Zinc smelter

A zinc smelter , i.e. a steelworks for zinc production, is understood to be an industrial company for the extraction of metallic zinc . The raw materials used for this can be both primary raw materials , i.e. ores , and secondary raw materials such as recycling materials , zinc-containing waste, slag, etc. The term hut encompasses the entirety of all machines , facilities, apparatus and systems , including the buildings and the property. In Germany only one smelter, Nordenhamer Zinkhütte GmbH in Nordenham , produces zinc metal from ores.

Slag tapping on a zinc smelting furnace

Driveway to the Harlingerode zinc smelter

The history of zinc smelting

Although zinc occurs in relatively large quantities in the earth's crust among metals , it has long been unknown as a pure metal in technical applications. This was mainly due to the fact that the usual metal extraction through the carbothermal reduction of zinc is difficult. At the reduction temperatures of over 1000 ° C, it is already in vapor form (boiling point 907 ° C). The base metal is difficult to precipitate from the gas phase without reoxidation .

In India , a production of zinc in larger quantities for the 13th and 14th Century, can be detected in China for the 17th century. The merchant shipping made it into the cabinets of rarities in mansions in Europe.

In the Central European mining centers (e.g. Harz ), zinc was a coincidental product. When lead was melted in the charcoal shaft furnace, smaller amounts of zinc condensed on the cooler upper part of the furnace shaft. Zinc is often associated with lead in the ores. The name of the metal is said to come from the prongs that formed as approaches in ovens. By installing a slate, the zinc chair , zinc was finally extracted systematically for the first time in the 16th century.

In contrast, the use of zinc as an alloy component of copper alloys ( brass ) is older : Since the activity of zinc in a melt with other metals declines, the production of brass with up to 23% zinc was possible as early as 1400–1000 BC in the Middle East. In the early modern period, this alloy was melted in Central Europe from carbonate furnace attachments (furnace gally) and copper .

On a larger scale, zinc smelting succeeded after successful attempts by the metallurgical engineer Ruberg at the Wessola hut in Upper Silesia from 1798 to 1800. The first pure zinc smelters were built alongside those in Upper Silesia in the 19th century in Belgium and in the Stolberg-Eschweiler area .

Zinc smelters, like other metal smelters, were mostly built either near zinc ore deposits (e.g. Stolberg , Harz ), in a convenient location (e.g. Nordenham ) and / or near centers of hard coal mining ( Ruhr area ).

Due to its low melting point of 419 ° C, zinc was particularly suitable for cast utensils (e.g. pots, fittings) and was easy to roll into sheet metal. It was also used in brass production. In the 20th century, the special suitability of zinc for active anti-corrosion coatings on steel / iron materials was discovered. As a result, zinc smelting increased significantly.

The process of industrial zinc smelting

In nature, zinc never occurs in its pure form, but is bound to oxygen , sulfur and / or silica in zinc minerals . It is only through the chemical separation of the zinc in the metallurgical processes that a metal is obtained that is suitable for further processing or technical application.

The most important zinc minerals

Surname	chemical formula	Zinc content in%
Zinc cover	ZnS	67
Marmarite	(Zn, Fe) S	45
Smithsonite (zinc spar, calamine)	ZnCO 3	52
Hemimorphite (diatomaceous earth)	Zn 2 SiO 4 * H 2 O	54
Willemite	Zn 2 SiO 4	58
Zincite (red zinc ore)	ZnO	80

Ores from natural deposits usually only contain 4 to 20% zinc. After a wet mechanical treatment (flotation), concentrates with a zinc content of 45 to 60% are obtained. Lead and zinc ores are very often found together. Since the two metals can only insufficiently be separated from one another in the flotation , the respective mixed concentrates are obtained. After appropriate pre-treatment, slag from lead smelting is often used as a raw material for zinc extraction.

Preparation of the ores (roasting)

Schematic representation of the Dwight-Lloyd process

Fluidized bed roasting furnace

All known zinc smelting processes require a pretreatment of the ore concentrates. Sulphidic zinc ores are desulphurized by roasting , carbonate zinc ores are burned ( calcined ) to remove the carbon dioxide . As a rule, lump ores are roasted on sintering belts ( Dwight-Lloyd process ), and fine ores in fluidized-bed roasting ovens. Floors - or Handfortschaufelungsöfen are outdated equipment, which have not been used more in recent decades. When roasting, the sulfur is volatilized as sulfur dioxide and is replaced by atmospheric oxygen:

${\displaystyle \mathrm {2\ ZnS\ +\ 3\ O_{2}\ \longrightarrow 2\ ZnO\ +\ 2\ SO_{2}\uparrow } }$

In a catalytic process, sulfuric acid is produced as a by-product from sulfur dioxide .

