economically profitable use of a loader used in loading - transport is below 150 to 200 m. depending on the driving conditions.
When the materials of the track are sharp tire wear is important and the profitability of such transport decreases rapidly. It is the same if the track is in bad condition because number of rotations / hour decreases. To improve track stability in poor condition most modern machines are equipped with an anti-pitching system. However, it is worth remembering that obtaining a good cost price is inseparable from the good maintenance of the slopes.
Distances greater than 200 m. but less than 2000 m.
This diagram represents the most common configuration for medium to large flows. On
a rough terrain with slopes not exceeding 8 to 10%, rigid dump trucks
will be chosen which are more economical to buy and less expensive to
maintain than articulated haulers. These
funds will be reserved for TP construction sites in muddy or hilly
terrain or the exploitation of gravel pits in unstable terrain with low
traction.
The
choice of the loading machine, shovel or loader, is related to the
grain size, the density and the proliferation of the felled materials,
as well as the quality of the loading area, declivity, adhesion. We will return to these particular conditions in the detailed study of the productivity of these machines.
This
deserted system, simple and flexible to use, has the disadvantage of
being expensive as soon as the tonnages to be transported become
important and the distance becomes longer.
Important distances 1500 m. and more - Big Flow
A first configuration consists in limiting the transport by trucks at reasonable distances, 800 m. For example, for this we will regularly bring the primary crusher to the felling fronts. The materials are then conveyed in strips to their points of use or treatment. In general one builds at the end of the band a buffer stock with automatic recovery by tunnel. This system ensures a great regularity of the supply and maintains the same flexibility as the direct service by dumpers
This configuration ensures large flows, more than 1000 t / h. over very large distances. In general, the direct operating cost for opencast mining is distributed as follows:
- 40 to 50% for drilling operations, logging, load
- 50 to 60% for transport operations
Transport therefore has a great importance in the total cost of exploitation.
However, it is often observed that if the operations of drilling and slaughtering are
perfectly optimized, little attention is paid to downstream loading and
transport operations. There is one anomaly that nothing justifies since, as we will see later ,
we currently have the computing tools that allow such a search.
If the transport is done by dumpers the energy expended is distributed on average as follows:
- 60% for the dumpers
- 40% for the loading machine
if the transport is done in bands the energy balance becomes:
- 20% for the conveyor
- 80% for loading
To transport 1 tonne on 1 km, flat it takes on average 8 liters of diesel with
dumpers or 12 kW with a transport by band. In total, the useful tonne-kilometer (TKU )
is 2 to 4 times cheaper with belt transport than with dumpers.
However it does not lose sight of the fact that the transport by band will be profitable only under
certain conditions of distance and flow.
Another technique derived from the previous one, is to make the primary crusher
movable loading is then directly into the hopper of the mobile installation.
Some armored models can be placed in close proximity to the front of the felling
and can be loaded directly with a hydraulic shovel.
This technique has been developed for twenty years in France and
mainly by cement companies in their limestone quarries. This technique applies
all the better :
* the deposit is regular in geometry and quality since the rapid change from oneloading point to another becomes difficult
* the materials must in any case be crushed, it takes little or no no sterile
* the transport distances and the flows to be carried out make it possible to absorb the high cost of the
equipment and installations necessary for the extraction.
b) The materials are brittle and / or shiftable
work stripping
For relatively high flows and distances greater than 200 m. the solution of mechanical stripping ( scrapping ) is the most economical. The scraper operates in the range of 20 to 25 cm. depth It ensures both extraction and transport.
The ability of the material to be "stripped" is often determined by the depression of the tires which must be between 2 and 12 cm.
The scraper can be towed, bucket from 5 to 20 m3, or self-propelled, skips from 5 to 40 m3 ..
In the loading phase, a bulldozer or other scraper is often used as a pusher. This quickly makes filling the bucket a short distance.
The flat speed can reach 50 km / h which allows this type of machine to reach high flows over distances between 200 and 2000 m. The maximum permissible slopes are :
unladen: 12% solo, 20% tandem
load: 6% solo, 12% tandem
The average characteristics of the machines used in France are:
Empty weight: from 30 to 80 tonnes.
