The effect of temperature on the interaction of MPC with a PGM matte

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Platinum group metals (PGMs)

The world’s reserves of platinum group metals/elements (PGMs) are estimated at 100 million kilograms, of which over 80 percent is contained in South Africa’s Bushveld Igneous Complex [Junge et al. 2015]. Summary of the PGMs reserves by country is shown in Table 1-1 [Jewell et al. 2015]. PGMs are a group of six elements namely; iridium, osmium, platinum, palladium, rhodium and ruthenium [Glaister et al. 2010]. PGMs together with gold and silver are classified as noble metals because of their high corrosion and oxidation resistance [Glaister et al. 2010, Xiao et al. 2004]. PGMs are used in a number of industrial processes and commercial applications, such as; automotives, jewellery, electronics, dentistry and investments amongst others [Jewell et al. 2015, Jones 2005]. The principal sources of PGMs are sulfide and arsenide minerals such as PtAs2, PtS, (Pt, Pd)S, RuS2, PdSb and elemental ruthenium [Xiao et al. 2004].

Noble metals together with cobalt (Co), copper (Cu), iron (Fe) and nickel (Ni) belong to the class of transition metals in the periodic table [Jones 2005]. Geologically, PGMs associate with base metal sulfides such as chalcopyrite (CuFeS2), millerite (NiS), pentlandite (Fe, Ni)9S8, pyrite (FeS2) and pyrrhotite (Fe1-xS) [Xiao et al. 2004, Jones 1999]. Gangue minerals associated with PGM containing minerals are feldspar, biotite, plagioclase and pyroxene [Jones 1999]. To liberate the PGM-rich minerals from gangue minerals the PGM ore is crushed and milled prior to concentration processes [Habashi 2002]. The milled PGM-ore is treated using gravity separators and flotation to produce a sulfide-rich PGM concentrate [Jones 2005, Mudd 2012]. To separate the PGM-rich sulfides from the gangue minerals, smelting is used. Rectangular six-in-line or circular three-electrode electric furnaces are typical in the PGM industry [Jones 1999].

During smelting of the PGM containing concentrate two liquid phases of distinct relative densities form, a lighter fayalitic-fosteritic ([Mg, Fe]2SiO4) slag and a denser matte. Matte is rich in iron-copper-nickel sulfides and it serves as a collector for PGMs [Liddell et al. 1986]. Due to high operating temperatures (1350 °C to >1600 °C) [Eksteen 2011] associated with PGM smelting the smelter has to be lined with refractories at the hot-face. To prolong the service life of the refractories, sufficient cooling of the refractories is required at the cold-face of the refractory wall. Copper waffle coolers are typically used on the cold-face of the refractories to extract heat away from the refractories [McDougall 2012].

Due to high operating temperature and corrosiveness of the PGM containing melt, failure of the copper waffle coolers has been experienced in PGM smelters in the upper sidewall region [Thethwayo 2010]. Failure of copper waffle coolers causes explosions, loss of production and costs associated with furnace rebuild. The failure of copper waffle coolers was preceded by the consumption of conventional refractory bricks (MgOx-CrOx) that were used to form the furnace lining. To prevent the occurrences of copper waffle cooler failures, conventional bricks have been replaced by the graphite blocks in recent designs of PGM smelter refractory walls. Graphite blocks are only applied at the hot-face upper sidewall (against the concentrate and the slag-zone). Graphite blocks have increased the service life of the copper waffle coolers [Thethwayo 2010]. It is desired to extend the graphite blocks to the lower sidewall of the PGM smelter refractory wall against the matte zone. There is virtually no data published on the behaviour of graphite when it is in contact with liquid PGM matte.

