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Dredging at Marcopper in the Philippines

Source: Asia Pacific Mining

Marcopper's porphyry copper open pit mine is located near the geographic center of the island province of Marinduque in the Philippine archipelago. The 780 square kilometer island is 170 kilometers south of Manila. (Marcopper was until recently a subsidiary of Placer Dome.)

B890 Wheel-Dragon Dredge
Ellicott® Series B890 Wheel-Dragon Dredge

Marcopper Mining Corporation was commissioned with a rated mill capacity of 15000 tonnes per day. Mill tailings were disposed of into the nearby San Antonio valley 4 kilometers from the concentrator. Two dams were built with compacted laterite at both ends of the valley to form an impoundment or basin. The main part of the two dams was built with underflow from the cycloned mill tailings to an eventual crest height of 330 meters above sea level. In some places the dam crest was 50 meters above the valley floor. The downstream construction method of building up tailing dams resulted in sandy beaches near the dams and slime accumulation near the center of the pond.

After the initial capital investment of the plant had been recovered, planning commenced to construct a disposal system to transport thickened slurry by gravity to the sea. Tailing thickeners were constructed to recover water for recycling and to control slurry density between 48% to 50% solids by weight. Fresh water reservoirs expanded, and the concentrator capacity was increased to 25,000 tonnes per day. The new disposal system was made up of pipelines, concrete flumes, drop boxes, and drop tanks. The tailing line had an initial length of 13 kilometers from the concentrator of which over 4 kilometers was through a tunnel specially driven for that purpose. To avoid settling and at the same time reduce water to a minimum, the flume gradient was set at -1% and the pipe gradient not more than -1/2% to minimize wear. Concrete launders were constructed where topography allowed it and as made possible with tunnels. Sections of rugged terrain were built with drop boxes in non-pressure areas and with steel drop tanks where head was required to drive the flow out into the sea.

After drilling 139 diamond drill holes in the pond and adjoining areas, an ore body was outlined and was found to contain a total geologic ore reserve of 200 million tonnes averaging 0.56% Cu using 0.40% copper cut-off grade. Feasibility studies warranted the mining of the near-surface high-grade portion of the deposit. This required the early removal of the accumulated mill tailings from the pond by dredging and transporting the repulped slurry to the tailing disposal facility in Calancan Bay.

Selection
After a worldwide study of all dredging systems, an Italian harbor desilting system using a pneumatic dredge was considered. The method required the least wastage of water and produced a slurry at a controlled density between 40% and 60% solids by weight. To establish the suitability of the dredge, a small unit was tested in the mine site. After several months of intensive testing, results were found satisfactory.

Foreign and Marcopper engineers designed and built two pneumatic dredges of much larger capacity. These were commissioned soon after. Likewise, the tailing disposal facility was expanded to accommodate the additional dredge materials. The two dredges operated efficiently in the coarser and more free-flowing beach sand at depth where the intake is submerged between 15 and 20 meters below the water surface.

At shallower depths and in denser and more cohesive material, the pneumatic dredge was less effective because the hydrostatic head was insufficient to force the material into the dredge intake. It was concluded that more positive acting pneumatic dredges encountered great difficulties. Studies showed that a newly developed bucketwheel dredge could be more effective in terms of production and flexibility to handle a wide range of materials in shallow water.

An Ellicott® Series B890 dredge was put into operation. The bucketwheel dredge as expected performed remarkably and proved to be much superior in performance and cost as compared to the earlier pneumatic dredges. On the average, the bucketwheel dredge production rated 8000 tonnes per day as against the combined rated 9500 tonnes per day of the two pneumatic dredges. Two years later a second Ellicott® dredge was placed in operation.

 

B890 Wheel Dragon Dredge
Ellicott® Series B890 Wheel-Dragon 

Rapidly falling water level prompted the shutdown of one pneumatic dredge. As the dredging approached the original ground contour, the use of the pneumatic dredge became less applicable in maintaining high-density slurry.

A deteriorating cashflow position brought about by the recession of metal prices left an estimated 5.2 million cubic meters of tailings scheduled for removal after the termination of dredging operations.

A subsequent financial turnaround for Marcopper (due to improved metal prices), and the expected depletion of the Tapian orebody, made the San Antonio copper project worthy for extensive feasibility studies.

Tailing Characteristics
A typical analysis of mill tailing deposited into the pond shows the following size distribution:

Mesh Fraction

% Cumulative

 

+20

0.2

-20

+28

0.8

-28

+35

3.6

-35

+48

9.7

-48

+65

19/4

-65

+100

30.4

-100

+150

40.1

-150

+200

49/4

-200

+325

58.5

-325

 

100.0

An arbitrary division between sand and slime is set at 70% passing 270 mesh. Of the 22.5 million cubic meters of tailing predominantly of kaolinitic clay and chlorite. During the preliminary test period, the pneumatic dredge head easily penetrated the different types of tailing materials at depth. They flowed towards the inlet pipe of the dredgehead forming cone-shaped holes in the silt bottom.

