A safe jump in temperature
Plate heat exchangers made of high-grade materials in operation

Plate heat exchangers are normally only one of many components in a large, complex plant, but they are very important. Without this tech-nology, many processes would not be feasible, as we can see for example from an application involving a number of plate heat exchangers at a modern copper mill. These devices, which contain special materials and reflect many years of engineering experience, play an important role in gas cleaning applications.

Seven years ago, Bulgaria’s largest copper complex was in a poor state. Output was low, the quality of the copper was mediocre and environmental protection was only in its infancy. Today Pirdop, which is located about 80 km east of Sofia, is home to a copper complex that produces the red gold to international standards. e150 million were invested between 1998 and 2002, and the money was used to retrofit key process equipment and improve environmental protection. At a cost of about e50 million, Lurgi Metallurgie modified part of the plant on behalf of Union Minière Pirdop Copper to bring equipment into compliance with European environmental protection standards. Work carried out by Lurgi included a retrofit of three existing copper converters and construction of a new sulfuric acid line. The Essen based company installed the off-gas cleaning equipment in this section of the copper complex, and it was responsible for installation of the heat exchangers. Varitherm VT80 heat exchangers are located at a number of strategic locations around the complex. Three heat exchangers cool the converter hoods. Another two are used in the process of quenching roaster gas downstream from the flash smelting furnace. Besides copper, another important raw material is produced in Pirdop for the chemical industry, namely sulfuric acid. It is used, for example, for the production of fertilizers, for pigments and in fiber manufacturing. Sulfuric acid has other important applications in the petroleum processing industry, in the extraction of non-ferrous metals and in the production of explosives, plastics, viscose and washing agents. Elementary sulfur is normally used for production using a contact process, but sulfur dioxide produced from sulfide ores can also be used as a raw material. A number of costly techniques and processes are needed to produce industrial-grade sulfuric acid. It begins with the supply of copper ore, continues with processing of the ore and ends in complex steps to purify the gas, the copper and the sulfuric acid. As a result, a modern copper mill resembles a chemical complex.

The refining process
A very complex range of processes is used at the Pirdop copper complex. The raw material is delivered from far-away mines in Chile and Indonesia. About half comes from domestic sources. A mixture of concentrates and sand is turned into copper matte and slag in the flash smelting furnace. Blister copper forms in the converter, and oxygen is removed from the blister copper in the anode furnace. The copper content of the anode plates is 99.6 percent. However, to make the copper suitable for use as a raw material in cellular phones, cars or cables, it must undergo a further refining process. Electrolysis now enables Pirdop to supply copper which is 99.99 percent pure. A contact process is used to almost completely convert the sulfur content in the copper concentrate into sulfuric acid. The off-gas from the flash smelting furnace, which contains nitrogen dioxide, is another source of sulfuric acid. The exhaust gas cannot be routed directly through the contact furnace, because the gas has to be cleaned first. It includes airborne dust and arsenic compounds which would damage the catalyst. Electrofiltration is currently used to clean the roaster gas. The engineering experts for this process work at Lurgi Bischoff, the company which supplied the wet gas electrofilters and electrostatic gas cleaning equipment to the copper smelting complex. Plate heat exchanges help cool roaster gas exiting from the dust collector down to 20–40° C. The cleaned, dried gas is fed through the contact furnace. Sulfur dioxide is oxidized to form SO3 with the help of a catalyst. Because the reaction is strongly exothermic and cannot be completed in a single step, oxidation is performed using several contact layers, between which the gas mixture is cooled in a heat exchanger. The SO3 is adsorbed and converted to sulfuric acid. In addition to removing harmful nitrogen oxides from the exhaust gas, this process produces a valuable raw material. The sulfuric acid line can produce around 1,950 tons of sulfuric acid per day while complying with the latest environmental protection regulations.
Special material
The modernization process presented a huge challenge. “Despite ongoing construction work, the production of copper continued and was only halted for four weeks at a time to integrate the new equipment, which was installed next to the old systems.” This meant that a tight schedule had to be maintained. Carsten Reuter, Pirdop Project Manager at GEA Ecoflex in Sarstedt/Germany, said that the “normal lead time for plate heat exchangers made of standard materials is four weeks”.
However, the use of special material for the plate heat exchangers in the cleaning and cooling unit on the sulfuric acid system presented a special challenge on this project. The medium that flows through the heat exchanger can contain up to ten percent sulfuric acid plus other acids which are removed from the gas. Each heat exchanger has 221 plates with a plate thickness of 0.6 mm and an EDPM seal, providing a surface of 184 m2. Hastelloy C276 was selected because of its high durability. “The lead times for this material are very long, and you have to make provision for that when you are putting together your plan”, explained Reuter. Lead time for the material is up to five months. GEA Ecoflex has a stock of high-grade materials, but even special materials like Hastelloy are available in a number of variations. “Because the material is so hard, it is more difficult to work with, and even the stamping process takes longer”, Reuter said. You need the right equipment, and you have to have an instinctive feel for the work. GEA Ecoflex has a press line that is regarded as one of the most modern lines of its type in the world. It has a coil line for making the blanks, a pallet changing system to hold the blanks and a 15,000 t stamping press with integrated foil coating line. The line also has an automatic tool change system and a 630t punch press. According to Reuter the field of thermal transfer engineering is very complex. “We are specialists who very consciously limit ourselves to the production of plate heat exchangers.” Despite many years of experience (the company has been working in the field for 70 years), nearly every plate heat exchanger is a one-off item. The Varitherm heat exchanger is available with more than 40 different plate types which are then assembled to make an individual system. Inside the plate pack, the plates are in metallic contact at the support points. This ensures that the characteristics of the plate gap remain stable. Besides the dimensions and materials, various connections and company-specific colors have to be provided. Even the documentation is different. A common feature of all GEA Ecoflex products is low investment and maintenance cost, which ultimately results in improved efficiency