There are no major innovations in any of these plants. In fact, Australia is really only beginning to get started in CIP research and in process and design development. The plants described here have borrowed from technology, primarily in the United States and South Africa, and although the plants were carefully designed and based on sufficient (if generally minimal) testwork, they are not generally the product of years of research and analysis. The plants are, however, well-designed plants which operate efficiently.
Adsorption
The peripheral screen system uses 8 screen panels per tank, curved to the radius of the adsorption tank. These panels are fixed vertically to the top frame of the tank providing about 90% screen area for overflow.
The airwash system consists of perforated air pipe covered with a Linotex sleeve containing trans verse slits at 30mm (1 in.) intervals. The pipe, supported and fed at each quadrant of the tank, is located 55 mm (2.2 ins.) below and 25 mm (1 in) from the screen. The screens, as currently operated, pass a maximum of 64 tph of solids at about 46% density through 7.3m (24 ft.) of 20 mesh peripheral screen using 7m³/min (240 cfm) of 280 kpa (40 psi) air.
In addition to its reasonable air consumption and flow rates, the Windarra Nickel Project system has a number of favourable operating points. The system is, to a large extent, self regulating. If the pulp level in the tank rises, the air bubbles blow against a larger area of screen and tend to bring the level down again. Should the level in the tanks still continue to rise, simply increasing the air volume will usually clear the screens and drop the pulp level in a few minutes.
The screens are reasonably readily removed. Also, as shown in Figure 2, the air bubbler pipes are connected for quick complete removal if required.
Elution
Initially there was some concern about the possibility of caxbon wedging into the bottom screen and eventually blinding it. In fact, initial operating procedures called for keeping at least 1 meter of solution in the elution column before beginning to transfer carbon, to minimize the possibility of plugging the screen with the first few particles of carbon transferred. Since start-up, however, there appears to be no such problem with pegging.
One problem, however, related to the fines in the system (see Carbon Movement – Windarra) did occur. After about three weeks of operation it became increasingly difficult to move carbon from the elution vessel. Washing with large quantities of water, either from the top or bottom, had little effect. Opening the vessel showed that carbon at a very steep angle of repose had built up on the wedge wire cone, completely blocking all of the screen except for a small area immediately above the backwash inlet.
Electrowinning
The Zadra-type electrolytic cells at the Queen Margaret stripping plant are simple and inexpensive to construct and operate, and give good operating efficiencies.
For better contact between the cathode grid and the steel wool, a bit of wool is teased up through the grid and twist-tied in about four or five places in each quadrant of the grid.
The cathode grid is sandblasted after every strip to clean it, and lasts 20 or 30 strips.
The Haveluck operation uses two rectangular cells in series, and reports a 90% efficiency in the circuit.
The cells are made of mild steel, painted with an epoxy paint, and are large enough to hold six cathodes and three anodes. The cathodes are of stainless steel, 680 x 850 mm (27×33 in.), with 80 mm (3 in.) spikes to support the steel wool. The steel wool is held to the cathode by an open-backed polyethylene cathode box into which the cathode can be mounted. The cathode forms the back of the box 10 mm holes are present in the front of the cathode box. The cathodes in their boxes (or baskets) fit loosely into the cell, with about 6 mm (½ in.) clearance in each side. The stainless steel anodes slip tightly into slide guides in the cell. In each cell one of the anodes is unperforated, and two have 10 mm (3/8 in.) holes on their bottom one third.
Reactivation
Regeneration is done in General Furnace Construction rotary kilns made in Sydney, Australia. Both have given good service from the start, with minor mechanical and electrical problems. The system started up with no difficulties, and has presented absolutely no problems.
The feed hopper, as shown, is free to drain contained water from the carbon at all times, but the drain capacity is small. The hopper is open to the atmosphere and would have to be covered if it were to be used during the infrequent rainstorms we have. The fact that Windarra has an extremely arid atmosphere seems to have little bearing on the feed system. Although the carbon dries almost immediately on top, it will remain damp under the top layer for an extended period. It feeds well with no need whatsoever for water sprays or washing.
When carbon is being educted to the feed hopper the Linotex valve is fully closed and the 50 mm (2 in.) gate valve to the overflow (slops tank) is fully open. When excess water to the carbon is drained away the Linotex: valve is opened. The 25 mm (1 in.) valve with the quick-disconnect fitting is used when carbon hangs up in the hopper.
Acid washing is performed on every cycle. Concentrated hydrochloric acid is diluted with hot water to produce 3% acid at 80°C, and pumped through the carbon which is held in a rubber-lined storage vessel. The acid is kept in the carbon for one hour. Live steam from the boiler is injected into the bed to maintain the 80° temperature. After a 20 minute drain cycle the bed is flushed with 12 kl (3200 gal) of fresh water and the carbon is then educted to the autoclave. Both the rinse water and the spent acid are pumped out with the adsorption tails.
Instrumentation and Process Control
The elution plant is designed for minimum manning and has a relatively high degree of automation. Loaded carbon transport and treatment are effectively controlled by a CPU (Central Processing Unit) working in conjunction with timers, digital counters and automatic controllers.
- Lime Feed Control
- Cyanide Feed Control
- Surge and Conditioning Tank Level Indicator
- Leach Pachuca Feed Density and Flow
- Leach Pachucas Total Air Flow, Individual Air Flow
- Air Lift Controller – Adsorption Vessels
Particular Problem Areas
A refining problem, peculiar to CIP, came to light in some of the initial refining. The problem was related to the presence of fine carbon in the elution vessel. Initially, as elution continued through the pressure vessel, some extremely fine carbon was washed out. This carbon was effectively removed from the system at the electrowinning cell: both by being caught in the steel wool and by being dropped to the bottom of the cell as solution velocity decreased through the cell.
For refining, the steel wool is normally acided, then filtered and dried before mixing with fluxes. When carbon was present with the acided gold sludge, the carbon floated to the top of the melt in the furnace and did not melt even at the highest temperature possible with the furnace. When poured it formed a thin layer on the slag.
The presence of such high levels of Ni has definitely adversely affected the gold loadings possible. Gold loadings at both plants seldom exceed 4000 gm/t (130 oz/T). Soluble losses at both plants are acceptably low (vic. .02 gm/t) (0.5 mo/T)).
A detailed systematic study of the effects of high nickel levels on CIP at Western Mining operations has not been carried out, although shorter studies have been made in the course of starting up and operating the plants.