Two years after the tests, pneumatic dredging revealed that the slimy layers had become compacted and gelatinous. Dredging in the sand portions however, gave satisfactory results. The dredgehead easily burrows into the bed creating a cone that collapses and flows easily towards the dredgehead. This resulted in high production and acceptable levels of pulp density.

The Pneumatic Dredge
The pneumatic dredge system utilizes the hydrostatic pressure of the overlying lake to push material through a buried pipe into a pressure tank vented to the atmosphere. When the tank is filled, compressed air is introduced into the cylinder, closed the inlet valve and forces the slurry to be discharged to surface through an outlet at the top of the tank. The air is then vented to the atmosphere and the cycle repeats itself at approximately 20-second intervals. In practice, 3 tanks are clustered together to form a dredgehead. Compressed air is fed to each cylinder through an air distributor. The slurry discharged from each tank passes into a manifold common to the three tanks. The material is therefore discharged as continuous stream rather than in batches. The dredge equipment is mounted on board compartmentalized steel barge measuring 12 meters wide, 32 meters long and 2.5 meters deep. The dredgehead hangs from a 15-meter high tower that is mounted over an opening in the dredge floor.

Two methods of dredging were employed with the pneuma-pump namely: "holing" and "trailing". In holing, the suction openings are fitted with vertical pipes and these are driven into the material by the pump's weight. In trailing the suction openings are provided with shovels and the whole pump is dragged along the pond's bottom. As the water level dropped in the pond, the operation of the pneumatic dredges became restricted to smaller areas. As the dredging got deeper, the deposited slimes became more compacted and cohesive and less free-flowing. The operation of the pneumatic dredges became increasingly difficult as it got nearer the original ground. Eventually, after dredging some 16 million tonnes of tailings, the first pneumatic dredge was shutdown in early 1982 due to typhoon damage while the second was also shutdown at the end of the same year. The two dredges were later dismantled and sold.

The Bucketwheel Dredge
The feasibility of dredging the tailing with the use of other dredging equipment aside from the pneumatic dredges was investigated. This led to the purchase of a suction dredge for the final layers of slimes and sand over the uneven bottom of the tailing pond.

A bucketwheel dredge was chosen the standard suction-cutter dredge because it can apply direct digging action in both swing modes. This feature of the bucketwheel dredge makes it possible for the dredge to directly discharge its products into the tailing disposal system. After evaluating several quotations from various manufacturers, a decision was made to purchase an Ellicott® Series B890 "Wheel-Dragon" Dredge.

Ellicott® "Wheel-Dragon" Dredge
The Ellicott® Bucketwheel dredge is a non-self propelled hydraulic pipeline cutterhead dredges whose entire operation can be controlled by one man in a central control room. The dredging plant is housed on three sectional pontoon-type rectangular hulls made of heavy-duty steel.

The main dredge pump plant is installed in the cutter hull pontoon.

A heavy duty, tubular steel dredging ladder mounted on the forward end of the center hull pontoon carried the revolving bucketwheel for excavating the materials to be dredged.

The speed of the bucket can be varied depending upon the requirement. The bucketwheel is powered by a hydraulic motor of high horsepower and torque mounted on the forward section of the ladder.

A suction pipe is mounted underneath and is secured to the tubular ladder. A heavy-duty flexible connection is provided between the ladder suction pipe and the hull.

There are two tubular steel spuds with steel points, one installed in a fixed spud well for holding, and one installed in a traveling carriage for working. The spuds are lifted by means of individual single-drum winches with free-fall provisions for good penetration. The spud carriageway system allows for a three-meter forward travel of the dredge and is positioned by a travel rail assembly.

The prime movers are two diesel engines: one to drive a 14-inch centrifugal dredge pump that discharges material to a floating pipeline connected to the stern of the dredge, and one to drive heavy duty hydraulic pumps which supply separate circuits to the respective hydraulic motors driving the bucketwheel, the swing and spud winches, plus the ladder winch.

Dredging Operation
As the bucketwheel revolves at the end of the dredging ladder, material is scooped and fed into a suction system. The design of the cutter wheel is such that once the material is cut, it can only escape via the suction pipe and delivery line.

The dredge is swung from side to side across the cut with the aid of individual direct line winches, cable lines and anchors. The vessel pivots on the heavy-duty large cross-section working spud. The advancing motion into the cut is provided by a hydraulically operated traveling carriage upon which the working spud is mounted.

When a cut is completed after the carriage has reached it maximum travel, the holding spud is lowered. The working spud is raised and the carriage travels 3-1/2 meters forward. The working spud is then lowered when the desired travel is reached after which the holding spud is raised. The dredge is now ready to make another cut.

Dredging Components
The dredge is made up of the following:

Structural Components - The hull is made of three rectangular welded steel pontoons, two side pontoons and one center pontoon, which are rigidly connected to provide stability. An A-frame with rigid backstays is provided at the forward end of the hull which in conjunction with the ladder hoist which provides for hoisting, holding and lowering of the dredge ladder.

