By: Róger Asunción Saldaña, Benito Quiñones Kong y Sergio Vicuña Diaz, Newmont Yanacocha.

Abstract

The Yanacocha mine is located in northeastern Peru in the province of Cajamarca.

Approximately 25 km to the north. The mining facilities are located between 3,700 and 4,100 masl and occupy a total area of approximately 125 km².

The Yanacocha mine is operated under the jurisdiction of Newmont Yanacocha. The operation comprises open pits, leach pads and ponds, processing plants and excess and acid water treatment.

Continued operation of the mine will require water treatment for discharge to the environment that complies with the new ECA maximum permissible limits and conforms to the requirements of future Peruvian regulations.

These new processes for water treatment include the INCO process, Ultrafiltration and Reverse Osmosis.

Introduction

Water management at Newmont Yanacocha has allowed us to meet the requirements and quality standards defined in Peruvian regulations, but also in the quantities required to meet the needs of the communities near the mine.

The constant changes in our operation and the adaptation to new processes and standards, make it necessary to continuously reconfigure the water management and treatment strategy. Thus, the appearance of new maximum permissible limits (MPL) in the ECA, or the need to adapt the plants for new projects such as sulfides, motivates us to identify solutions that meet the these changes, such as the incorporation or adaptation of INCO technologies, ultrafiltration and reverse osmosis.

The challenge to achieve the MPLs defined in the ECA is the treatment of the exhausted solution (effluent) coming from the carbon-in-column (CIC) plants that enters the water treatment plant, this means that high values are expected with respect to cyanide, copper, iron, mercury, ammonia nitrogen, TDS; and that they must be treated to be discharged to the environment within the accepted parameters.

INCO process

This process of cyanide destruction by SO₂/air is based on a catalytic reaction involving cupric ion as a catalytic agent. According to the chemical reaction, the cyanide is oxidized to the form of cyanate ion OCN-, which is a harmless compound. The oxidation of free cyanide and weak cyanide complexes which are part of WAD cyanide occurs according to the following reactions:

Reaction 1:

CN- + SO₂ + O₂ + H₂O → OCN- + H₂SO₄

Reaction 2:

M(CN)₄^-2+4SO₂+4O₂+4H₂O→ 4OCN-+4H₂SO₄+M²⁺

M = metal forming a weak bond with cyanide

(Ag, Cd, Hg, Cu, Ni and Zn)

In the case of the La Quinua EWTP plant design, the SO₂ ion is obtained by using the chemical reagent sodium metabisulfite (Na₂S₂O₅), the oxygen comes from the air which is added by means of blowers and the cyanide is found in the aqueous medium of the barren solution to be treated coming from the La Quinua carbon-in-column plant. The chemical reaction considering sodium metabisulfite and WAD cyanide concentration for the plant design is as follows:

Reaction 3:

Na₂S₂O₅+2O₂+H₂O+2CN-(WAD)→2CNO-+2NaHSO₄

The design considers two reaction tanks with a residence time of 30 minutes in each tank. As can be seen in chemical reactions 1 and 2, sulfuric acid is formed, so it is necessary to add lime to maintain the optimum pH. The dosing of reagents must be done in the first reactor, the second reactor is to continue giving residence time to the reaction with oxygen injection.

Cyanide Destruction

The objective of this process is to "destroy" the cyanide (CN-) in the feed. The barren solution is pumped to the two reactor tanks arranged in series, with a processing capacity of 700 m³/hr. As the WAD cyanide in the feed is expected to fluctuate, an on-line WAD cyanide meter monitors the feed and adjusts the dosage of sodium metabisulfite solution.

The reaction is normally carried out at pH 9.1 to 9.2, with lime normally required for pH control, and oxygen is supplied constantly by air blowers at a flow rate of 680 CFM. The sodium metabisulfite solution is dosed at a 10% concentration, with the flow rate based on the cyanide content in the feed.

Mercury precipitation and removal using sodium polymer thiocarbonate 

This mercury precipitation process is carried out by adding the sodium thiocarbonate (NaPTC) polymer chemical reagent, the chemical reaction is as follows:

NaPTC dosing and mercury precipitation control

In the second reactor, NaPTC is added in 5% solution, which is pumped from the dosing system to the overflow of the first tank to the second reactor in order to have a good mixing and reaction and to generate a more stable sludge.

The objective of this process is to reduce dissolved Hg from 120 ppb to 1.2 ppb, no additional residence time is required for this reaction.

Reaction time: 30 minutes per tank pH: in the range of 8.2 - 8.3

Lime slurry dosing: to maintain pH in the specified range NaPTC dosing: The reagent arrives in liquid form at 100% concentration and is dosed at 10% concentration, after dilution with water in the dosing line. To control the dosage of this reagent, samples are taken and sent to the laboratory for total and dissolved mercury tests.

Air dosing: constant at a flow rate of 600 CFM.

Clarification of the solution and discharge of sludge generated

The generation of solids, product of precipitation, requires a solid/liquid separation and for this purpose a vertical lamella system is used, equipped with a small rake that helps the formation of the filtration bed.

