Copper Smelting Technology Trends

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Abstract

Copper smelting has evolved over time with various process routes culminating in the development of the Outokumpu Flash Smelter (Flash), which was one of the most significant developments in the twentieth century. Subsequently, this Flash process has been improved over time. Other process developments are the Ausmelt submerged lance smelting, the Mitsubishi continuous smelting, slag cleaning and blister copper production. Other innovations have recently been commissioned in China, including the Noranda top smelting and the Chinese bottom blowing (BB) furnaces. Energy, environmental and copper and precious metals recoveries are key factors in selecting a new copper smelter process. This paper outlines the findings of a recent investigation into the consideration of Flash or Chinese bottom blowing processes for a new smelter project. A site visit was arranged to some of the largest Chinese smelters and the findings taken into account for the smelter process study.

Introduction

A History of Copper Processing

Pyrometallurgy involves the treatment of ores at high temperature to convert ore minerals to raw metals, or intermediate compounds for further refining. Roasting, smelting and converting are the most common pyrometallurgical processes.

Roasting

Historically, copper concentrates have undergone roasting treatment processes. The roasting process is generally undertaken in combination with reverberatory furnaces. In the roaster, the copper concentrate is partially oxidised to produce "calcine" and sulphur dioxide gas. The stoichiometry of the reaction that occurs is: 

2 CuFeS2 + 3 O2 → 2 FeO + 2 CuS + 2 SO2

Roasting is no longer common in copper concentrate treatment due to its use of energy inefficient reverberatory furnaces and the formation of dilute SO2 roaster off gas that is not economically viable to capture. Due to these limitations of roasting, direct smelting is now the favoured route.

Smelting

The initial melting of the material is usually referred to as the smelting or matte smelting stage. It can be undertaken in a variety of furnaces, including the largely obsolete blast furnaces, reverberatory furnaces, flash furnaces, Isasmelt furnaces and more. The enriched copper product is known as matte or copper matte, and is typically a mixture of copper, iron and sulphur. The purpose of the matte smelting stage is to eliminate as much of the unwanted iron, sulphur and gangue minerals (such as silica, magnesia, alumina and limestone), while minimizing the loss of copper. This is achieved by reacting iron sulphides with oxygen (in air or oxygen enriched air) to produce iron oxides (mainly as FeO, but with some magnetite (Fe3O4)) and sulphur dioxide off gas and a fayalite slag. Copper sulphide and iron oxide can mix in the furnace, although can be separated when sufficient silica is added. The separate slag layer is subsequently formed, which contains the iron oxides. Adding silica also reduces the melting point of the slag.

The slag forming reaction is:           FeO + SiO2 → FeO.SiO2

Slag and matte form two distinct layers once separated. Slag is less dense than matte and therefore constitutes the upper layer, with the matte sinking and ultimately forming the lower layer. Copper can be lost from the matte in three ways: as cuprous oxide (Cu2O) dissolved in the slag, as copper sulphide dissolved in the slag or as tiny droplets (or prills) of matte suspended in the slag.

The phases associated with smelting copper concentrates has been shown in Figure 1. This diagram maps out the changes from sulphide minerals to metal, which is the basis of all copper smelting technologies.

A typical smelter has a life of 40 years thereafter technology changes. Operating temperature for this process varies between 200 to 2000°C, with 95% of the world’s metal production benefiting from smelting processes. Smelting is the oldest and most effective processing technique with a long history of efficient operation. The changing environmental legislation with respect to off gas has changed the way smelting is currently being viewed. The technology to produce clean off gasses in order lower emission levels requires increased capital for scrubbing, electrostatic separators and bag filters. The advantages and disadvantages of smelting are listed below:

Advantages

-       Reaction kinetics allow for high capacities

-       Solid waste is easily separated and disposed

-       Low OPEX

-       Suited to large tonnages

Disadvantages

-       Production of large volumes of toxic gaseous products

-       Environmental issues require the cleaning of gaseous products

-       Unable to treat low grades or complex concentrates

-       High CAPEX (the minimum size is 400,000 tpa copper metal)

In addition, any implemented technology should be investigated from 3 main aspects: social licence, environmental impact and occupational health and safety.

