Article originally published in Mineria Pan-American Mining Magazine, April 2019. Reproduced with kind permission.

In 2010, Ausenco began working on the optimization of the Constancia copper project which was owned by Norseman Mining at the time. A feasibility study showed unfavorable productivity and the global firm was asked to carry out an optimization study to improve the viability of the project.

The Constancia mine is a porphyry copper-molybdenum system located in the Andes in south-eastern Peru and is positioned 4,100m above sea level. The mine consists of two deposits, Pampacancha and Constancia, and is located with the processing plant 1 km west of the Constancia pit.

During the first years of production, the performance of the Constancia plant averaged 80,000 tons per day (t/d) (compared to a capacity of 76,000 t/d), and copper production in concentrate averaged 82,000 t/a.

Ausenco continued with the project after ownership transferred from Norsemont to Hudbay, and delivered EPCM services to design, build and commission a 25 Mt/yr concentrator and associated infrastructure for the project.

“The optimisation study was based on Ausenco’s cost effective design principles and resulted in a net present value (NPV) improvement of $US800M when compared with the feasibility study. This NPV improvement was due to a combination of increased capacity, optimised equipment selection, lower capital costs, improved construction schedule and optimised mine plan” said Ausenco’s Simon Cmrlec, President, Americas Project Delivery. Construction commenced in late 2012 and the plant started commissioning in late 2014 with first concentrate produced in December 2014.

Interview with Ausenco’s Simon Cmrlec, President, Americas Project Delivery

Why is there continued strong interest in the Constancia project?

The Constancia project was the first in Peru to use cost effective design principles that were first developed in Australia. By applying these principles, Ausenco designed a concentrator for a relatively low grade copper deposit with a low capital cost and consequently an improved rate of return on capital invested. The project provided a “proof of concept”, demonstrating that these design principles were applicable in high altitude locations with high seismicity. As such, the Constancia project provided a template for how other low grade copper projects in Peru can be advanced economically and sustainably.

Discuss the technical characteristics of the design, construction, and commissioning of the 25 Mt/year concentrator and its associated infrastructure at high altitude in the Constancia mine.

A key objective in Constancia’s design was to maximize capital efficiency (minimizing capital and operating costs) without compromising safety, operability and maintainability. During the optimisation study Ausenco identified several opportunities to add value to the Constancia project through improvements in plant throughput and layout.

Plant throughput at Constancia is constrained primarily by ore competency. The feasibility study called for the use of a single gearless SAG mill with 21,500 kW of installed power and two ball mills with 13,000 kW of installed power each. This would have allowed for a grinding P80 of 106 microns at a throughput of 76,000 t/d in the first five years followed by a reduction to 50,000 t/d due to increasing percentages of competent hypogene ore in the plant feed.

Ausenco performed a number of trade-off studies to determine which arrangement would return the optimum project NPV. These studies considered the use of a single gearless SAG mill, with installed power between 20,000 and 26,000 kW, against two lines of pinion-driven SAG mills, with installed powers between 13,000 and 16,000 kW each. The optimum NPV was achieved with the latter option resulting in the selection of a grinding circuit consisting of two lines of pinion-driven SAG mills, with installed powers of 16,000 kW each, followed by pinion-driven ball mills, with installed powers of 16,000 kW each. This yielded a total of 64,000 kW of installed grinding power, a significant increase over the original feasibility study grinding power, and allowed the plant to maintain a throughput of 76,000 t/d throughout the mine’s life.

Although the grinding circuit was designed at 76,000 t/d, Ausenco determined that the circuit would be capable of treating 86,000 t/d in the first five years of operation due to softer, less competent ore in the plant feed during this time. Minor modifications were made to downstream circuits to match this capacity, allowing for maximization of overall plant throughput.

The original feasibility study layout was modified with the following key objectives in mind: minimize plant footprint, take full advantage of the site’s natural ground contours, minimize the use of fully enclosed buildings, minimize the height of all structures, optimize the location of sub-stations and motor control centres (MCCs), and allow for the use of tower cranes (mobile and permanent) to support construction and operations. For example, (a) the primary crusher was relocated to a nearby hill that provided the necessary height for the circuit with minimal earthworks. This allowed the primary crusher and stockpile to align with the grinding circuit along a relatively flat spur, which reduced overall project earthworks quantities, (b) the copper flotation circuit was relocated and the footprint was made more compact. Rougher cell tailings were allowed to gravity flow down-grade to the tailings thickener, which was placed at a local low point and (c) the contact water collection pond was placed at the low point in the plant, allowing for site-wide drainage to a single settling pond.

The construction execution plan was tailored to suit the methodologies commonly used in the Peruvian construction industry and to suit the location of the site at 4100 m A.S.L. Particular attention was paid to personnel logistics, shift rosters and health monitoring to ensure that the construction personnel could work safely at all times. The site also experiences regular thunderstorms and resultant lightning strikes so procedures were developed to deal with these contingencies.