Aims of study

Owing to high infiltration of furnace melt in the slag-matte tidal zone (with graphite lined smelters), the refractory lining of the lower side wall has to be redesigned such that the refractory material at the hot-face has high resistance towards melt penetration. It is envisaged that using carbon-based refractory at the hot-face of the matte zone (lower side-wall) will improve the service life of the furnace lining in PGM smelters. The behaviour of graphite when in contact with liquid PGM matte has not been covered in the literature. The objectives of this work are the following: o Determine the most prominent refractory-wear mechanism when selected carbon-based refractories are in contact with liquid PGM-furnace melt at typical furnace operating temperatures. In this context; prominent wear mechanism of a refractory is a wear mechanism that causes the most degradation/dissolution/penetration or erosion of a refractory. o Determine the compatibility of carbon-based refractories (graphite and micropore carbon) with PGM melt (matte and slag).

Table of Contents

1 Introduction
- 1.1 Platinum group metals (PGMs)
- 1.2 Aims of study
  - 1.2.1 Approach
  - 1.2.2 Rationale and significance
  - 1.2.3 Thesis organization

2 Background
- 2.1 The PGM ore
- 2.2 PGM ore processing: an overview
- 2.3 PGM ore smelting
  - 2.3.1 Industrial PGM-furnace matte
  - 2.3.2 Industrial PGM-furnace slag
  - 2.3.3 Challenges associated with smelting high chromite concentrate
- 2.4 PGM smelter
- 2.5 Sidewall lining design of a typical PGM smelter
  - 2.5.1 Challenges with the conventional lining design (MgOx-CrOx brick)
  - 2.5.2 The freeze lining concept
  - 2.5.3 The current design of a PGM smelter refractory wall (graphite block)
- 2.6 Summary
3 Literature
- 3.1 Refractories
  - 3.1.1 Carbon-based refractories
  - 3.1.2 Synthetic (moulded) graphite
  - 3.1.3 Micropore carbon (MPC)
  - 3.1.4 Refractory wear mechanisms
  - 3.1.5 Refractory-wear testing methods
  - 3.1.6 Prevention of refractory wear using cooling
  - 3.1.7 Summary
- 3.2 Factors affecting matte flow rate through a graphite block tap-hole
  - 3.3 Phase relations in the Cu-Fe-Ni-S system
  - 3.3.1 Fe-Ni-S system
  - 3.3.2 Cu-Fe-S system
  - 3.3.3 FeS-FeO-Fe2O3 system
- 3.4 Interaction of carbon with sulfides
- 3.5 The solubility of carbon in the FeNi-Si
- 3.6 Interaction of silicates with sulfides in the presence of carbon
- 3.7 Interaction of sulfides with gas bubbles
- 3.8 Melt foaming
- 3.9 Summary
4 Materials and methods
- 4.1 Characterization Techniques
  - 4.1.1 Carbon and sulfur analyser
  - 4.1.2 Electron Probe Micro Analyser (EPMA)
  - 4.1.3 Inductively coupled plasma-optical emission spectrometry (ICP-OES)
  - 4.1.4 Optical microscope (OM)
  - 4.1.5 Scanning Electron Microscope (SEM)
  - 4.1.6 X-Ray powder diffraction (XRD)
  - 4.1.7 X-Ray fluorescence spectroscopy (XRF)
  - 4.1.8 X-Ray computed micro tomography (XRM)
- 4.2 Materials
  - 4.2.1 Carbon-based refractories
  - 4.2.2 Industrial PGM matte
  - 4.2.3 Synthetic matte (Matte-S)
  - 4.2.4 XRF analysis of industrial and synthetic matte
  - 4.2.5 Industrial PGM slag
- 4.3 Apparatus
  - 4.3.1 FactSage
  - 4.3.2 Coal Ash fusion furnace
  - 4.3.3 Vertical tube furnace
- 4.4 Experimental procedures
  - 4.4.1 Wettability test measurements
  - 4.4.2 Prominent wear mechanism of graphite by a PGM melt (matte and slag)
  - 4.4.3 Prominent wear mechanism of a micropore carbon by a PGM matte
