Large Metallurgical Acid Plant – Reference Site – Mt Isa

At the time in 2000, the Metallurgical Acid Plant built in Mt Isa (NW Queensland) was the largest in its size in the world. Below is an article that appeared in “Sulphur” Journal (May – June edition).


WMC Fertilizers Limited (WMCF) has recently brought into operation the world’s largest metallurgical sulphuric acid plant at Mount Isa, NW Queensland, Australia. The plant will provide feedstock in excess of one million tonnes/year for downstream use in the manufacture of ammonium phosphates, 150 km SE of Mt Isa at Phosphate Hill. Steven Campbell, project manager at WMC Fertilizers, describes the project.

WMC acquired the world class deposit from Broken Hill South some two decades ago and since that time, many feasibility studies have been carried out to determine a product mix to provide the best return on capital. In December 1996, the WMC board sanctioned capital well in excess of $700M AUD to produce in excess of one million t/y of ammonium phosphates.

The Phosphate Hill complex consists of the following:

  • 300 t/h beneficiation plant designed by SKM and constructed by Thiess.
  • 1500 t/d phosphoric acid plant constructed by a Mitsui/ABB/Clough consortium using Hydro Agri technology (single stage hemi-hydrate process).
  • 600 t/d ammonia plant constructed by Linde.
  • 135 t/d granulation plant constructed by Fluor Daniel using Jacobs technology.

The final product is transported by rail to the Townsville port, a distance of some 1000km. The complex was commissioned during the last quarter of 1999 and has currently produced in excess of 20,000 tonnes of product.

Mt Isa

Projected acid requirements for the fertilizer complex dictated that the sulphuric acid plant design would include the provision for burning sulphur whilst also treating metallurgical off gas from MIM’s copper smelter. The copper smelter consists of a primary smelting facility- the proprietary Isa-smelt bath furnace producing copper matte- and secondary smelting of the matte, which takes place in four Pierce Smith converters. The design/construction contractor for the job was Lurgi Metallurgie/ Lurgi Umwelt. The project was executed via a lump sum turn turn key basis. There were numerous challenges to this job. This was the largest metallurgical acid plant ever built and only the second time that separate companies intended to operate a smelter and the downstream acid plant. The design was based on design data provided by Mt Isa Mines Limited (MIM), WMCF validated all design inputs. The plant was built within 180 m of the local community. Mt Isa is a small town with a population of some 24,000 people. A decision had to be made about what to do with the waste heat (export or internal use) and a cost effective transport solution was required to transport the large volumes of acid to Phosphate Hill.

The Process

The Mt Isa plant site comprises five main areas:

  • Primary gas cleaning
  • Secondary gas cleaning
  • Oxidation Plant
  • Sulphuric Acid Storage
  • Sulphuric Acid loading and transport

Primary Gas Cleaning

Primary gas cleaning, located at the base of MIM’s copper smelter, is divided into two sections; gas from the copper Isasmelt waste gas scrubbing section and gas from the copper converter aisle. In the first section, copper Isasmelt waste gas, containing sulphur dioxide passes through an electrostatic hot gas precipitator which partially removes dust contained in the gas. The gas then enters a two-stage scrubbing system. The first stage is a quench tower where the gas is cooled from a temperature in the range of approximately 220-300°C to approximately 68°C by a spray system, while also being partially cleaned of impurities. During the second stage, a radial flow scrubber ( a Lurgi proprietary design) further cleans the gas where it exits with final temperature of approximately 66°C. The gases are drafted through the scrubber by an Induced Draft fan. In the second section, there are four MIM copper converters, each with a separate radial flow scrubber. The gas is cooled form approximately 450-550°C to approximately 68°C by a spray system, while also being cleaned of dust and other impurities. The four gas streams merge to a single duct. Each radial flow scrubber has an ID fan at the outlet to draft the gases. The gases from both streams are then combined before entering the secondary gas cleaning section.

Secondary Gas Cleaning

Secondary gas cleaning removes the remaining gaseous, liquid and solid impurities, leaving purified sulphur dioxide in a wet state. The gases first pass through two gas cooling towers which operate in parallel and cool the gas from approximately 67°C to approximately 40°C by condensing water from the gas phase and further removing impurities. The next stage cleans the gas using electrostatic mist precipitators (EMPs). This area has four primary and four secondary EMPs.

Oxidation Plant

The oxidation plant consists of an acid circuit, catalytic conversion system integrated with sulphur preparation (melting, filtration and storage) and burning. A waste heat boiler system is also incorporated in the plant. The wet SO2 bearing gas is taken from the secondary gas cleaning section and is passed through a drying tower which is irrigated with 95% sulphuric acid to remove the water from the gas. The dry sulphur dioxide gas is then passed through a three-pass catalytic converter where the sulphur dioxide reacts with oxygen in the present of a catalyst and is converted to sulphur trioxide. Prior to entering the catalytic converter, the gas is combined with sulphur dioxide produced from sulphur burning. Initially the sulphur (pastille form) is melted and filtered to remove impurities. The molten sulphur is then fed to a furnace where the sulphur reacts with oxygen to produce sulphur dioxide. The sulphur dioxide produced is combined with the gas from the exit of the drying tower. The sulphur dioxide from sulphur burning provided supplementary acid for the plant. Finally the sulphur trioxide passes through the absorption tower which is irrigated with 98.5% sulphuric acid. The sulphur trioxide is absorbed into the acid by reacting with the water content to produce the final product, 98.5% sulphuric acid in a liquid state. The heat generated from the catalyst conversion reaction and from the sulphur burning is used to raise steam. The plant incorporates a waste heat boiler and superheater for this function. The steam raised is used to drive a turbo-alternator set for the production of power. Power produced by the turbo-alternator set will deliver +65% of the site’s internal requirements.

