Source: World Dredging Mining & Construction
Over the years a wide range of physical tests have been developed to allow prospective gold mining organizations to analyze various aspects of intended mine areas. These tests vary depending on the intended method of extraction. In the case of dredges, specifically wheel excavator equipped dredges, the following tests for evaluation are required:
- A full granularmetric curve which is representative of the entire ore body is required, particularly if the body might contain material which is beyond the capabilities of the dredge to excavate or transport.
- To generate the information satisfactory to present this granularmetric curve, test holes should be dug and all material from these test holes sized.
- Typically, the test hole or casing used to extract the samples should be sized to accommodate no less than the maximum ring size of the dredge pump that will pump the material.
- In the case of possible use of a 14" (356mm) pipe size Ellicott mining dredge (1000 total installed HP and 150 HP on the excavator), a sample tube of no less than 12 inches in diameter should be driven, with all material retained within the tube tested for dimension. This can be accomplished by using a tube and auger testing device. Barring the availability of this type of equipment, test holes may be dug with a backhoe, dragline, or, if the soil conditions permit, a front end loader. A test pit can also be hand dug. The hand dug pit requires an appropriately reinforced test hole.
All material taken from the test hole should be sized, particularly if there is any indication of material too large to pass the dredge pump. If such oversized material is discovered, additional testing should be undertaken to establish the percentage of material which exceeds pumping capabilities. This information should then be used to further delineate the selection of the appropriate size of equipment.
- Next, the deposit must be tested for signs of clay or silt, and for indications of cementation which might effect excavation rates as well as the selection of processing equipment.
If the material can be easily dug to bedrock with a backhoe or dragline, it is safe to assume that a bucketwheel dredge can satisfactorily complete the excavation phase of mining. If cementation is encountered, an analysis of the degree of effectiveness of the test machine is required. This analysis must be balanced against the excavation capability of the wheel excavator equipped dredge to be utilized. The presence of clay may require the utilization of a flushing system on the wheel excavator to insure continued dredge production.
Beyond the possible need for a flushing system, the presence of clay will bear heavily on the selection of processing equipment.
Appropriate testing of the ore body requires at least three test holes. These holes should be located at the upstream, midway point, and downstream point of the deposit. Assuming that the analysis of material from these holes indicates that a dredge is suitable for excavation and hydraulic transport of the material, it is reasonable to assume that the entire project will lend itself to exploitation with a dredge. In the event that results indicate a dredge is inappropriate, further testing will be required. Alternatives such as increasing the dredge size or abandoning some area of the deposit may be considered.
In addition to the above sampling, it is absolutely necessary that holes be dug to bedrock or the bottom of the deposit. This is necessary for two reasons:
- Most ore bodies tend to have the highest values at the bottom.
- These high values which are resident at bedrock are often included in the irregular surface at the bedrock. As a result, this surface must be excavated by the dredge, if possible.
A sample of this bedrock is necessary to determine if the dredge can excavate the bedrock. Elevation differentials must be established if they exist. If significant elevation differentials occur, a thorough study of the bedrock condition should be undertaken to develop an appropriate mining plan. Exploration of bedrock elevation may minimize the potential loss of high values. Assuming the full testing of the ore body is completed and proves satisfactory, a specific dredge may be selected for the project.
Dredge size will be controlled by material dimension, volumetric requirements of the processing plant, digging depth, and the distance between the mining site and the processing system. The processing system should be as close to the dredge as practical to minimize the horsepower and pipeline handling resultant from long pipeline distances.
As stated above, assuming all these conditions are thoroughly reviewed, appropriate dredge size may be determined.
Following the selection of dredging equipment, a mining plan should be developed which allows operators to analyze the overall project layout. The mining plan should insure that the ore body is exploited in such a fashion as to maintain a continuing and profitable operation for the life of the project. The plan should also be scheduled to maintain a constant rate of excavation and processing with as near as possible a constant recovery rate for the gold mineral being extracted. If significant variations are found in gold content, dimension, or variations in materials which may impede the recovery of gold, the project should be laid out to provide a uniform extraction process.
In planning the job or project layout, it is necessary to define value bearing areas where there are significant differences in make-up. Areas where the extraction of the mineral is easy should, if at all possible, be separated and mined apart from areas which contain clay and limes. This separation is required if the presence of slimes or clay significantly affects processing equipment setup and the rate at which the processing system can operate.
If the variations are such that easily processed material can be blended with materials that are harder to process and still provide an efficient and uniform extraction rate, the project may be laid out accordingly.
Beyond this, if areas of high concentration are discovered, it may be necessary to adjust the excavation and processing rate of the dredge beneficiation plant. This will insure that values are not lost by overfeeding the system with an unusually high concentration of gold or mineral, some of which may pass through the processing system and be lost.
Areas with high concentrations of boulders or material difficult to dredge should be identified and either avoided or considered for a different method of exploitation.
Sampling the Alluvial Body
In addition to analyzing the ore body to establish the compatibility of dredging equipment, it is necessary to understand and define the values in the deposit. These values should be established by a recognized and disinterested geologist who is familiar with the specifics of an alluvial survey. This survey is particularly necessary if investors are to be solicited for development of the project.
Relative to project layout, the survey of the alluvial deposit may be used to establish the relative position of values and areas which may be barren or of insignificant value. The mine plan should allow stripping of barren areas or low value area prior to mining the gold bearing strata. If there are significant areas of barren ground with high value areas beneath them, overall project performance can be improved by high speed stripping followed by normal rate mining.
In addition, a fully developed and completed sampling study allows optimum selection of processing equipment. It can also guide excavation and extraction rates to maintain a continually profitable operation. The integration of the value study with the mine plan will maximize the recovery rate throughout the life of the project. In addition, if relatively high cash flow is necessary at an initial stage, the project plan based on known values will produce relatively high returns at project start-up.
Separation equipment should be selected based on the information developed by the sampling and dredge selection process. This equipment should be designed to handle the full range of material sizes delivered by the dredge. The equipment should be designed to allow primary separation of the gold-or mineral bearing material from the stone, gravel, and clay balls if they exist. (NOTE: If clay balls exist and there are significant quantities of gold nuggets, it may be necessary to mill all clay balls to prevent carry over of the nuggets by the clay balls).
Primary separation may be accomplished by a trommel appropriately sized to separate the gold bearing dimensions from the oversize and trash with oversize, and trash being delivered to the stackers or tailings delivery system. Following prime separation, the gold bearing material should be passed on to jigging equipment or other gravity separation equipment as is deemed appropriate.
The material retained from the jigging system or sluices is further collected and processed as required. Assuming all equipment operates as designed and described above, gold will be recovered.
Reprinted from World Dredging Mining & Construction