- 4.5 Summary: Materials and methods
5 Results-A: Wear of graphite by a PGM melt (matte and slag)
- 5.1 Wettability of SG graphite
  - 5.1.1 Wettability of graphite by pure sulfides
  - 5.1.2 The melting behaviour of pure sulfides
  - 5.1.3 Wettability of graphite by a synthetic and an industrial PGM matte
  - 5.1.4 Melting behaviour of synthetic and industrial matte
  - 5.1.5 Wettability of graphite by industrial PGM-slag
- 5.2 Graphite wear by Matte-S
  - 5.2.1 Graphite dissolution by Matte-S
  - 5.2.2 Penetration of Matte-S through SG graphite
  - 5.2.3 Mechanism of Matte-S loss through SG graphite
- 5.3 Graphite wear by industrial matte
  - 5.3.1 Penetration of liquid industrial matte through graphite
  - 5.3.2 Chemical interaction between SG graphite and Matte-A
  - 5.3.3 Dissolution of graphite by industrial matte
- 5.4 Graphite wear by industrial melt (matte and slag)
  - 5.4.1 Overview-specimen layout
  - 5.4.2 Slag interaction with graphite
  - 5.4.3 Penetration of slag through graphite
  - 5.4.4 Interaction of graphite with matte
  - 5.4.5 Graphite erosion
- 5.5 The effect of cooling graphite on the behaviour of liquid industrial matte
6 Results B: Wear of micropore carbon by a PGM matte
- 6.1 Wear mechanism of the MPC refractory wall
  - 6.1.1 The microstructure of the MPC wall
  - 6.1.2 Disintegration of the MPC wall
- 6.2 The interaction of MPC with a PGM matte
  - 6.2.1 Interaction of MPC with Matte-S
  - 6.2.2 The interaction of MPC with industrial matte
- 6.3 The effect of temperature on the interaction of MPC with a PGM matte
  - 6.3.1 Matte loss _ Matte-S
  - 6.3.2 Matte loss_Matte-A
  - 6.3.3 Carbon dissolution
- 6.4 Mechanism for matte loss – analysis of the gas condensate
7 Discussion
- 7.1 Wettability of SG graphite
  - 7.1.1 Melting behaviour of sulfides
  - 7.1.2 Melting behaviour of a Matte-S
  - 7.1.3 Melting behaviour of industrial matte
  - 7.1.4 The change of the volume of the droplet
  - 7.1.5 Deductions
- 7.2 Wear of SG-graphite by a PGM matte
  - 7.2.1 Graphite dissolution
  - 7.2.2 Mechanical loss of matte during exposure to graphite
  - 7.2.3 Chemical interaction of graphite with matte
  - 7.2.4 Deductions
- 7.3 Wear of graphite by industrial melt (matte and slag)
  - 7.3.1 Physical penetration of graphite by matte and slag
  - 7.3.2 Chemical interaction between graphite and the industrial melt (matte and slag)
  - 7.3.3 Contrast between the behaviour of industrial matte and slag toward graphite
  - 7.3.4 Graphite erosion
  - 7.3.5 Deductions
- 7.4 Effect of cooling graphite
  - 7.4.1 The morphology of matte residue as a function of exposure temperature
  - 7.4.2 The effect of the cooling rate on the freeze lining
- 7.5 Wear of MPC by a PGM matte
  - 7.5.1 Interaction of matte with MPC
  - 7.5.2 Deductions
  - 7.5.3 Industrial implication
- 8 Conclusions and recommendations
8.1 Conclusions
- 8.1.1 Question 1: “Does a PGM melt (matte/slag) wet graphite?”
- 8.1.2 Question 2: “What is the most prominent wear mechanism of graphite when
- graphite is exposed to a liquid PGM melt (matte and slag)?”
- 8.1.3 Question 3: “Does matte form a protective frozen skull when graphite is cooled
- sufficiently?”
- 8.1.4 Question 4: “Does a micropore carbon perform better than the graphite when exposed to a liquid PGM matte?”
8.2 Recommendations
8.2.1 Recommended actions for successful application of graphite in industry
8.2.2 Future work
9 REFERENCES