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Publication > Issue > Articles

Sulphuric acid in the balance

Summary

J. H. Templeton and W. J. Schlitt discuss how the chemical reactions that occur within a ­copper-leach recovery system impact on the overall acid balance. Both oxide and sulphide ­mineralogy are addressed, and their effects on acid consumption, generation and solution buffering are presented. Two case studies that apply the discussed chemistry show the practicality of using a computer spreadsheet acid balance model for the early planning and design of copper leach systems.

Abstract

A major consideration in any new copper-leaching project is the net acid balance over the expected life of the operation. Thus, when copper is to be leached from various ores and wastes, project personnel must understand and apply the chemistry of acid generation, con­sumption and buffering by the mineral species present. Also important is the net acid generation or consumption during the copper-recovery operation.

The net acid balance is likely to have significant operational and/or environmental consequences that must be addressed as the project develops. For deposits that are net acid consumers (typically those that have predominantly nonsulphide mineralization), the availability and cost of acid become important considerations. On the other hand, for deposits that are net acid generators (typically sulphidic mineralization), the problem may be too much acid. For these deposits a high acid level in the pregnant leach solution (PLS) can accelerate decrepitation of the host rock and can reduce the efficiency of copper transfer to the organic phase in solvent extraction. Neutralization of the excess acid can represent an ancillary operation with extra cost if the solution must be treated and discharged to maintain a water balance or as part of the final mine closure and site remediation.

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End of the Santa Fe trail

Summary

They've shut down Culberson. The mine's closure is a significant moment in the current decline of the Frasch sulphur industry, but its opening was a major event for the world's sulphur trade. Chris Cunningham looks back on a distinguished corner of the history of the brimstone business.

Abstract

Neil Armstrong had barely shaken the moondust from his NASA issue boots when another Houston-based organisation began to break new ground, this time for the sulphur business. September 1969 marked the start of operations at Rustler Springs, Culberson County, where Duval Corporation launched the largest single producing site in the world, as a guarantee against under-supply of brimstone while the world moved into a new decade. The mine represented a breakthrough in mining technology by successfully adapting Frasch recovery techniques to unpromising geological conditions. But worldwide production of sulphur prior to the opening of Culber­son stood at less than 18 million t/y, with plenty of potential for under-supply. The mine has closed at the end of the century with total world output at around three times that level, only a fraction of it native sulphur.

Culberson County, West Texas is a region barely more inviting than the lunar surface. And Rustler Springs isn’t the sort of place you would get to unless you were going there. These are the Badlands of Wild West lore. But 30 years ago the Santa Fe Railroad signed contracts with Duval Corpo­ration and struck out with a branch line into the desert. The site’s remoteness and the sheer technical difficulty of getting sulphur out should have been slim encouragement to go ahead at all.

But the potential wealth of this deposit led rapidly to a production complex rated at 1.5 million t/y of mined sulphur, rising to 2.5 million t/y.

Along with a new rail branch line, there emerged an investment in rolling stock on an ambitious scale and, at the end of the run, a major new coastal terminal to serve local and ocean-going shipping.

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Cutting emissions from sulphuric acid production

Summary

Technology improvements over the past decade have made today's stringent emissions limits for sulphuric acid plants a reality. In this article, Lisa Connock reports on some of the recent developments by plant constructors, engineering companies and catalyst manufacturers.

Abstract

Ever increasing demands for clean air requirements regarding the SO2 emissions from metallurgical, chemical, and power plants have also influenced the production of sulphuric acid. This has been a challenge to both plant constructors and catalyst manufacturers. In response to these demands, there was a revival in the mid 1950s of the double absorption sulphuric acid plant, which reduces SO2 emissions by 75% compared to conventional single absorption plants. During the last decade several new catalyst types have been introduced enabling smoother and more flexible operation with improved performance and faster and cleaner start-ups.

In recent years a new trend has emerged. Many plants are now required to achieve 99.85% conversion which corresponds to only 50% of the tradi­tional EPA limit of 4 lbs of sulphur dioxide per s.ton of acid produced (2 kg per metric tonne). In some areas requirements are even more stringent with acid plants required to achieve more than 99.9% conver­sion, corresponding to a 100-150 ppm sulphur dioxide emission level. In the most extreme cases emissions levels as low as 40 ppm sulphur dioxide in the stack have been stipulated. Existing technologies such as tail-gas scrubbing or triple absorp­tion have the disadvantage of increased capital costs. New plants may be designed with 5-bed converters in a 3+2 layout but this is less attractive for existing plants equipped with 4-bed con­verters.

The introduction of the first commercial caesium-promoted vana­dium catalyst in the late 1980s meant a tremendous step-for­ward in reducing tail-gas emissions. High performance caesium catalyst has made it possible to reduce emissions by 50% or more, with a conventional 3+1 converter.

Caesium-enhanced catalyst allows a lower bed inlet (ignition) temperature, resulting in higher conversion and lower SO2 emission than with conventional catalyst. Figure 1 shows the effect of lower bed inlet temperatures on equilibrium conversion and the improvement possible using caesium catalyst.1 The lower ignition temperature of the caesium catalyst also accelerates start-ups when applied as top layers, reducing the time taken to reach emission compliance.