Control of clarification in vertical pretreatment lamella for handling of solids generated

The overflow from the second reactor tank is sent by gravity to the pretreatment lamella in order to control the turbidity of the solution by dosing ferric chloride to obtain a turbidity below 30 NTU; this clarified solution enters the UF feed tank and is then pumped to the Arkal filter system. The clarified solution from Arkal filters contains solids below 130 microns and feeds the ultrafiltration process.

Removal of residual solids through ultrafiltration 

The objective of the UF system is to remove suspended solids as a pretreatment prior to reverse osmosis, and to send the sludge with high mercury content to the hold-up tank and then to the discharge lamella.

Control of permeability and transmembrane pressure

There is a relationship between permeability, flux and transmembrane pressure, therefore a follow-up must be carried out to control and prevent these membranes from becoming prematurely saturated; backwashing, CIP washing helps to keep the membrane filtering properly.

An appropriate permeability value is 4LMH/psi; if this value is lower than 4LMH/psi, the system is prematurely saturated and requires chemical cleaning.

Sludge recovery system for UF washings and backwashes

The waste solution from the UF automatic self-cleaning strainer is sent to a solid/liquid separation process, the flow with solids is sent to the sludge tank and the liquid is recirculated to the UF feed tank. A coagulant may be necessary to improve sedimentation based on the recommendations of the final supplier's equipment.

Sludge pumping system and disposal in the leach pad Turbidity and pH control and sludge management

The sludge is pumped to the Hg sludge pond located at the LQ Pad. Slurry pump control will be determined by the plant operators as required.

Reverse osmosis process

The objective is to remove dissolved solids from the water, and the permeation and especially the ammoniacal nitrogen, this is achieved by controlling the pH so that if it meets ECAS - water quality standards it is sent to the Buffer Pond LQ and then discharged to the environment and the concentrate is sent to the recirculation tank to the LQ pad, located in the CIC LQ plant.

Conductivity, pressure differential and pH control

Keeping track of the conductivity of each of the Reverse Osmosis units cannot help to know how these membranes are filtering and find some problems such as contamination by faulty connectors, membrane fractures, O-ring condition.

On the other hand, monitoring the differential pressure in stages that do not exceed 40 psi will help prevent membrane oversaturation and chemical cleaning will be sufficient to restore membrane performance.

Discharge of water into the environment

Treated water is returned to the environment through discharge control points (DCP) authorized by Administrative Resolutions to: reservoirs, ponds, dams.

Monitoring and discharge of water to the environment

Newmont Yanacocha has 13 water discharge points into the environment, all of which comply with current Maximum Permissible Limits and the General Water Law applicable to each point.

Participatory monitoring of these discharges by the communities and authorities is continuous.

Conclusions

1. The INCO process is an effective process for the destruction of cyanide, in this oxidation process it was possible to destroy cyanide up to concentrations of 60 ppm in the feed and due to the nature of the solution it does not require the addition of copper sulfate, since the feed solution has copper concentrations of up to 8 ppm, with respect to the stoichiometric it works at 175%, this can be adjusted according to the operating variables of the INCO process.

2. With respect to the use of NaPTC in the precipitation of metals, it can be concluded that at a pH of 8.9 mercury is precipitated and the precipitated compound is more stable to pH changes.

3. The implementation of vertical lamellae for clarification has provided us with an opportunity to look for new ways to perform solid-liquid separations that can be competitive in operations, but the original design must be maintained or it will not achieve its purpose in clarification.

4. In the ultrafiltration stage, it can be concluded that this separation at the macro molecular and molecular level helps to eliminate some colloids, viruses, among others.

5. Chemical CIP cleaning for both Ultrafiltration and Reverse Osmosis is very important because if this task is not performed the membranes can be prematurely damaged and their recovery can be irreversible.

6. With respect to Reverse Osmosis, it can be concluded that a filtration technology at ionic level and that these membranes reject up to 98.5% of the metallic content, the use of the antiscalant plays a very important role in the operation since without it, performance would not be maintained within the expected range.

7. The innovation of using new processes for effluent control for the mining industry and conditioning them to it, guarantees us to achieve very good water both in quantity and quality and to comply with legal regulations.

Acknowledgements

Special thanks to the entire LQ process team and those who contributed to the development and review of this document.

References

Bucknam, C. H. 2005. “A Green Chemistry Approach to Mercury Control During Cyanide Leaching of Gold,” Presented at the SME Annual Meeting, February, 2005, Denver, Colorado.

Bucknam, C. H., McComb M. 2007. “Study of stability of mercury and silver Polythiocarbonate Nanofilms generated during optimized mercury removal from Cyanidation Pregnant Eluate,” Presented at the SME Annual Meeting, February, 2007, Denver, Colorado.

Pocock Industrial, Inc. 2002. “Mercury Precipitation on Yanacocha Carbon Stripping Eluate Using Sodium Polythiocarbonate,” Report to Newmont Mining Corporation, September 2002.