Improved recovery, reliability and wear after flotation upgrade at Nui Phao, Vietnam

Background

Masan Resources, one of the largest private-sector natural resources companies in Vietnam, acquired a controlling interest in Nui Phao in 2010, a world-class polymetallic project in Northern Vietnam. Masan Re­sources achieved commercial production in Q1 of 2014, with steady-state conditions of throughput and recovery levels achieved in Q4 of 2014. 

Nui Phao is now the largest producer of tungsten outside China, and amongst the largest producers of acid-grade fluorspar and bismuth in the world. Masan Resources undertook various projects to improve operational profita­bility at Nui Phao, but the copper and bulk sulphide flotation (BSF) rougher circuits still experienced inefficiencies. 

Since commissioning, Nui Phao’s metallurgical perfor­mance of the copper and BSF roughers was below expectations. In March 2013, the sulphur grade in the BSF tails was 3.5% S (on average), significantly higher than the design value of <1.5% S. The higher sulphur tails grades affect the downstream performance of the tungsten gravity plant. Coarse sulphur competes with tungsten in the gravity plant, and both tungsten grade and recovery suffer. The lower sulphur recoveries (particularly in the coarse +106 μm range) were attributed to insufficient flotation capacity and poor particle suspension (striation) in the flotation cells. 

Nui Phao undertook various remedial steps and the BSF tails were reduced to 2.5% S. However, the copper and BSF circuit experienced significant structural and mechanical vibration and reduced mechanism wear life. Short-term solutions were implemented but it was recognised that a longer-term solution was required.

Scope

In 2015, NMPC approached Metso Outotec to investigate opti­mising recovery in the copper and BSF circuits, as well as reducing structural vibrations and improving mechanism wear life. Metso Outotec also investigated options to increase the sulphide minerals recovery from the BSF rougher tails stream. Mineralogy reports indicated that there were still significant coarse sulphide particles recoverable by flota­tion to warrant further investigation. The objective was to achieve a BSF tail grade (feed to tungsten gravity circuit) of ≤1.5% S. 

FloatForce upgrade

After reviewing the existing copper and BSF circuit, Metso Outotec recommended replacing the internal components of the Metso cells with the FloatForce mechanism. In the Metso mechanism design, the stator is suspended from the lower shaft, and this arrangement was identified as the cause of the mechanical and structural vibration. The FloatForce stator design is bolted to the tank floor and was predicted to deliver a significant reduction in circuit vibration. It was expected that metallurgical performance would be unaffected by the mechanism change. 

In October 2015, a FloatForce trial upgrade was under­taken on one Metso cell, resulting in significantly lower vibration, with no adverse metallurgical performance. In March 2016, during a 3-day shutdown, all remaining copper and BSF rougher cells were upgraded with Float­Force mechanisms (plus new bearing assemblies with drive shafts). 

Structural improvement

The FloatForce mechanism significantly reduced the structural and mechanical vibration, see figure 1. Previ­ously in the copper and BSF circuit, the 12 points surveyed indicated that the structural vibration ranged between 6 and 17 mm/s. Seven points were above the permissible level. After the conversion to FloatForce, all 12 points indicated a structural vibration <4mm/s and were within acceptable levels. 

OPEX cost saving

The FloatForce mechanism significantly improved the mechanism's wear life. Previously in the copper and BSF circuit, all flotation rotor and stator units in this circuit were replaced every six months. This resulted in a signifi­cant operational cost in spare parts and labour. Following the conversion to FloatForce the mechanism wear life has increased from 6 to 15 months. The FloatForce mechanism is operating at a similar rotor tip speed to the previous design. 

Metallurgical performance

After the conversion to FloatForce, the BSF rougher circuit initially experienced poor particle suspension affecting sulphur mineral recovery. Outotec worked closely with Nui Phao to improve performance. By October 2016, all recommended changes (including FlowBooster-secondary impeller, vertical baffles, froth stabilising fins) were implemented. The BSF rougher tails grade returned to the target value of <2.5% S. 

