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VSF at RT.

John Chadwick reports on Outokumpu's Vertical Smooth Flow (VSF) copper solvent extraction at Radomiro Tomic, Codelco's newest copper mine.

Radomiro Tomic (RT), at 180,000 t/y (already expanded from the start-up at 150,000 t/y) of cathode copper, is one of the largest SX-EW plants in the world. It is the lowest cost of all of Codelco's operations, with cash production costs of US$ 0.38/lb. Although it is Codelco's fifth Division, it is the first large-scale project that Codelco has developed from start to finish.

The mine lies some 45 km north of Calama in Chile's Region II, 3,000 m above sea level. RT is a resource of 2,510 Mt of cupriferous mineralisation with an average grade of 0.59%, of which 970 Mt is oxide and considered leachable. The rest is refractory ore - sulphides and exotic minerals. The project itself is based on a reserve of 671 Mt at a grade of 0.73% Cu. Conventional truck and shovel mining produces 90,000 t of ore and 135,000 t of waste each day. The average SX-EW recovery rate is some 78%.

The mine as a whole is a zero-discharge facility. To ensure that no pollution at all is introduced to the region's groundwater, fail-safe technologies prevent any discharges, even if something does go wrong.

The establishment of the Radomiro Tomic division and project was an important milestone in Codelco's history and of major significance for its future. RT, in addition to being of considerable strategic importance to the company's future development, is also of significance in that it is the first 'megaproject' that Codelco developed entirely on its own, from initial exploration through to mining and marketing the production. The project required an investment of over US$640 million.

Three P&H 4100A electric shovels, two delivered in 1996 and one in 1998, load 218 t capacity trucks. The shovels have 44.3 [m.sup.3] dippers and get long-term, life-cycle management support under a six-year repair and maintenance agreement by P&H MinePro Services. Each of the older shovels is averaging about 105,000 t/d of material, with an availability of over 93%.

RT uses re-usable (on-off) heap leach pads. First run-of-mine ore is reduced in size by a 7,000 t/h Fuller primary crusher. Ore from here is conveyed onto a 60,000 t live stockpile which feeds the secondary 5,500 t/h Nordberg Symons crusher. This produces the minus-51 mm material for the leach pads.

For the heap leach, RAHCO designed and has supplied two 305 m long crawler-mounted, self propelled, conveyors that operate at a capacity of 10,000 t/h. One conveyor is a Mobile Stacking Conveyor (MSC) while the other is a Mobile Reclaim Conveyor (MRC). These are the largest capacity MSC and MRC ever built by RAHCO. Equipped with 1,828 mm wide belts, both are designed to handle a minimum of 10,000 t/h of copper ore. The project employs the re-useable 'racetrack' leach pad operating concept. Computer programmed control systems, including Programmable Logic Controllers and GPS, make this equipment very easy to operate accurately. The operator need only monitor the system and, if needed, can override the computer controls.

The two parallel leach pads, each 300 m wide by 1,300 m long have been established 50 m apart to allow for a solution collection ditch. They have been designed to be stacked in 8 m lifts.

The full feasibility study of the Radomiro Tomic project was undertaken using the services of various companies, under the overall management of Bechtel. The cost estimates used are shown in the table.

VSF extraction

The most significant new technology at the mine is Outokumpu's VSF (Vertical Smooth Flow) copper solvent extraction circuit[2]. When the pregnant leach solution (PLS) is first mixed with an organic solution to extract the copper from the PLS, this mixing process is where VSF technology differs radically from the norm. Key features of VSF technology include separate pumping and mixing, low head pumping by a dispersion overflow pump and gentle low-speed mixing maintained by a helical Spirok mixer. The gentle pumping and mixing eliminate overmixed dispersion, which is characteristic of conventional blade stirrers.

VSF mixer tanks are sealed and a slight overpressure is maintained in them, which prevents air intake and helps to cut the formation of crud to 20-30% of conventional designs, as well as reducing organic reagent losses. The carryover of impurities to electrowinning is low, which provides high copper quality; the VSF process can produce up to 99.999% pure copper. According to Outokumpu, VSF further features overall low power consumption and variable throughput rates. The need for auxiliary equipment is reduced, which decreases the initial investment cost and maintenance investment.
Radomiro Tomic - capital cost estimate

Capital pre-production

                                                $US million

Geology and mining                                      137
Crushing and heap construction                          144
Solution reticulation and SX-EW plant                   131
Infrastructure and indirects                             55
EPCM and owner's costs                                  123
Contingency                                              51

Total                                                   641

From Behn et al(1)

Radomiro Tomic - operating cost estimate/lb fine Cu

Area/process                                       US cents

Geology and mining                                    14.73
Crushing and heap construction                         4.44
Solution reticulation and SX-EW plant                 15.87
General services and administration                    2.32
Operating costs                                       37.36
Sales costs and marine transport                       2.13
Direct costs                                          39.49
Depreciation                                          10.87

Total cathode costs                                   50.36

From Behn et al(1)                                        1


Mine development, having been on and off in the early years of the decade, was given the go-ahead again in 1995. Between October and December that year, Outokumpu made a proposal and Codelco placed an order for two solvent extraction trains as a turnkey delivery. The facility was to include the latest process improvements based on field experience, in particular from the Outokumpu/Placer Dome Zaldivar joint venture (MM, November 1994, pp. 271-277), and ongoing work at Outokumpu's Research Centre. Because of Codelco's desire for rapid delivery, Outokumpu developed element technology to construct the settlers, settler area and some of the DOPs (dispersion overflow pumps) and mixer tanks (pre-fabricated element technology).