Social Licence: The resource sector is generally accepted by the public due to the role it plays in the supply of necessary goods and services. There can be no doubt as to the historic role the natural resource industry has played in the advancement of societies needs and well-being, economic growth and industrialization of specific countries. Today, in order to effectively have the Social Licence to Operate, there is an obligation to gain and maintain the support of all stakeholders impacted or influenced by any given project; failure to gain and maintain this Social License can lead to conflict, delays or cost for the proponents of the project.

Environmental Impact: Australia’s main national environment law is the Environment Protection and Biodiversity Conservation Act 1999 or EPBC Act. This legislation is designed to protect and manage matters that are of national significance. These include:

-       World heritage properties

-       National heritage places

-       Wetlands of international importance (Ramsar wetlands)

-       Nationally threatened species and ecological communities

-       Migratory species

-       Commonwealth marine areas

-       Great Barrier Reef Marine Park

-       The environment where nuclear actions are involved (including uranium mines)

-       A water resource in relation to coal seam gas developments and large coal mining developments

While all levels of government regulate activities to protect the environment, the Federal Government’s role is specifically focused on protecting these matters.

Occupational Health & Safety: In the past smelting had a number of adverse health legacies. Lung diseases related to long-term dust exposure are now rare in our workforce, demonstrating the effectiveness of dust control programmes. Significant strides in reducing the number of new cases of occupational asthma within smelters have been made, although the potential for chronic disease due to smelter fume exposures remains. Heavy equipment tends to be noisy, which is why noise-induced hearing loss is still a problem. Musculo-skeletal disorders remain a common form of new occupational illnesses, despite advances in technology rapidly reducing physical demands on employees.

SMELTING AS AN OPTION

Reverberatory furnace smelting: The reverberatory furnace feed is added to the furnace through feed holes along the sides of the furnace. Additional silica is normally added to help form the slag. The furnace is fired with burners using pulverized coal, fuel oil or natural gas and the solid charge is melted. Reverberatory furnaces can additionally be fed with molten slag from the later conversion stage to recover the contained copper and other materials with a high copper content.

The main equilibration reaction is:        Cu2O + FeS = Cu2S + FeO.

The slag and the matte form distinct layers that can be removed from the furnace as separate streams. The slag layer is periodically allowed to flow through a hole in the wall of the furnace above the height of the matte layer. The matte is removed by draining it through a hole into ladles for it to be carried by crane to the converters. This draining process is known as “tapping the furnace”. The matte taphole is normally a hole through a water-cooled copper block that prevents erosion of the refractory bricks lining the furnace. When the removal of the matte or slag is complete, the hole is normally plugged with clay, which is removed when the furnace is ready to be tapped again. Reverberatory furnaces were often used to treat molten converter slag to recover contained copper. The furnace feed would be introduced by pouring ladles carried by cranes. However, the converter slag is high in magnetite, some of which would precipitate from the converter slag (due to its higher melting point), forming an accretion on the hearth of the reverberatory furnace. This ultimately requires the furnace to be shut down in order to remove the accretion. The accretion formation limits the quantity of converter slag that can be treated in a reverberatory furnace. While reverberatory furnaces had very low copper losses to slag, they are not very energy efficient and the low sulphur dioxide concentrations in their off gases made its capture uneconomic. Consequently, smelter operators devoted a lot of money in the 1970s and 1980s to developing new, more efficient copper smelting processes. In addition, flash smelting technologies had been developed in earlier years and began to replace reverberatory furnaces. By 2002, 20 of the 30 reverberatory furnaces worldwide had been shut down.

Flash Smelting: Flash smelting was developed by Outokumpu in Finland and it is one of the most significant technical developments in the 20th century. Flash smelting accounts for over 50% of copper matte smelting globally. It involves blowing oxygen-rich air, dried Cu-Fe-S concentrate, silica flux and recycled materials into a 1250oC hearth furnace.

Once in the furnace (Figure 2), the sulphide mineral particles react rapidly with the oxygen gas. The products are (i) a molten copper-iron-sulphide matte with approximately 65% copper concentration, (ii) molten iron-silicate slag containing 1-2% copper and (iii) hot dust-laden off gas containing 30-70% SO2. A great deal of heat is generated in the shaft due to the oxidation of the sulphides.

Mitsubishi Technology: The Mitsubishi process is a continuous copper smelting and converting technology using three furnaces. The three furnaces are linked with covered launders, through which all the molten materials are continuously transferred by gravity. Copper concentrate (Cu: 30%, S: 30%, Fe: 25%, Gangue minerals 15%) is fed into the smelting furnace through a lance pipe with oxygen enriched air, is then oxidized and melted by an exothermic reaction to form molten mixture of matte (Cu: 68%) and slag. The matte is separated from the slag by difference of specific gravity in the slag cleaning furnace. The matte is further oxidized to form blister copper (Cu: 98.5%) in the converting furnace.