The commissioning team was mobilized in mid-2014 with first ore to the primary crusher on 24 October, 2014. This was followed by first ore through grinding line two on 16 December, 2014 and first copper concentrate production on 27 December, 2014. In March both grinding lines were operating simultaneously and the plant met design throughput of 76,000 t/d for three consecutive days on March 29, 2015. The start-up effort was characterised by collaboration between the project team, construction and commissioning crews, and the owner’s operations group allowing for an effective transfer of knowledge between all parties. The result was an efficient ramp-up to name plate capacity with commercial production being declared in May 2015.

What were the main challenges for the Constancia Project during the construction phase?

Construction on site commenced in mid 2012 and was complete by March 2015. The timing of the construction phase of the project coincided with two other major resource construction projects in the region which meant competition for labor was extremely high.

In-country manufacturing was also a major challenge with key materials like structural steel, platework, pipe spools and rubber lining all being manufactured by the same manufacturers. Expediting strategies were implemented early to ensure timely delivery of these materials which were free issued to the site contractors.

The remote location of the project created immense challenges for the transport and logistics teams and detailed planning and communication was critical for success. Located high in the Andean ranges, the site is subject to severe weather events – in particular, electrical storms. A Lightning Standard was developed to ensure the safety of workers and included scheduling high risk tasks earlier in the morning before the typical start of the weather events.

Did Ausenco apply any innovations to the Constancia Project?

Given the location of the project, every effort was made to utilise equipment that had previously been proven commercially. Some new approaches were used, for example, we used a Jameson Cell for gangue rejection in the last stage of molybdenum flotation which had not been done elsewhere at the time. The real innovation that drove the success of the project was in the concepts developed by the engineering and construction teams to ensure that the plant could be built cost effectively and then ramped up and operated without complexity throughout the mine life.

The footprint of the plant, the bulk materials quantities (steel, concrete, piping, cable) and the CO2 footprint were all significantly lower than other copper concentrator projects in South America on a pro rata throughput basis.

How does Ausenco’s experience designing the Lumwana and Phu Kham concentrators apply to the Constancia Project?

The Constancia project uses design concepts developed in the design of small (1 Mt/y) gold projects and extended into large copper concentrators like the 12 Mt/y Phu Kham copper concentrator in 2006-08, and 20 Mt/y Lumwana copper concentrator in 2006-08. These concepts involve the optimum equipment selection for the specific project to generate the best returns possible, the reduction in the project footprint and optimization of bulk material quantities. These principles are at the heart of everything Ausenco does and the same concepts have been utilised on OZ Minerals’ Carapateena project, Marcobre’s Mina Justa project, Hudbay’s Rosemont project and several other large-scale copper projects around the world

Was there any assembly of large equipment on the project site? Comment in depth on the execution of these processes. (Machinery)

The assembly of the mills, crushers and conveyors on site were completed by large mobile hydraulic and heavy lift crawler cranes. As the layout for the plant was compact, access into these areas where large equipment had to be installed, needed to be well managed and planned well in advanced. The installation and use of a Tower Crane aided in the construction process by simplifying crane planning, adding flexibility and providing quick access to internal plant locations.

Laydown areas on site were established for specific major componentry with allowance for preassembly and preparation being completed away from the confined plant site, then delivered to the plant site just prior to installation. This process significantly reduced many high risk tasks and the number of personnel within the condensed plant site. This approach also minimized lifting and vehicular interactions of the componentry to smaller, more specialized, task specific teams that could manage these critical tasks safely and efficiently.

The design of the of the flotation area and the close proximity to the main grinding building provided challenges during the installation process. The design included the installation of a tower crane for the maintenance of the cells and associated equipment. This key piece of infrastructure was procured and installed right at the start of the construction phase and was utilized in the assembly of the flotation cells, saving time and reducing the amount of additional cranage required on the site.

The construction of the primary crusher structure and the main grinding building was completed in a level by level approach around the main componentry. This was not just for structural support but also to aid safe and efficient access to these areas during construction and to also minimize the use of scaffolding and boom lifts which can often constrict a construction site creating issues with productivity and reducing work fronts available for construction teams.

What are some other highlights/achievements for the Constancia project?

The Constancia project was completed on time and in line with the project budget. The project went from mobilization to first copper in 27 months and ramped up to commercial projection soon after. A key focus for both Hudbay and Ausenco was to ensure that the work performed by Peruvian engineers and contractors was maximized and to that end, all of the construction was carried out by Peruvian contractors. The project was carried out at 4,100 m A.S.L and both the design and the construction execution planning was specifically tailored to the challenges that come with operating at these elevations.

During the first years of production, the Constancia plant throughput averaged 80,000 tonnes per day (t/d) (compared with a name plate capacity of 76,000 t/d), and production of copper in concentrate averaged 82,000 t/y. The plant has continued to exceed nameplate capacity even as hypogene ore makes up a greater percentage of plant feed.

From the outset of the project, there was a strong focus on developing a culture that promoted safe working, caring for coworkers and protecting the environment. The safety performance of the project was outstanding with more than 8 million man hours being carried out without a lost time injury. We consider this one of our biggest achievements.