Outokumpu provided basic and detailed engineering and construction of the SX area, training, implementation, startup and follow up on the process. Basic design and manufacture of the proprietary equipment started soon after Codelco's order was received. Basic engineering plans were ready by March and all proprietary equipment was on site by November 1996. Civil works had already started in mid-1996, along with element fabrication in the nearby city of Calama. The two SX trains were ready for their first water test runs in June 1997 and commissioning was completed towards the end of that year.

Good leaching

The good leaching characteristics of RT's oxide ore, composed primarily of atacamite and chrysocolla, led to a leaching time calculation of 45 days. Further estimates for the process design were a copper content of 6.5 g/litre in the PLS, at a total flow rate of 3,000 [m.sup.3]/h. The main contamination is chloride, expected to be some 23 g/litre in the PLS when purely atacamite is the ore being leached, falling to 13 g/litre when mixed with chrysocolla.

High copper and chlorine contents in the PLS, together with a low pH set strict requirements for design and construction, particularly in material selection. For example, only 316L stainless steel could be used in the electrolyte circuit. All tanks, settlers, recirculation valves, PLS and LO (loaded organic) valves and instrumentation had to be chloride resistant.

The solvent extraction (SX) process layout is 2L (leaching) +W (washing) +S (stripping), with a loaded organic (LO) tank. The external O/A ratio in extraction is 1.07 to lower the amount of the reagent and to ensure organic continuity. As in the first pilot plant at Codelco's Chuquicamata operation (MM, November 1994, pp. 275-277), organic solution leaving the LO circuit is guided via organic coalescers to the LO tank. From there it is pumped back to SX, the washing settler. The electrolyte circuit has one stripping stage in SX and in the tankfarm and electrowinning area flotation columns, a filter feed tank, a recirculation tank, scavenger cells and commercial cells. The leach circuit has four PLS ponds, two extraction stages and one raffinate pond.

Originally VSF for copper started from the design of the pump mixers and secondary mixers, but now includes process and settler design. The DOP and the mixers are an integral part of the settler with connecting pipes, thus representing a whole new SX system, including the tank farm.

The most important operating criteria for an SX plant are its operating capacity and the stage efficiency of specific mixer-settlers. The washing efficiency of the wash mixer-settler is also important because of RT's high chloride content. If too much chloride enters the electrolyte circuit, it will not only attack all stainless steel but also affect the handling and quality of the cathodes.

The mixer-settlers were designed for solution flows and tenors determined in the feasibility study. In these design calculations, the balance value of the PLS flow through one train was 1,500 [m.sup.3]/h and the total flow through one mixer-settler 3,500 [m.sup.3]/h. Considering this and a conservative specific flow of 3.8 [m.sup.3]/[m.sup.2]h a settler size of 28.5 x 28.5 [m.sup.2] effective settling area was chosen by Codelco. The organic linear speed of the settler is thus 52 mm/s with an organic band thickness of 300 mm. With a total retention time of three minutes in the DOP and two Spirok mixers, a stage efficiency of 98% is achieved at the extraction stage and even higher in the stripping stage.

The primary aim in designing the SX equipment was to ensure that all pumping and mixing are as smooth as possible. Similarly, the DOP turbine and Spirok mixer are designed for specially gentle handling of solutions and dispersion. To ensure pumping and stage efficiency in a conventional system, the pump mixer has to be run at high rpms. Naturally, this leads to over pumping with respect to capacity and entrainment considerations. This also causes a great deal of carryover of impurities to the copper electrolyte. This is why pumping and mixing are separated in VSF technology.

The heat generated in the DOP is very low because of the need to forward the solutions without excessive shear. In normal process conditions, the head generated is in the order of 200-400 mm [H.sup.2]O, for both suction and outward flow. This places strict demands and limits on settler and piping design. Also, unnecessary flow resistances cannot be tolerated anywhere in the system.

Hydraulic balance calculations provide the main parameters affecting the DOP-pump performance. Interconnecting pipes have to be of a sufficient diameter without unnecessary bends. Furthermore, inner circulation should not be choked by pipes that are too small or constrictions for flowmeters or control valves.

In summary, these mixer-settlers, with a throughput capacity of 3,500 [m.sup.3]/h are among the largest such installations in the world. The divergent process conditions such as a high Cu content of 6-10 g/litre and a Cl content of 13-23 g/litre set their own requirements for smooth operation. With VSF technology, the solutions are pumped and mixed separately. Settling takes place in a 28.5 m wide settler; relatively deep compared with conventional design.