Sulphuric Acid Storage

Acid from the sulphuric acid plant is delivered via a 200mm stainless steel pipeline to two 10,000 tonne mild steel storage tanks and a 450 tonne stainless steel scavenger tank.

Sulphuric Acid Loading and Storage

Acid from the storage tanks is pumped in two 250mm mild steel pipelines ( including leak detection systems ) to the rail loading station. Queensland Rail transports sulphuric acid to WMCF operations at Phosphate Hill by acid tank cars designed by American Transport Company, GATX. Each trainload is between 22 to 26 tank cars, each holding 58 tonnes. A rail siding has been constructed providing two lines for acid loading plus a line for the locomotive to bypass parked acid tank cars. The advantage of the GATX system is that the number of filling points is greatly reduced. WMCF has only two filling points. The GATX tank cars are interconnected by hoses so that once a car is filled, the acid will then pass through the car and into the next and so on. Malfunction of equipment and or / specific process conditions will initiate an emergency stop of the loading sequence. The displaced vapour is returned to the storage tanks so they are kept in equilibrium.

Reduced Emissions

The introduction of the sulphuric acid plant by WMC Fertilizers Ltd at Mount Isa will substantially reduce levels of sulphur dioxide emissions from the smelter operations. In the past, sulphur dioxide gases were emitted from the 270m lead smelter stack and the 153m copper smelter stack. The sulphuric acid plant has been designed to capture 80% or more of the sulphur dioxide emissions from the copper smelter. The guaranteed conversion efficiency is 97.4% at nominal capacity. Although performance testing has not been fully completed, data collected to date indicates in excess of 97.4% for acid production rates less than the nominal rate of 3700 t/d. Following the removal of the majority of the sulphur dioxide component, the tail gas from the sulphuric acid pant is exhausted via a 110m stack. The tail gas consists of minor traces of sulphur dioxide, oxygen and carbon dioxide with the majority being nitrogen.

Technical Aspects of the Project

The sulphuric acid plant is designed for a dry gas flow of 91.7Nm3/s at 10.1% SO2 at the inlet of the oxidation section. This is composed of gas from the Isasmelt, a large Pierce Smith converter on copper blow, a small Pierce Smith converter on copper blow, minimum sulphur combustion and dilution air. Although it is currently a single absorption plant, tie-ins and space have been provided in the acid plant design to facilitate a retrofit to double absorption if required. The off-gases will provide of the order of 800,000 tne/yr equivalent of sulphuric acid. To make up the shortfall, a 25 tph sulphur furnace consisting of 2 x 12.5 t/h Luro burners has been installed. The intention will be to run the sulphur burning train continuously at low turn down rates. This allows continued acid production in the case the smelter is shutdown. The presence of the sulphur burner also meant that a dedicated preheater was not required.

Primary gas cleaning consists of five proprietary Lurgi radial flow scrubbers similar to those installed at Magma, Arizona. Due to the fact there are no hot gas precipitators in between the converters and the gas scrubbers, these radial flow scrubbers were the preferred choice to previous operating experience. The stainless steel scrubbers were manufactured by NDA Engineering (NZ) with the variable speed ID fans on each of the gas scrubbers being supplied by Rothemulle. A brick lined mild steel quench tower is installed downstream of the Isasmelt hot gas precipitator. The counter current flow of gas to weak acid spray allows the arsenic to crystallise in the gas phase and be removed from the weak acid circuit via bleed control.

The EMPs were sourced from Chemical Process Equipment Limited (India). The drying and absorption towers were supplied by Evans Deakin Industries (EDI). There are 2 KKK blowers with electric drives rated at 4.9MW each and have installed soft starters to limit the starting current. The boiler system was supplied by Bertsch(Austria) with the SS 3-pass converter with an internal hot heat exchanger supplied by EDI. The converter is 20m high and 16m in diameter. Conventional vanadium catalyst was used for the first two passes with caesium impregnated catalyst used in the third pass for higher recoveries.

The pipework around the drying tower is anodically protected 316L SS, the absorption tower pipework is Sandvik SX, and the distributor is constructed of Superferrit. Lurgi’s proprietary NOx removal system has also been installed. The sulphur filter was supplied by AMA Filters. Waste heat is recovered and sent to a 9.7MW steam turbine supplied by Shin Nippon with the alternator supplied by ABB. Simon Carves Australia won the installation contract worth $5M for these works. The acid storage tanks and rail loadout facility were constructed by Stork ICM. The storage tanks are mild steel with anodic protection supplied by Kvaerner Chemetics.

Impact of the Community

Measures that were taken to alleviate community concerns due to proximity of residences included:

  • A 5m noise mitigation barrier running the length of the site. The alone cost $1.8m.
  • A commitment to provide an extensive vegetation barrier so as to minimise any visual affects.
  • Containment measures that would cater for the catastrophic failure of both acid storage tanks.
  • An acid resistant impermeable liner under the rail loadout facility designed to contain and collect any spillages.
  • Dedicated noise mitigation wall around the steam turbine/dump condenser.

Current Status

The fist sulphuric acid was produced from sulphur on 9 September 1999 and from metallurgical off gas on 29 September 1999. Commissioning commenced during July and ran through until early October. The name place capacity of the plant is 3700t/d. Although some short term high rates have achieved around 3,500t/d, performance testing had not been completed at the time of press and the gas sources had not yet been combined together in order to achieved the maximum rate.