SkimAir Flash flotation cell

Flash flotation (SkimAir SK500) was also investigated by Metso Outotec as part of the BSF upgrade. The objective was to increase the recovery of liberated coarse (+106 μm) sulphur minerals and lower the BSF tails further from <2.5% S to ≤1.5% S. 

The justification for flash over conventional flotation was based on the ALS mineralogy findings. The coarse sulphide minerals in the tails were predominately liberated or rich in binary associations. Thus, under the optimum flotation conditions, it was expected that relatively short residence times would be required to achieve additional sulphur recovery. 

In May 2015, Metso Outotec conducted laboratory flash flotation test work at Nui Phao, followed by NPMC’s own pilot plant trial. The plant trial indicated a sulphur unit recovery of 54% (on average) at a sulphur grade of 27% S, which was consistent with Metso Outotec performance guarantees, so the full-scale flash circuit was installed. The BSF rougher tails were pumped to a new cyclone cluster and the cyclone underflow was fed to the Outotec flash cell. The flash concentrate was sent to concentrate thickener, whilst the flash tails combined with the cyclone overflow and were pumped to the tungsten gravity circuit. It was noted that the Nui Phao flash circuit would be unique, as normally flash cells are located in the comminution circuit. 

Metallurgical improvement

During the first few weeks of the flash cell operation, the metallurgical results were significantly below expecta­tions. Flash survey results in April 2016 indicated a sulphur unit recovery of only 30% (on average), lower than the predicted sulphur recovery of 50 – 60%. 

Metso Outotec attended the site and observed that the froth velocity to the concentrate launder was lower than ideal. Sizing results confirmed poor coarse sulphur recovery when compared to the finer-size fractions. There was a sharp drop-off in coarse particle recovery, which is not typical of flash circuits. 

Optimising Flash flotation

To improve performance, Metso Outotec recommended a new double internal concentrate launder with a larger froth cone, to reduce the FSA (Froth Surface Area) and increase the FCR (Froth Carry Rate) to its optimal rate. In July 2016, the new concentrate launder was installed and the improvement in sulphur recovery was immediate. 

At a feed density of 1.52 S.G, the flash sulphur unit recovery increased to 55%. The -180 +150 μm size fraction sulphur recovery was ~70%, and comparable to the finer size frac­tions. The results confirmed that the new froth launder had been successful in improving coarse sulphur mineral recovery. As a consequence, the BSF final tails grade was reduced to the target value of ≤1.5% S. 

Figure 2 presents the CUSUM chart for the tungsten circuit recovery (based on the monthly averages), indicating a tung­sten recovery improvement since March 2016. It is impor­tant to note that from April to October 2016, the tungsten circuit underwent other optimisation projects, resulting in increased tungsten recovery by 4.5%. Since October 2016, the BSF circuit upgrades have improved the tungsten recovery by a further 2.7%. 

Summary

The Outotec FloatForce mechanism in the existing copper and BSF rougher circuits, combined with the new flash flotation (SkimAir SK500) circuit, have achieved the metal­lurgical recovery improvement targets at Nui Phao. The recovery of the coarse, rich sulphur particles has resulted in an improved sulphur recovery. The overall sulphur mineral recovery has increased by 4.7% and has produced the desired improvements in the tungsten gravity circuit. Since October 2016, the tungsten recovery has increased by 2.7%. 

In addition, the Metso Outotec FloatForce mechanism design, operating at similar impeller tip speeds to previous, has achieved the desired operational benefits. The structural vibration has reduced to acceptable levels (< 4.0 mm/s) and the OPEX costs have also reduced, with mechanism wear life increased by 150%. Based on the metallurgical and operational benefits, the BSF circuit upgrades achieved the project objectives and no further technology upgrades are required in this area.