Good results from Baja Mining pilot programme

The Demonstration Phase 2 Pilot Plant campaign at SGS Lakefield Research treating ore from Baja Mining’s Boleo property, Baja California Sur, Mexico, has been completed. The six-week pilot plant campaign treated a composite sample totalling 4.2 t of ore. The purpose of the campaign was to:

  1. Finalize design criteria for recoveries of metals and consumption of key consumables to be used in the Definitive Feasibility Study being prepared by Bateman Engineering Canada.
  2. Provide a design basis for Bateman to give process guarantees in the next phase of the project which is detail design and procurement.
  3. Demonstrate that the high clay content of the Boleo ore does not present a problem for washing in the Counter Current Decantation (CCD) circuit.
  4. Produce suffiicient quantities of copper cathode, cobalt cathode and zinc sulphate to enable off-take buyers to evaluate the quality of product(s).
  5. Produce a saleable manganese product (manganese carbonate);
  6. Demonstrate that the limestone located at Boleo is suitable for use as the neutralizing agent.
  7. Confirm that CSIRO Direct Solvent Extraction (DSX) technology is capable of giving good separation and recovery of cobalt and zinc into saleable products.
  8. Provide an opportunity for equipment vendors to work with Bateman on equipment design.

Test results met or exceeded the desired objectives in all aspects. The extractions of key metals in the leaching circuit under design conditions were: copper 91%; cobalt 82%; zinc 55%; manganese 97%. A number of specialist assays of final products are still pending but it is expected that the copper cathode grade meets LME Grade ‘A’ specifications (as was the case in the November 2004 pilot plant) and the cobalt metal is suitable for sale without requiring further refining.

The overflow of the metal bearing solution from the CCD circuit was very clear and leach residues settled quickly. The company’s locally available limestone was used for the entire test period. Several equipment vendors were on-site for the test period and the data collected is being used for design of several of the solid/liquid separation steps in the anticipated flowsheet. Good separation of zinc and cobalt was achieved and the zinc solution collected is suitable for production of zinc sulphate that can be sold in the fertilizer and animal feed markets.

The demonstration pilot plant flowsheet encompassed all of the unit processes of the expected full scale circuit and comprised the following steps:

  • Acid oxidation leaching of the ore with sulfuric acid addition
  • Acid reduction leaching of the ore with sulphur dioxide and sulphuric acid
  • Partial neutralization with Boleo limestone (local, low-cost limestone, available on the Boleo mining lease)
  • CCD washing of the leach residue in thickeners (to separate the metal rich aqueous solution from the clayey waste)
  • Copper SX/EW
  • Iron removal by pH adjustment, with Boleo limestone, and oxidation with oxygen (and polish with hydrogen peroxide)
  • Thickening and filtration/washing of the iron residue
  • DSX technology for selective recovery of cobalt and zinc (and small amounts of residual copper)
  • Manganese carbonate precipitation from the raffinate (barren solution) emanating from the DSX solvent extraction circuit
  • Thickening, filtration and washing of the manganese carbonate by-product
  • Purification of the zinc and cobalt strip solutions from the DSX solvent extraction circuit using zinc dust cementation and ion exchange
  • Recovery of cobalt cathode from the DSX cobalt strip solution using an SX and EW process.

The pilot plant was constructed by Lakefield technical staff to the specification of Bateman and the Baja Mining. It was operated by a team of approximately 60 Lakefield staff (working on 4 shifts), backed up by Lakefield’s in-house analytical team. Bateman and the company’s staff were present at Lakefield to observe and help in overall supervision/control of pilot plant operation. Additional technical specialists from the following organizations were present during the pilot plant to make measurements on key parts of the process flowsheet. Technical specialists present included:

  • Pocock Industrial – performed specialist testwork on solid/liquid separation and rheology of slurries
  • Mixtec – performed agitation studies
  • RPA Process – performed filtration studies
  • Outokumpu Technology – performed specialist testwork on high rate thickener design and flocculant selection and dosage.


In addition to on-site specialists, the Saskatchewan Research Council (SRC) has received a sample of leach slurry and is testing the slurry pumping characteristics. Dorr-Oliver Eimco is performing paste-thickening studies on a sample of leach slurry. Separate reports from each of these technical groups will be received and incorporated in the final engineering design of the Boleo metallurgical facility.

The CCD circuit to wash barren solids worked very well. The CCD was set up to simulate the use of high rate type thickeners with recirculation of overflow solution to dilute the feed slurry prior to flocculation. The leach residue settled quickly producing clear overflow solutions to advance to copper, cobalt, zinc and manganese recovery.