Noranda Smelting: Noranda Smelting was developed in Canada and is performed in large cylindrical furnaces. This produces a super high-grade copper matte (72-75% copper) in addition to a high copper slag and low SO2 off gas. These furnaces are steel barrels lined with around 500 mm of chrome-magnesia refractory. Industrial furnaces are 4.5 to 5.5 m in diameter and 18-26 m long. Air is injected through 35-65 tuyeres along the length of the furnace (Figure 4).

Sirosmelt: This process was developed by CSIRO in Australia. Most of the smelting energy comes from oxidizing the concentrate charge. Additional energy is provided by combusting oil, gas or coal fines blown through the vertical lance, as well as coal fines in the solid charge feed (Figure 5). The lance is submerged within the molten bath that results in the oxidation occurring directly in the melt.

Drying the feed is not necessary as the smelting reactions take place in the matte/slag bath rather than above it. Air, or oxygen enriched air, is blown into the Sirosmelt furnaces via a lance. Oxygen enriched air is typically 50-60 vol% oxygen, with significant levels of lance wear occurring if the oxygen content is higher. These furnaces are suited to small projects and are flexible in the regard that they can be fed concentrate to produce matte, or alternatively be fed matte to produce blister copper. This process can however result in foaming from certain concentrates.

Chinese Bottom Blowing Technology: China Engineering Corp. (ENFI) developed the technology and installed it at several smelters to test the concept beyond a pilot level. An oxygen bottom blowing copper smelting process has been developed at Dongying Fangyuan Nonferrous Metals Co. Ltd. This is the first modern copper smelting technology developed in China, with the advantages of high oxygen enrichment, high productivity and low energy requirements. After three years in operation, the new technology has shown to be one of the best copper smelting technologies in the world. The main feature of the bottom blowing smelting process is that high grade matte (up to 72 wt. % Cu) can be produced at relatively low temperatures with only 2-3 wt% Cu remaining in the slag. No extra fuel is required to smelt the concentrate containing 20 wt% Cu by using oxygen enriched air (70-75%). Quenched slag analysis shows that significant amounts of magnetite crystals are present at the operating temperature, indicating that the slag temperatures were much lower than the matte.

This study focuses on understanding the option of copper smelting, in particular, using Chinese bottom blown furnaces. This study has been performed based on prices and a Feasibility Study on using both Outotec Flash Smelting and Pressure Oxidation hydrometallurgy to produce copper metal. Two site visits with Chinese smelters and a separate meeting with ENFI were arranged to discuss costs and utilising the technology. Another meeting with Canadian smelting experts was arranged to discuss the Noranda technology and their views on the Chinese bottom blown smelting technology.

KEY PROJECT DRIVERS

Smelters have high CAPEX in long life projects and generally operate at very high overall efficiencies. The paid costs for the concentrate and applied penalties have great effects on the process economics. Highly desired concentrates have high copper grades, are quite clean and have gold and silver credits. The key project drivers are listed below:

Payment: The typical payable copper is based on copper content. If the copper in the concentrate is <22% a deduction of 1.1% is made. If it is less than 32% Cu, the payment is for 96.5% but it is subjected to a minimum 1% deduction. If the copper amount is 32%, the payment is for 96.65%. If copper is >38%, the payment is for 96.75%. Typical precious metals payments are zero for the concentration of 1g/t. Generally gold payment is 90% to 98% depending on gold grade. But the payment for silver is zero for the grades < 30 g/t and it is 90% for the grades above 30 g/t.

TC/RC: Treatment and Refining costs are deducted from the above payment. The cost to convert a tonne of concentrate to metal is market driven and can be negotiated in the commercial term. Prices are negotiated annually. Typical treatment costs (TC) are subject to supply and demand. Typically over the last 10 years, TC has varied between $415/t to $105/t. Refining costs (RC) are typically between $0.02 to $0.12/lb.

Environmental Considerations: The change in environmental legislation has caused smelting to be less favoured over hydrometallurgical routes. Traditional smelting is still the most efficient process for copper metal production, however tighter environmental gas emission legislation is changing the landscape. The cost of meeting tighter off gas legislation means large gas volumes are needed to be cleaned. This aspect has been working against new smelters and favouring hydrometallurgical facilities.