Calamine was distilled in shaft ovens or rotary kilns :

${\displaystyle \mathrm {ZnCO_{3}\ \longrightarrow \ ZnO\ +\ CO_{2}\uparrow } }$

Secondary zinc precursors

Among secondary Vorst open is understood to intermediates, residues and waste materials , which allow an economical metal recovery due to their metal content. In the last few decades in particular, the recovery of valuable substances from such materials has increased in importance. This is mainly due to the fact that the content of the usable metal in most cases exceeds that of natural ores and further processing is often not necessary before smelting. In addition, ever stricter environmental laws stipulate recycling to save energy and raw materials, as well as to avoid landfilling that is harmful to water and soil .

The most important secondary raw materials for zinc are:

• Metallic scrap such as gutters, zinc plates or die-cast products.

• Top slag . These consist of an intermetallic compound between zinc and iron ( hard zinc ) and occur during continuous strip galvanizing .

• Zinc-containing slag from the extraction of other metals, in particular lead or zinc itself ( clearing ash ).

• Dross , trass or other oxidic zinc residues that arise from the surface oxidation of zinc melts during hot-dip galvanizing, remelting or zinc extraction.

• Electroplating sludge or residues from wastewater treatment in zinc processing companies.

• Steel mill dust , cupola dust and blast furnace top dust . The zinc gets into the iron metallurgy through the high proportion of galvanized scrap, partly also through external input of die cast parts from the automobile recycling. Due to the prevailing temperatures, it is transferred to the filter dust .

The metallic secondary raw materials scrap and top slag are used directly in the zinc smelter and, if necessary, crushed or packaged beforehand so that they fit into the metallurgical apparatus. As a rule, a simple remelting followed by refining is sufficient (see below).

Slag is mainly treated as a liquid by blowing in atmospheric oxygen and coke ( anaconda process ). This creates a dust rich in zinc oxide. Clearing ashes used to be enriched in rotary kilns or in shaft furnaces . Nowadays, however, this is no longer economical, or clearing pockets are no longer produced.

If the zinc content is high enough (30 to 40%), dross, trass and flue dust are fed directly into the zinc smelting process. If the chlorine content is high (e.g. due to pickling residues from galvanizing), prior treatment is necessary. Up until the 1980s, this was carried out in a rotary kiln, where the chlorine content was volatilized and collected in a wet scrubber with subsequent soda precipitation . In the last decades a wet treatment (leaching) with heated soda solution has been used for this purpose .

The rolling process has established itself for poorer oxidic zinc precursors, especially steel mill fly ash . The Contop® process in the cyclone furnace and the scan dust process in the plasma furnace are of lesser importance because of the high expenditure on equipment and energy requirements .

Pyrometallurgical zinc smelting process

The basic principle of pyrometallurgical zinc extraction is based on the reduction of zinc oxide by carbon monoxide. Carbon monoxide is created when carbon carriers are burned to carbon dioxide according to the Boudouard equilibrium .

${\displaystyle \mathrm {ZnO\ +\ CO\ \longrightarrow \ Zn\uparrow +\ CO_{2}} }$

Reduction of zinc

${\displaystyle \mathrm {CO_{2}\ +\ C\ \longrightarrow \ 2\ CO} }$

Boudouard equilibrium

The zinc smelting in the lying muffle

Using the lying muffle process , metallic zinc was produced from roasting blende, desulphurised (= oxidic) zinc ore or calamine. It was the oldest industrial process and was used and continuously improved from the beginning of the 19th century until the beginning of the 1970s. The zinc raw materials were poured into muffles around 2 m long together with lump coke . These were clay tubes with an oval cross-section, closed on one side. The muffles had a capacity of 68 to 110 liters. They were indirectly heated in units of 120 to 132 pieces lying at a slight incline in a furnace housing at approx. 1400 ° C with gas. The last smelters using this process had 20 such furnaces and a capacity of around 60,000 tons of zinc per year. The zinc was formed in vapor form and was condensed in a receiver. From there it was tapped into transport pans about once per shift . Downstream of the capacitor was the Düte or Allonge . Zinc dust was deposited there, which was technically generated by reoxidation. The residue left in the muffle, the cleaning ash , had to be removed periodically (approx. Every 24 hours). At the same time, there was a renewed loading of raw materials. For this purpose the template was dismantled (= zinc maneuver ).