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New prospects for copper-zinc leaching

Summary

New leaching technology is currently being developed by the Canadian company Nitrosyl Technologies Corporation for the treatment of copper-zinc concentrates. The Nitrosyl metal sulphide leaching process offers a low pressure, low temperature route for the production of zinc and copper products from complex ores, without the need for prior separation of the ore into individual concentrates.

Abstract

While existing smelter technology is well suited to the treatment of relatively clean copper and zinc concentrates, the treatment of concentrates produced from complex ores frequently results in high penalty charges due to impurity concentrations. Smelters cannot process combined copper-zinc concentrates and the flotation separation of these metals to produce separate concentrates frequently results in high metal losses. In some deposits the ores are too fine grained to make saleable concentrates.

Vancouver-based Nitrosyl Tech-nologies Corporation, a wholly owned subsidiary of Expatriate Resources Ltd, was created to develop patented technology for the production of sulphuric acid and metal sulphide leaching.

The Nitrosyl Metal Sulphide Leaching Process offers a low temperature, low pressure alternative for the treatment of copper-zinc concentrates or concentrates of the individual metals, which may have high impurity metal concentrations.

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A legacy of relevance to the sulphur industry

Summary

Dawn Lee Wakefield* pays tribute to the late Sir Derek H. R. Barton who in recent years researched, developed and patented a novel process which combines the ­synergistic oxidation of hydrogen sulphide to sulphur with the oxidation of saturated hydrocarbons to useful chemical products, thus potentially making sulphur recovery from hydrogen sulphide less costly since another more valuable product is also formed.

Abstract

Sir Derek H. R. Barton is well known to the international world of scientists and researchers. A Nobel laureate in organic chemistry in 1969, Sir Derek spent his lifetime inventing new chemical reactions. In fact, in his final faculty position at Texas A&M University in College Station, Texas, USA he continued to invent new chemical reactions, emphasizing new ways of looking at problems of long standing to develop solutions.

One of his last major research projects for which he subsequently obtained his final patent focused on research into the synergistic oxidation of hydrogen sulphide to sulphur. In his own words, Dr Barton noted that this was “the first time a reaction had been invented in which the detoxification of hydrogen sulphide to give sulphur is synergistically coupled with the oxidation of saturated hydrocarbons to ketones and alcohols in satisfactory conversion and quantitative yield.”

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Norddeutsche Affinerie makes the grade

Summary

Since August 1998, Norddeutsche Affinerie (NA) in Hamburg, Germany has been producing a "premium quality" sulphuric acid from its copper smelting operation. The new grade has a purity equivalent to sulphur-based acid. New technology has been installed to reduce mercury levels to below 0.1 ppm. Lisa Connock reports on recent developments at NA and the company's strategy to increase the volume of acid marketed directly to customers.

Abstract

Norddeutsche Affinerie, one of Europe’s largest copper producers, operates one of the world’s most modern copper smel­ters with an annual production of about 370,000 tonnes of copper cathodes. The main copper products are electrolytic copper, continuous-cast rod, continuous-cast shapes and copper powders. NA is located on the southern outskirts of Hamburg, Germany, and has been in existence for more than 130 years. In addition to its core business, the production and refining of copper from primary (copper concentrates) and secondary (scrap) raw materials, NA also produces and sells major coproducts from copper production. In 1998 sales consisted of 740,000 t sulphuric acid, 500,000 t iron silicate stone or slag (used predominantly for river embankments), 7,500 t lead, 300 t silver and 8 t gold.

Since 1910, the production plant has been located on 980,000 m2 of land on the Peute, in the immediate vicinity of Hamburg’s harbour, about 5 km from the city centre of Ham­burg. Because of it’s close proximity to the city, the implementation of state of the art pollution control measures has been important from an early stage. In 1936, NA was the first company in the world to install a sulphuric acid contact plant at a metallurgical production site (at that time a single absorption plant) for environmental protection reasons.

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Port of convenience?

Summary

Plans to build a new sea terminal shipping molten sulphur from Canada's Pacific North West have still to become reality. Could this option, novel to Canada, become an important safety valve for an export trade under pressure? Chris Cunningham reports.

Abstract

When Sulphur was recently in Calgary, Albertan sulphur producers looked to be on the point of acquiring a brand new export option in the shape of an extra sea ­terminal.

Our informal discussions with Sulphur Corporation of Canada (SCC) indicated that construction of a new terminal at Prince Rupert port, to the north of Van­couver, was about to begin. More to the point, this would be the major producing and exporting regions first opportunity for the export by sea, from a domestic port, of mol­ten sulphur.

Subsequently, news of the projects progress has been difficult to track down, even though – according to Prince Rupert Port Corporation’s latest annual report – construction was due to start by the middle of 1999. The idea of a terminal at Prince Rupert handling molten product has a long history, and it is not unfair to say that the Calgary-centred, Van­couver-focused sulphur industry is more than a little sceptical about it.

We aim to provide more news of the project following our imminent trip to Calgary for the Sulphur 99 conference (although we have collected some introductory detail; see Boxes 1 and 2). However, there is value in looking at the current plan for Prince Rupert from a prespective of the current state of the Canadian sulphur industry.

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