Open Pit Mining and the Dredge
Open pit mining, always a demanding task, faces two relatively new problems: (1) availability of equipment and (2) environmental restrictions. The modern dredge offers relief in both problem areas. Delivery on a major dredge might run to 18 months rather than 24 months or more for a drag line; and lowering of the water table is precluded, thus minimizing environmental objections. In addition, a lower first cost and potentially increased efficiency may offer real advantages to the miner who is forced away from his traditional mining methods.
What are the objections of the use of a dredge in open pit mining? Perhaps first and foremost is the natural reluctance to change from a successful operation. Who can blame the miner who has developed expertise in the use of the dragline if he is reluctant to change to a new mining technique that requires new and different skills?
A part of the same problem is the miner’s desire to control the excavation visually in order to delineate sharply between overburden and matrix. This problem can be alleviated significantly by utilizing good boring data and a positive acquisition cutter. In the final analysis, however, if the miner is prohibited from dewatering the area, either he mines the deposit by dredge or not at all. There is little doubt that when the decision is made to remove overburden and/or matrix by dredge, the problems of delineating between overburden and matrix, creation of slimes, and development of mining plans to minimize double handling, etc., will be addressed or solved.
There are two broad categories of dredges: mechanical and hydraulic.
Mechanical Dredges
The mechanical dredge is characterized by the use of some sort of bucket which excavates and elevates the material to the surface.
The grapple dredge is frequently a dry-land clamshell or dragline machine mounted on a barge. It is essentially a derrick mounted on a barge and equipped with a "clamshell" bucket for dredging.
The dipper dredge is essentially a barge-mounted power shovel. Its main advantage is in the strong "crowding action" to the bucket as the dipper stick forces it into the material to be moved.
The bucket ladder dredge consists of an endless chain of buckets moving from the work face to a point above the surface of the water. Each bucket digs its own load, carries it to the surface and, as it rotates over the top tumbler, dumps its load and goes back for another. Bucket dredges are more efficient than dipper or grapple dredges because the work cycle is continuous.
Hydraulic Dredges
The hydraulic dredge is characterized by the continuous removal of bottom material by the suction of a dredge pump, supplemented by mechanical excavators where necessary. The dredge pump discharges the slurry, consisting of water and bottom material, through a pipeline to its ultimate site of disposition, thus combining all elements of the dredging operation into one continuous operation. There are several traditional-type hydraulic dredges available to the mining industry.
Plain suction is the simplest form of hydraulic dredge and utilizes no excavator.
The cutterhead pipeline dredge is the most versatile and widely used marine excavating unit. It is similar to the plain suction dredge but is equipped with a rotating cutter surrounding the intake end of the suction pipe. The cutterhead dredge, because of its versatility, capacity, efficiency and relatively low cost, has become the workhorse of the dredging industry. It does more dredging than all other types of dredges combined and has received the attention of a great deal of development.
More recently the industry leaders have developed a bucketwheel excavator for pipeline dredges.
The vacuum pulled by a hydraulic dredge pump provides an indication of flow in the suction line, but only a rough indication, since the vacuum (reduction of barometric pressure) includes all flow-related factors (entrance loss, friction loss, velocity head) plus the specific gravity head (hsg). This last term represents the force needed to lift the solids content of the slurry from the sea bottom to the dredge pump. Mathematically, it can be expressed as
hsg = Digging Depth X (Sp. Gr.slurry — Sp. Gr. water)
Calculations, testing, and actual dredge operation indicate that sand and gravel operations are most efficient at a slurry sp gr of 1.5. Using this figure, the parenthetical value in the above expression becomes 0.5 and then
hsq = 0.5 Digging Depth
At 50 ft digging depth, the hsg is 25 ft, or most of the vacuum the dredge pump is capable of pulling. This is obviously an impossible condition since the 5 ft of barometric pressure remaining is insufficient to create the necessary velocity in the suction line to carry the 1.5 sp gr slurry. Therefore, the operator is forced to reduce his solids content (his pay load) in favor of greater velocity. Note that he still pulls his maximum vacuum, but there is an enforced redistribution of the 30 ft barometric pressure between hsq and the velocity-related factors. Unfortunately, the operator is not normally provided with the necessary instrumentation to know what redistribution he makes. He sorely needs a production meter, which provides an instantaneous readout of the sp gr, velocity and production rate expressed in tons per hour. Also needed is a totalizer which keeps a running account of accumulated production. This instrument has resulted in significant improvements in dredge productibility (15% to 40%) by providing the information to the operator which allows him to optimize the capability of his equipment.
The production meter does not solve the problem of deep digging, however; it merely allows the operator to optimize his operating conditions regardless of depth. The answer to increasing productivity three to four times is a submerged dredge pump, which, when properly applied, eliminates for all practical circumstances the barometric restriction.
Requirements for submerged dredge pumps are as follows:
Head-capacity characteristics should be coordinated with those of the hull dredge pump in order to achieve the hydraulic transport system characteristics required.
It should be relatively maintenance free, and be capable of passing larger particles than the hull dredge pump to minimize downtime.
It should be installed as far down the ladder as possible, but never should it be located at less than one-third of the digging depth.
As a rule of thumb, the head provided by the submerged pump should be approximately one-half of the digging depth plus 15 ft.
In addition to production advantages, the submerged pump can solve gas problems in the suction by never allowing the released gas to expand appreciably. At 70 ft digging depth, 1 cu ft of gas liberated by the standard dredge expands to 10 cu ft at the dredge pump. This results from a simple application of Boyles Law, where P1 V1 = P2 V2. Experienced dredge men know the serious effect on production of such a situation. The properly installed submerged pump keeps the gas under pressure, precluding the debilitating gas expansion. Also, the higher concentration of solids per gallon of slurry significantly reduces the amount of water to be handled and the sedimentation problem of the flow from the disposal area, thus reducing ecological complains.
Conclusion
In summary, the seemingly disastrous market and environmental restrictions may turn out to be a blessing in disguise by forcing the miner to turn to the efficient modern dredge. The dredge manufacturing industry is much better prepared to serve the mining industry than it was a decade ago. An investigation of mining by dredge would certainly appear to be warranted.
Reprinted from Transactions of SME