In the extreme heat, working on the zinc furnaces was unimaginably difficult by today's standards. In recent years, machines have been available for loading and clearing the muffles, which has made work easier. At some locations, the zinc smelter itself included a so-called mufflery , in which the muffles were produced. The high expenditure of equipment and work, as well as the high specific energy consumption in connection with a low output of zinc metal of only about 60% with simultaneously high environmental pollution led to the extinction of this process. The last plants in Germany were shut down at the beginning of the 1970s.

The zinc smelting in the standing muffle

New Jersey retort

The standing muffle method , also known as the New Jersey vertical retort method after its first application , was a consistent further development of the horizontal muffle. A single muffle with significantly larger dimensions of around 8 m high and a flat cross-section of 2.6 m × 0.3 m now stood vertically in a furnace housing. Only the availability of a suitable material in the form of silicon carbide made the development of large muffles possible. Due to the vertical arrangement, the muffle could be charged from above and gravity could be used to discharge the scraping ash at the bottom of the retort. The upper 2 m were not heated and served as a reflux condenser for the unwanted lead. The filling consisted of briquettes , which were composed of zinc-containing raw materials (roasting blend , calamine or zinc-containing recycling materials) and coke . Before use, the briquettes were coked in special ovens . This made them more stable and porous, which is more favorable for the solid-gas reaction, and they were preheated. The zinc vapor generated in the muffle was deposited in a condenser. The condenser was connected to the actual muffle with an inclined channel. The more modern variant of the condenser, the spray condenser , consisted of a closed zinc bath that was kept at around 500 ° C by water cooling. A centrifugal wheel, the so-called impeller , was immersed in this bath . As it rotated, liquid zinc was atomized on which the zinc vapor could condense without re-oxidation.

The ovens stood in rows of 20 to 40 pieces on the zinc huts.

The process was last used in Europe at the Harlingerode zinc smelter in 2000 and is now technically obsolete. The difficult-to-use lead-containing scraping ash residues represented a major environmental problem.

The Imperial Smelting Process

The Imperial Smelting process , or IS furnace for short , was a shaft furnace in which zinc and lead could be produced simultaneously by setting defined furnace ratios. The process originated in the 1950s and was one of the first metallurgical processes to be developed entirely on a theoretical basis. The shaft furnace itself is constructed similarly to an iron blast furnace , only its dimensions are significantly smaller. The furnace is charged with preheated Möller from above via a pressure-tight seal on the furnace top. In the lower third, a hot breeze with 700 to 950 ° C is blown in from the side. This combustion air is heated by the furnace gases with a high carbon monoxide content via a cow heater or cowper . The reaction of metal oxide with carbon or carbon monoxide produces zinc vapor and liquid lead. While the lead and slag are tapped periodically, the zinc vapor is precipitated in a condenser by spraying liquid lead. For thermodynamic reasons, this is not possible in any other way, as otherwise the zinc would reoxidize through the top gas atmosphere. Due to the low solubility of zinc in lead, up to 400 t of lead are in circulation in the condenser, from which the zinc is separated out again when the temperature drops ( Seiger process ).

Despite the high expenditure on equipment, the process lasted for a long time because of the simultaneous recovery of lead and its good suitability for secondary raw materials. In Germany, the IS process ended with the insolvency of Metallhütte Duisburg (MHD) in 2005. There, up to 100,000 t of zinc and 30,000 t of lead were extracted in a single furnace per year.

Other rarer zinc smelting pyrometallurgical processes

KIVCET procedure

These are rarely or individually applied processes, some of which are no longer used.

