A Custom Fit
While it was rigorously testing a customized use of hydrometallurgical technology to assure commercial viability for its mammoth nickel-mining project, Vale Canada Ltd. was also testing a comprehensive effluent treatment program that incorporates new high-rate softening and clarification technology to help protect the environment.
International mining company, Vale, is nearing completion of its US$3.7-billion nickel processing plant at Long Harbour, on Newfoundland’s Placentia Bay. The Brazilian mining company’s wholly owned subsidiary, Vale Canada Limited, formerly known as Vale Inco, is directing the construction of the processing plant, which began in April 2009.
Start-up of the plant is scheduled for August 2013. Once fully operational, it is expected to annually produce 50,000 metric tons of nickel, 4,700 metric tons of copper, and 2,500 metric tons of cobalt.
The mined ore will first undergo a concentrating process at the Voisey’s Bay mine site in Labrador before it is transported by ship to the processing plant at Long Harbour. By processing ore concentrate at the plant, Vale anticipates achieving higher metal recoveries while also eliminating the time and expense of shipping to Ontario or Manitoba for refining.
The processing plant will use an innovative hydrometallurgical (hydromet) processing technology researched and developed by Vale in Canada. Although several Canadian operations use hydrometallurgical processes, Vale has tailored this processing technology specifically to treat the nickel-cobalt-copper sulphide deposits at Voisey’s Bay. The overall flow sheet, patented by Vale in 2000, results in an efficient and cost effective integrated process that makes high quality finished metals.
Hydromet Technology Commercially Viable
Because Vale had never used hydromet processing technology on the type of ore present at Voisey’s Bay, the company invested $200-million on a 10-year R&D feasibility program to test the technical, economic, and environmental viability of the process.
Hydromet testing was conducted in three stages. The first involved bench-scale testing, which proved that each separate chemical process worked individually at laboratory scale. The second stage featured the operation of a 1:10,000 scale pilot plant at the company’s Mississauga, Ontario, research centre. The third stage involved the design, construction and operation of a 1:100 scale demonstration plant in Argentia, Newfoundland
The objective there was to confirm technical and economic viability and to assist engineers in the selection of the most appropriate construction materials and specification of major equipment.
Results at the Argentia demonstration plant confirmed to Vale that the technology would be technically, commercially and environmentally viable for this project. Use of the water-based hydromet technology will enable the plant to process the nickel concentrate directly to metal products without first having to smelt the concentrate. It also will allow Vale to process higher yields of the nickel and valuable cobalt, both of which are lost to a great extent in traditional smelting processes.
Water Supply & Treatment Challenges
With water being a crucial resource in all steps of the process, Vale was also challenged with determining how to effectively manage water usage and treatment to meet the process and potable water needs of the facility as well as regulatory and environmental discharge requirements. Following performance testing of a proprietary water treatment technology at the 1:100 scale demonstration plant, Vale contracted Veolia Water Solutions & Technologies to design, supply and manufacture a complete effluent treatment package for the facility’s process effluent prior to discharge into the environment.
Vale also contracted Veolia for a water treatment plant to provide process and potable water for the entire complex.
The processing plant’s raw water treatment plant will include ACTIFLO® sand-ballasted clarification, a proven high-rate settling process that combines the advantages of ballasted flocculation and lamella clarification. The ACTIFLO process operates with microsand as a seed for floc formation. The microsand provides surface area that enhances flocculation and also acts as a ballast to aid rapid settlement, resulting in a high-rate settling process with a very small footprint.
For treating the facility’s process effluent water, the treatment system will include the ACTIFLO Softening process, a new technology developed by VWS Canada initially for pilot testing at Vale’s Argentia Demonstration Plant.
Process Overview
At Vale’s Long Harbour facility, slurry from the Process Effluent Neutralation (PEN) system will go into the treatment plant’s residual storage area (RSA) pond. Overflow from this storage pond or, if necessary, the plant’s site settling ponds, will be directed to the system to capture metals and Total Suspended Solids (TSS).
Pilot testing at Vale’s Argentia Demonstration Plant demonstrated the system’s effective high-rate desaturation capability, if it’s ever required at the Long Harbour facility. The system has a single treatment line that includes: 1) a dynamic mixing zone stage that creates rapid mixing conditions to destabilize anions by adding metallic salts, if necessary; 2) an enhanced precipitation reactor designed to improve the reaction of lime and carbonate ion with the hardness and natural alkalinity of water to form insoluble compounds; 3) an accelerated flocculation stage to provide rapid development of large settleable flocs; and 4) a settling unit providing both gravity and enhanced lamella clarification.