The performance of the DOP, Spirok-mixer and 3D-settler, and thus the SX process, is secured by using such advanced design details as.

* A guiding primary distributor in the feed end of the settler, allowing the flow to distribute evenly over the full width;

* Two sets of picket fences accumulate the dispersion in the feed end of the settler as dense layers. This also helps collect entrainment mist droplets into the mother solution;

* The aqueous inner circulation is taken from the settler bottom rather than from a collecting launder to prevent accidental organic flow to the electrolyte; and

* Very precise, non-leaking organic and aqueous launders are used.

Pilot optimisation

In the middle of last year Outokumpu Tecnica Chile, dispatched a VSF SX Pilot 100 plant to the Zaldivar mine in northern Chile, site of its first fully commercial VSF plant. This is the second such mobile pilot plant that Outokumpu has put together. The first, not automated like this latest plant, is now permanently sited at the Morenci copper mine in Arizona.

The Zaldivar pilot, 1/500th the capacity of the mine's actual VSF plant, is being used to conduct tests on process automation and optimisation. Housed in a 12 m long, open-sided container, the pilot plant's VSF copper SX train can be configured in a number of ways - two extraction, one stripping and one washing module, as shown in the diagram, or two extraction and two stripping, or two extraction and one stripping. The design PLS flow capacity is 3 [m.sup.3]/h.

In the control more, a Courier 30SX system analyses samples of process streams from the solvent extraction processes to make elemental analyses (Cu, Fe, Mn). This is supplemented by two OTI 99 titrators to analyse supplementary ionic contents such as Cl and [H.sub.2.]S[O.sub.4]. Results from the Courier 30SX and titrators as well as signals from field instrumentation, are sent to a Proscon 2100 NT process control system.

Although only used in copper recovery so far, VSF solvent extraction can be used for nickel, cobalt, molybdenum, zinc, magnesium and more. Its advantages include low organic and aqueous entrainments, a lower basic organic inventory than conventional plants, minimum crud formation, low requirement for electrolyte filtering and total air exclusion in pumping and mixing. In copper, VSF has shown it gives superior operating results than conventional systems, providing maximum recovery at minimum operating costs.

Bechtel - ARA

The Chilean engineering company Arze Recine y Asociados (ARA) is one of Bechtel's frequent partners within Chile, particularly on SX-EW projects. Bechtel had started work on Radomiro Tomic back in 1991 (when it was known as Pampa Norte), on prefeasibility and feasibility studies. The project was deferred in 1993, by which time Bechtel and ARA together had begun basic engineering based on the scope and concepts developed in the feasibility study. They came back in together when the project was resurrected.

This was a fast track EPCM contract. Once the basic engineering was completed, detailed engineering, major equipment procurement and construction developed simultaneously. There was a very specific sequence to be followed in which the largest, most critical items were committed first, before adding in all the secondaries.

The joint venture was not directly involved with the VSF facilities, but had to connect them in with the rest of the plant, working up to the VSF technology with all the piping and interconnecting interfaces. Bechtel had to be the co-ordinating engineers bringing all this together. Similarly, Codelco chose to install a filtering and coalescing process to purify part of the flow streams, and chose its own Chuquicamata technology for filtration. Chuquicamata was thus able to use RT to expand the use of Codelco's technology on a larger scale. Bechtel assisted with purchasing components for the system and made certain that it fitted into the project as a whole.

Radomiro Tomic was the first project in Latin America to use Bechtel's Safety Work Observation Process (SWOP). Rather than supervisors checking on the work, the workers themselves go around the various work sites, watching and commenting on their colleagues safety practices. In this way, they can both learn from and teach one another. Bechtel has found this improves communications and promotes a better safety among the workforce. The RT project achieved two long periods of 1.8 million and 1.9 million hours, without a lost time accident.

The contract was completed three months ahead of schedule in October 1997, allowing initial production to begin that November. It also came in some $53 million under the original budget predictions. Thus, the original capital requirements were cut to $588 million from $641 million and Bechtel claims some $22.5 million was in savings it achieved.

References

1. Behn et al, Estimation of resources and engineering of reserves, Radomiro Tomic project, Codelco-Chile, Transactions of the Institution of Mining and Metallurgy, May-August 1998, pp. A83-A101.

2. Taipale et al, Implementation of Outokumpu's VSF (Vertical Smooth Flow) Cu Solvent Extraction Plant Project at Codelco's Radomiro Tomic Mine, Chile; Randol's Copper Hydromet Roundtable, Vancouver, November 1997.

3. Bechtel 100 Years, The Weekly Mining News, Santiago, Chile.
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Title Annotation:vertical smooth flow copper solvent extraction at Radomiro Tomic copper mine
Author:Chadwick, John
Publication:Mining Magazine
Date:Mar 1, 1999
Words:2761
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