Payload: from 20 to 50 tons.
Power: from 300 to 700 kW
The most rational uses are :
The extraction of topsoil before exploitation
The earthworks of roads and platforms in public works
The extraction of friable cover waste material
Note: The stripping can be carried out after stripping the bulldozer but the profitability is
poor and this technique is used in troubleshooting when, for
environmental reasons , shooting is not possible.
Exploitation work
* The material does not require crushing, the distance is short.
We use the combination of a bulldozer that feeds a loader which ensures extraction and transport .
We thus realize a very economical operating system. This solution is frequently used in dry placer mining. The hopper is moved periodically. If the distance is important then must organize a transport by conventional dumpers.
* The material does not require crushing, the distance is greater than 200m and
less than 2000m.
A bulldozer feeds, usually a high-speed shovel, which loads dumpers. Unloading is done on a sterile spill in the case of a stripping, or on a stock if it is an extraction of all coming.
Large flows over long distances are possible with this organization. The main drawback is that the cost increases very quickly with the distance and the tonnage to be transported. Note that in this organization the excavator works in "butte" configuration. Simultaneous pushing by dozer does not create
the loading platform necessary for loading in "retro" configuration. Moreover, for safety reasons it is necessary that the excavator driver always has in his field of vision the position and the work phase (pushing or retreating) of the dozer.
* The material requires a primary crushing
If
the material is sufficiently friable to be extracted directly with the
hydraulic shovel and it is often the case, in alluvial deposit to dry,
one can organize a transport by dumpers on distances ranging from 200 to
1800 m.
However,
it will be advantageous to periodically move the primary crushing to
reduce the cost of transport which in this case may exceed 60% of the
total price of the item extraction / transport.
In stripping or irregular deposit it is preferred to use pushing by bulldozer with recovery charger. This
solution is less economical than the previous one, there is an extra
gear, but we gain in operating flexibility and selectivity.
This configuration is to be retained if one exploits deposits irregular in quality and hardness.
A more economical solution for mobile and low to medium production sites is to use a mobile crushing plant type "LOCOTRACK". A shovel provides extraction and feeding. Finished products, in limited numbers are put in stock as and when they are produced.
* Particular case of an alluvial deposit under water cover
As soon as the water cover is important, more than 5m. and that the deposit has a certain power, beyond a few meters it is necessary to use either the dragline bucket or dragline grapple
mounted barge. The diagram below represents a typical configuration.
The extraction in water imposes a deposit of the materials for draining.
A recovery, usually by loader, is necessary to serve rolling stock or feed hopper
of a conveyor belt.
Any attempt to remove this recovery usually leads to failures.
It is generally found that :
§ during transport by conveyors, the fine materials saturated with water clog the bands despite the presence of scrapers.
§ the water draining from the trucks soaks the tracks which can not then be held in condition § it is impossible to use the national road network § the excess of water seriously disturbs the subsequent treatment, crushing, screening § the dragline yield decreases rapidly Moreover
the existence of a buffer stock between the extraction and the
treatment gives flexibility to the exploitation while improving the
overall reliability of the production line.
c) Summary of the selection criteria
The
following synoptic makes it possible to orient the choice of a system
of service according to the primary fragmentation and the conditions of
deposit. The
materials being supposed sufficiently fragmented or having a natural
friability which allows their extraction by a mechanical machine, two
operations must be carried out as economically as possible : 1. Loading the ore or the material and transporting it to its point of use 2. Loading transport and store the waste rock or land that can be used for the future redevelopment defined by the impact study.
As
we have seen, the possible technologies are linked to the available
materials and the synoptic schematically schematizes various
combinations.
It can be seen that the loading and transport machines can be classified into two groups :
* Machines that work while moving: bulldozers, scrapers, loaders, dumpers.
* Machines working in stationary or slow moving stations: excavators,
draglines, chain loaders, mobile crushers, conveyor belts.
The
conditions of use, the calculation of the productivity and the
efficiency of the loading and transport equipment, except the conveyor
belts, listed in this chart are the subject of the following chapters.
Reference: Ceficem , 3 rue des Freres Chausson 92600 ASNIERES