Sludge is recirculated and reinjected into the precipitation reactor to improve the precipitation kinetics and optimize chemical consumption.
Testing
Veolia performed two series of jar tests for the Vale project, beginning in 2006, to initially validate the efficiency of the process in treating different water qualities. After successful jar testing, Vale requested Veolia conduct pilot tests at its Argentia demonstration plant.
Before these tests could proceed, however, Veolia was first challenged with designing and building a miniature pilot unit that could properly demonstrate the technology at the 1:100 scale demonstration plant. The then-current technology with pilot units required a minimum flow rate of 720 m3/d (30 m3/hr) of raw water. The miniaturized version required for operation in Vale’s demonstration plant had to efficiently operate at flow rates as low as 24 m3/d (1 m3/hr).
Veolia developed a unit with a “lab capacity” flow rate in order to demonstrate the applicability of this technology to the Vale demonstration plant’s raw water of limited stream capacity. This innovation, the “ACTIFLO Mini,” treats water with a flow rate 20 times smaller than the flow rate required for the standard ACTIFLO pilot unit.
Once developed and validated, the unit was performance tested at the Vale demonstration plant in 2007 to treat PEN water, with the goal of reducing TSS and heavy metals. The resulting data showed that the technology could meet or exceed the effluent quality criteria, producing a good clarified water quality and respecting regulation for each different stream.
For PEN overflow, for example, turbidity was reduced from 50 NTU to below 1 NTU in the clarified water, corresponding to a removal of 99%. Nickel was reduced from an average of 50 m
g/L to 0.2 mg/L in the clarified water, corresponding to a removal of 99%. Iron was reduced from an average of 1.4 mg/L to 0.2 mg/L in the clarified water, corresponding to a removal of 86%.
Additional tests were performed to verify the reproducibility of the results and showed clarified water quality with the same metal concentration ranges and same turbidity.
A pilot test at the 1:100 scale plant was conducted the following year using softening technology to treat CCD5 (counter current decantation) and PEN water. The main objective was to demonstrate the efficiency of the softening process in removing calcium and magnesium to reduce hardness. Tests were conducted both with and without sludge recirculation at various water qualities and flow rates.
Results showed the combination of softening and sludge recirculation achieved the water quality treatment objectives of TSS and calcium (Ca) concentrations lower than 15 mg/L and 300 mg/L as Ca, respectively, in the final effluent. Utilizing sludge recirculation in the softening process reduced chemical requirements by approximately 10 per cent.
The performance of the clarification and softening process was highly robust throughout the pilot trial and the technology reacted quickly to major changes in raw water quality and in TSS loading with sludge recirculation up to 3,000 mg/L. Treated water quality (turbidity, Ca concentration) was not affected by variations of inlet water quality.
A High-Rate Process
The precipitation reactor is a critical component in the high-rate process because it allows for complete homogenization of water and chemicals. The design of TURBOMIX™, a proprietary draft-tube reactor, combines the advantages of plug flow and complete mixing. It reduces the reactor tank volume by suppressing dead zones and decreases reagent loss by preventing short-circuiting. Sludge is re-circulated in the draft tube reactor so that large crystals form quickly and retention time is reduced. Recycling the sludge also increases the sludge waste concentration and thus reduces the volume of sludge for disposal.
With the addition of a draft tube into the system’s maturation tank, mixing and reaction kinetics are improved such that crystallization can occur more efficiently, requiring shorter reaction times and lower chemical demand. When this is combined with softening chemistry, calcium, magnesium, and other constituents may now be removed rapidly from water at high rates within a much smaller footprint.
The reactor immediately downstream from the precipitation reactor slowly mixes the precipitated particles with a flocculating polymer to promote the formation of large flocs. This stage includes the introduction of microsand to act as a nucleus for the flocs. The ballasted flocs settle rapidly and provide the system with its high rising velocity. The final stage includes a clarification tank providing lamella-enhanced settling.
Surpassing Standards
Based on the results of the demonstration trials, Vale selected two full-scale ACTIFO Softening units to treat the Long Harbour processing plant’s RSA decant water prior to discharge into the environment. It also selected three ACTIFLO high-rate clarification units to treat raw pond water to provide process and potable water for the facility.
Vale has stated that, through continuous investment in technology and processes, it aims to surpass international sustainability standards for its global operations. The water treatment systems currently being installed at its new Long Harbour processing plant will further help Vale meet this goal.
Marianne Dupla is Technical Services Commissioning and Piloting Supervisor, and Dave Oliphant is Director of Industrial Project Development, Veolia Water Solutions & Technologies Canada Inc.
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