Copper was recovered from the clarified solution using conventional SX/EW. An average extraction of 98.5% of the copper was achieved through solvent extraction (the remaining copper in the raffinate is partly recovered by recycle of the raffinate to the ore milling circuit). A total of 47 kg of copper metal was recovered by electrowinning at a current efficiency of greater than 97%. The copper SX raffinate was split with a portion of the raffinate recycled to leaching and a portion advanced to cobalt, zinc and manganese recovery.

The iron removal circuit was designed to remove iron, aluminum and other impurities from the solution prior to recovery of cobalt and zinc using DSX technology from CSIRO. The iron removal circuit consistently produced very low concentrations of key impurities in solution with negligible losses of cobalt and zinc. Boleo limestone was successfully used throughout the pilot plant for neutralization during iron removal thus confirming its suitability for use in the circuit at Boleo.

The CSIRO DSX circuit for cobalt and zinc recovery performed very well. The feed to the DSX circuit was the solution after iron removal. The average composition was 789 mg/litre Zn, 219 mg/litre Co, 44 mg/litre Cu, 1.6 mg/litre Fe, 10,038 mg/litre Mn, 600 mg/litre Ca, 6,612 mg/litre Mg and 14,900 mg/litre Cl. The raffinate (barren solution) from DSX contained on average 4.2 mg/litre of Zn and 0.95 mg/litre of Co. These values confirm that over 99% of both Zn and Co were recovered through the DSX circuit.

The two products from the DSX circuit were a zinc strip solution and a cobalt strip solution. The selective stripping of zinc and cobalt were gradually optimized during the pilot plant.

The zinc strip solution was generally very pure. The average Zn/Co ratio in the zinc strip solution was about 47:1. The small amount of cobalt impurity with the zinc strip solution would represent a loss of a small amount of cobalt from the circuit if allowed to go to final product (less than ~ 5% of the cobalt). A small bench scale test programme is underway at SGS Lakefield to demonstrate that the cobalt in the zinc strip can be recovered and recycled in the process to eliminate this potential loss. The purified zinc strip solution (after zinc dust cementation to remove cadmium) will be crystallized to form zinc sulphate crystals using evaporative crystallization.

The cobalt strip solution from the DSX circuit was rich in cobalt but still contained large amounts of zinc and other minor impurities. The cobalt strip solution was accumulated during the main part of the leach pilot plant. The strip solution was then treated by zinc dust cementation to remove copper and cadium followed by an anion exchange purification process step. The strip solution was then sent to a zinc/cobalt solvent extraction circuit for separation and purification of the zinc and cobalt. The purified zinc solution will join the DSX zinc sulphate solution as feed to zinc sulfate crystallization. The purified cobalt solution was electrowon to produce cobalt metal.

The results of the cobalt/zinc SX and cobalt EW processes were excellent. The Cyanex 272 solvent extractant was used to separate and purify these two elements. Over 99.5% of the zinc and cobalt was recovered through these solvent extraction circuits.

The cobalt strip solution was passed through two ion exchange columns for minor element purification prior to EW to deposit cobalt metal. The cell design used a lead anode with anode bag opposite a stainless steel blank for cobalt deposition. A total of nearly 2 kg of cobalt was deposited during the pilot plant at a current efficiency of 89%.

A portion of the cobalt strip solution (after ion exchange for minor element control) was also withdrawn and precipitated with sodium carbonate to make cobalt carbonate as a potential alternative cobalt product. The last product that was recovered from the demonstration pilot plant was manganese carbonate. Manganese carbonate was precipitated from the DSX raffinate solution using sodium carbonate. The manganese carbonate was thickened, filtered and washed. The barren solution after manganese precipitation was returned to the circuit as wash water for the CCD. The manganese precipitation was very selective with an average (for all days except startup day) manganese content of 44.2%.

In related news, Baja Mining has acquired an option to purchase four diesel driven generator sets from a US power company. Each unit is rated at 2.5 MW, providing a total of 10MW. The generation plant had been used to provide standby power for a US East Coast island. The units are in excellent condition as they have very low operating hours and are maintained in a hot standby state. The modules are skid-mounted and are thus easily re-locatable. The option provides for a series of payments that, in total, will amount to approximately 10% of the cost of equivalent new units.

El Boleo project will require an average of 34 MW of power when fully operational and its power supply will be totally independent of the power grid of the State of Baja California Sur. Most of the power will be provided through heat recovery and steam generation from the acid plant. The balance of the requirements will be generated from diesel fired units. The acquisition of the 10 MW generating plant will cover most of this additional power and will more than adequately provide power for the construction activities. The units will remain in storage, on hot standby, in the USA until required at El Boleo and will be moved to site prior to the start of construction in 2007.