• Fast process (rotating retort, heated inside or outside)
• Josephstown Process (St. Joe Lead Co. electrothermal process)
• KIVCET process (zinc and lead simultaneously, electrothermal)

The production of fine zinc (refining)

The production of die-cast alloys based on zinc and copper requires a particularly pure zinc of at least 99.99%. Even small amounts of lead and cadmium lead to intergranular corrosion and thus to destruction of the workpiece, also known as zinc plague . While no further refining is necessary for the hydrometallurgical extraction of zinc , metallurgical zinc produced by pyrometallurgy had to undergo post-treatment. Because of the comparatively low boiling points of zinc and cadmium, fractional distillation or rectification was obvious. This process was also used in the USA by New Jersey Zinc. Co developed in Palmerton and consisted of two stages. In a first step ( lead column ), liquid raw zinc was introduced into the upper third of a 15 m high distillation column made of silicon carbide bowls. The lower 2/3 were heated indirectly by gas to about 1100 ° C, the upper part served as a reflux condenser . At the foot of the column, an increased lead-containing, low- cadmium washing zinc leaked out. The impurity was separated from the washing zinc by Seigern in the form of a lead-rich zinc-lead mixture. Zinc that is rich in cadmium is precipitated in the condenser. This condensate was fed to a second distillation column (cadmium column), the combustion chamber of which was heated to 950 ° C. The reflux condenser was kept at temperatures between 765 and 907 ° C. in order to retain the cadmium-free zinc in the column and discharge it at the foot. The cadmium contaminated with zinc accumulated as dust in the condenser.

In the 1990s, a zinc refining column collapsed at Metaleurop SA in Noyelles-Godault in France as a result of overloading. The spontaneous combustion of the escaping zinc vapor led to the complete destruction of the system and cost the lives of several employees.

In order to remove any arsenic contained in the raw zinc , aluminum was initially stirred in. The resulting aluminum arsenide tended to form the highly toxic arsine gas when it came into contact with air humidity . Therefore, sodium was used later with the formation of sodium arsenide . This oxidizes to the safer sodium arsenate .

The hydrometallurgical zinc extraction

The hydrometallurgical zinc production was in the time of World War I developed. The first company to use this method in Germany was the Magdeburg zinc smelter owned by Georg von Giesches Erben . Nevertheless, it was not until the 1960s and 1970s that electrolysis had established itself as a zinc extraction process worldwide. There were two main reasons for this:

• The availability of large amounts of electrical energy.

• Zinc is a metal with a high electronegative potential (−0.763 V). First of all, all noble metals, theoretically also the hydrogen of the aqueous cell solution, are deposited on a cathode .

The procedural solution to these problems and the expansion of the power grid ultimately led to the fact that in the 1980s over 80% of world production was already made using the zinc electrolysis process.

Basic principle of hydrometallurgical zinc extraction:

${\displaystyle \mathrm {ZnO\ +\ H_{2}SO_{4}\ \longrightarrow \ ZnSO_{4}\ +\ H_{2}O} }$

Leaching

Here dissociated zinc sulfate in aqueous solution to Zn ²⁺ and SO ₄ ^2- .

${\displaystyle \mathrm {Zn^{2+}\ +\ 2e^{-}\ \longrightarrow \ Zn} }$

electrolysis

Leaching and caustic cleaning

Scheme of zinc leaching using the jarosite process

While a contaminated raw metal is always produced in pyrometallurgical metal production and the unwanted accompanying elements are separated out in a subsequent refining process , this is basically the other way around in hydrometallurgical zinc production. In the case of leaching with sulfuric acid , zinc is brought into solution in a targeted manner and foreign metals that are also captured are precipitated again . For this purpose, precisely defined acid concentrations must be set in various process stages. In the early days this was not yet fully controllable, therefore purer, especially low-iron ore types were preferred. The largest amount of ore concentrates on the market are sulphidic zinc ores ( zinc blende ), which have to be desulphurized / roasted before leaching , see section Preparation of the ores (roasting) . This results in poorly soluble zinc ferrites. With a strong leach, i. H. With high sulfuric acid concentrations, a lot of troublesome and difficult to precipitate iron goes into solution, with a weak leaching the zinc loss is great (up to 20%). Only the discovery of jarosite precipitation made a high zinc output possible. After the leaching of the roasted material, the iron is converted into the mineral jarosite (NaFe ₃ ³⁺ [(OH) ₆ | (SO ₄ ) ₂ ]) by adding ammonium or sodium salts and deposited. Landfill problems with jarosite, which is contaminated with heavy metals, led to the development of newer processes ( goethite or hematite processes) that enable recyclable waste. Lead and precious metals remain in the residue after leaching d. H. are not brought into solution and given to appropriate huts.

The lye cleaning d. H. the more noble metal ions that have dissolved are usually removed by cementation with zinc. Metallic zinc dust is added and stirred in in several steps. The metals such as B. Copper , cadmium or cobalt precipitate one after the other and can be sold on to the respective smelters. The various leaching steps, precipitations and lye cleaning are carried out in acid-proof lined stirred tanks. The required solid-liquid separation takes place after pre-thickening on filter presses .