Dredging Components - The bucketwheel module consists of a rotating bucket wheel which is driven by two radial pistons, slow speed 106 shaft horsepower hydraulic motors which provide the cutting bucket with a total of 1,400 pounds of cutting force. A cast steel suction elbow is connected to the receiving hopper of the bucketwheel at one end and to the suction pipe on the ladder at the other end. The ladder suction pipe is connected to the suction pipe in the center hull pontoon by means of a flexible reinforced rubber hose, permitting ladder inclination to desired digging depth.

Prime Movers - The Series B890 "Wheel-Dragon" is equipped with two diesel engines as follows: One 520 HP, CAT Series 3412, to drive the main dredge pump, and one 210 HP, CAT 3306, to drive the hydraulic pump.

Hydraulic Power system - The hydraulic system is made of three independent open-loop circuits. One circuit for the bucketwheel; one circuit for the ladder hoist and swing winches; and one for the spud winches and spud carriage cylinder. The pump draws oil directly from the low-mounted oil reservoir through a heavy-duty filter.

Electrical System - The electrical system is designed to provide for the following circuits:

24 volt starting system on the engine

Battery recharging systems

Dredge pump and auxiliary engine gauges and shutdowns

Solenoid operated hydraulic control valves

24 volt internal and external lighting system

Control center and operating controls. To promote safety in operation, flexibility, and efficiency the controls are mounted on a console in the control center to provide for one-man operation of the dredging sequence.

Support & Auxiliary Equipment

A single engine 235 HP, 10m long X 3m wide X 1.3m deep tugboat. This is used to move the dredges over a long distance and to pull heavy steel anchors.

A single engine 50 HP, 8m long X 3m wide X 1.3m deep service boat. Heavy parts for the dredges are loaded on this boat. It is used by mechanical maintenance personnel.

A hydraulic monitor and a 12x10 centrifugal pump powered by a 304 HP engine. All mounted on a heavy barge 23m long X 12m wide X 1.25m deep. This is being used to sluice high steep banks of tailing above the water surface that may collapse into the bucketwheel when being dredged.

A barge 10m long X 4.5m wide X 1.8m deep with one 15-ton winch for use in lifting/pulling anchors, reflow lines and others.

A service boat with 9.9 HP outboard motor.

A welding machine/cutting outfit set abroad a small floating deck.

A floating fuel tank with 15,000 liters capacity.

Monitoring of Tailings
The discharge from each dredge is pumped through 16" diameter reflow lines of pipes and flexible hoses into a three compartment bypass box prior to being discharged into the mixing box leading to the tailing flume. It is at the discharge point in the bypass box that the pulp pumped by each dredge is monitored as to its suitability for transport through the tailing disposal system. A nuclear gauge measures the density of the pulp being discharged into each compartment. As an added check on density, a manually activated sampler cuts and collects the pulp being discharged. These samples are then checked for density and settling rate. If the density is below 45% solids and settling rate exceeds 4 feet per hour, the dredge discharge is channeled back to the pond. Discharged to the flume of material with a settling rate in excess of 5 feet per hour or less than 40% solids density has resulted in flume handling and overflowing. During these conditions, the dredge is moved to a new location for better dredging materials where production will be continuous. The practice is to blend the coarse sand from one dredge with slime from the other to produce a pulp mixture of proper density and particle settling rate.

Dredge Planning

A planner-side plots the location of the dredges by triangulation with the use of the sextant. The location of the dredges is plotted on a map and the dredge movements for the next 24 hours are indicated on the map and in the field.

The production of the bucketwheel dredges is based on a volumetric survey made at the end of the month.

In monthly planning and long range scheduling, the average mechanical and operating performance during the past months, the topography of the entire pond considering the thickness of the remaining tailing, the pond's bottom configuration and the physical distribution of sand and slimes are all considered

Accompanying the dredge production schedule is a projected monthly water balance which indicates the water level in the pond based on the outflow discharged by the dredges, evaporation loss and seepage. Make-up water into the pond is provided as necessary depending on the water requirement. The rate of expected inflow is dependent upon the projected monthly rainfall based on the average rainfall over a period of ten years.

Project Status
Experience gathered in the first phase of dredging using the pneumatic dredges and in the final dredging utilizing the bucketwheel dredges, showed that the Ellicott® Series B890 "Wheel-Dragon" dredges are very capable of successfully completing the removal of the mill tailing that was deposited over the San Antonio mineral deposit. On several occasions the dredge was used in digging compacted laterite and gravel within pond without difficulties except for the handling of the laterite and gravel at the mixing bowl.

Subsequently, the two Ellicott® Bucketwheel dredges were bought by a mining contractor who used them successfully mining hard compacted salt formations in the Dead Sea. The dredges are still in use there today.

Reprinted from Asia Pacific Mining

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