Precious Metals Recovery: Smelters have great lengths to recover all of the precious metals. The precious metals are recovered in the tank house slimes from electrowinning anodes to cathode copper. The slimes are smelted to produce Dore, which is then leached to separate gold and silver. This represents an important revenue stream.

Energy Efficiency: Energy is a major cost for the smelting process, and keeping the energy cost at low levels lead to efficient smelting. The sulphide oxidations generate lots of energy that can be applied to concentrate melting. It is not uncommon to have waste heat boilers to generate electricity.

Copper Loss in Slag: The slag from the smelting process is cooled, crushed and ground before flotation to recover copper sulphides and copper metal. Typically, slags may contain 3% copper but after the treatment the copper loss is as low as 0.3%.

Process Flexibility: Smelters are designed with a fixed capacity and specific design criteria. Typically smelters blend concentrates from various sources to maximise efficiency and productivity. Lower grade concentrates produce more slag and are more expensive to process. Higher precious metals content is highly desirable to process.

Acid By-product: 2.5 tonnes of sulphuric acid can be produced per tonne of copper concentrate depending on sulphur grade. Sulphuric acid has many uses and its price currently varies between $100 to $200/tonne. For a smelter, it is mandatory to recover the sulphur dioxide and produce acid - it is a valuable by-product with a significant impact on the smelter economics.

Project Ramp Up: The ramp up for a large project, such as a smelter, is critical for establishing a cash flow positive venture. If the project ramps up quickly then the full economic potential will be realised sooner (Figure 11).

CONCLUSIONS

There is little doubt that the Chinese have developed very good technology in regards to their bottom blowing furnaces. The technical catch up progress with Chinese smelters is quite remarkable to the point where, in some areas, they are now leading the world. The cleanliness and high level of automation, particularly at the Henan Yuguang Group’s new copper smelter, was very impressive. The plant was commissioned in 2014 and fully integrated with a large sulphuric acid plant and electrowinning tank house to produce copper cathodes and slag cleaning. In addition, there was a slimes recovery plant to produce gold and silver. Environmentally the plant was excellent with no detectable sulphur dioxide odour.

The Dongying Fangyuan Nonferrous Metals copper smelter in Dongying city was six years old, which was evident when compared to the previous copper smelter. At the same time the larger, partly constructed, smelter is very impressive also. This plant was fully integrated with a large sulphuric acid plant and electrowinning tank house to produce copper cathodes and slag cleaning. In addition, there was a slimes recovery plant to produce gold and silver, like that seen at Henan Yuguan Group’s new copper smelter. They were constructing a bond warehouses and workshop facilities for what is becoming a very large copper smelting complex. Environmentally the plant was also excellent with no detectable sulphur dioxide odour. These furnaces use oxygen, not air, and therefore an ancillary oxygen plant is included.

The stockpiles of concentrates at the smelters were very large and it appears the Chinese Government assists with soft loans to finance the purchases and holding costs.

The meeting with ENFI in Beijing was very informative. The standout message was a feasibility Study would be required before the project could be seriously considered. They said the capital costs for smelters were very high and the economic return depended on efficiency and the terms and conditions for concentrate purchase. The profit margins for smelters are small and require tight economic and process management. They were not surprised the previous study utilising Outokumpu Flash Smelting show a poor financial outcome. They also indicated very long lead times for engineering, procurement and construction of any new smelter. They were evasive over likely CAPEX and with regards to process guarantees. They said there was an opportunity for modularisation and shipment to site to facilitate the project construction.

Our discussions in Canada were disappointing because the Canadian Experts dismissed the Chinese claims even though they had not been to China and did not have firsthand knowledge. They were very old school and promoting the Noranda Smelting technology.

The overall conclusion was that significant CAPEX savings could be achieved if the Chinese technology was employed and equipment sourced out of China and at the same time allow for sufficient controls to be in place to manage quality and schedule. The bottom blowing technology is functional, as evidenced by the smelters we visited, and they appear to have advantages over flash smelting hence the trend towards these smelters.

Environmental permitting would be a slow process and should begin early to avoid delays.

ACKNOWLEDGEMENTS

The author wishes to acknowledge our Client for the opportunity to present this paper, and would like to thank all Midas Engineering staff and consultants for their contributions and Vendor companies for their conversations and contribution.

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