Zinc electrolysis

Electrolysis (general)

In contrast to copper refining electrolysis, zinc electrolysis is a so-called extraction electrolysis .

After the nobler impurities have been removed from the solution, they are used in the electrolysis baths. Less noble cations such as Na ⁺ , K ⁺ or Al ³⁺ remain in solution during electrolysis, but increase the viscosity. This slows down the diffusion in the bathroom. The entry is therefore taken into account when selecting the raw material. Theoretically, hydrogen would be deposited before zinc, i.e. the electrolyte would be decomposed. However, since hydrogen is a gas and therefore a higher energy is required for nucleation at the cathode, what is known as hydrogen overvoltage occurs . The formation of hydrogen is made more difficult by smooth cathode sheets, which is why polished aluminum sheets are used. The cathodes are taken out of the bath after just 2 days and freed of the deposited zinc (= stripping ) on peeling machines so that the zinc layer does not simplify the formation of hydrogen. Furthermore, the hydrogen overvoltage decreases with increasing temperature, so that lower temperatures would be more favorable. However, this leads to a lower conductivity and thus a lower current yield. Bath temperatures of 35 to 40 ° C have proven to be a compromise. Due to the high cell voltage, the electrolyte heats up by itself above this temperature and must be cooled. When cooling, gypsum often falls out and clogs the pipes and fittings, which therefore have to be cleaned often.

The anodes consist of perforated lead sheet and the anode-cathode distance is 90 mm. The cathode sheets are about 1 m × 1.3 m in size. The cell voltage is 3.3 - 3.5 V with a current density of 400 to 600 A / m². The specific energy consumption is 50 GJ / t zinc. For comparison:

• Standing muffle: 60 GJ / t
• ISP process: 40-45 GJ / t
• Josephstown process: 54 GJ / t

Zinc smelting sites in Germany

Surname	state	place	Construction / commissioning	Shutdown	Procedure used	Notes / sources
Bensberg-Gladbach zinc hut	North Rhine-Westphalia	Bergisch Gladbach	1853	1931	Lying muffle
Billwerder zinc smelter	Hamburg	Hamburg-Billwerder	1905		Lying muffle
Birkengang zinc hut or Friedrich Wilhelm hut	North Rhine-Westphalia	at that time Eschweiler district Birkengang	1845	1926	Lying muffle
Borbeck zinc works	North Rhine-Westphalia	Essen district Borbeck	1847	1968	Lying muffle
Ruhrzink dates	North Rhine-Westphalia	Dates	1968	2008	Zinc electrolysis
Metalworks Duisburg (MHD)	North Rhine-Westphalia	Duisburg district Wanheim	1906	2005	Imperial smelting process	most recently Sudamin MHD GmbH
Harlingerode zinc works	Lower Saxony	Harlingerode	1936	2000	New Jersey Vertical Retort Process
Zinc works Magdeburg	Saxony-Anhalt	Magdeburg		1946	Zinc electrolysis	First zinc electrolysis in Germany, operator Georg von Giesches Erben
Zinkhütte Mülheim an der Ruhr	North Rhine-Westphalia	Mülheim an der Ruhr	1845	1873	Lying muffle
Zinkhütte Münsterbusch or Heinrich-Hütte	North Rhine-Westphalia	Stolberg district of Münsterbusch	1834	1967	Lying muffle
Nievenheim zinc works	North Rhine-Westphalia	Nievenheim		1971	Lying muffle
Zinkhütte Nordenham	Lower Saxony	Nordenham	1908	still in operation (2009)	until 1972 lying muffle, then zinc electrolysis	belongs to the company Glencore
Norzinco GmbH (Harz zinc oxide)	Lower Saxony	Harlingerode	2002 (1974)	still in operation (2009)	New Jersey fine zinc distillation	Predecessors were VHZ ( Heubach ) and HZO GmbH . On the grounds of the zinc hut Harlingerode
Steinfurt zinc smelter	North Rhine-Westphalia	at that time Eschweiler district Steinfurt	1850		Lying muffle
Zinc hut Velau	North Rhine-Westphalia	then Eschweiler district of Velau	1819	before 1922	Lying muffle

See also

• List of steelworks in Germany

literature

• Wolfgang Mehner: History of zinc metallurgy on the Harz: a chronicle of zinc production from 1900-1990 . 2nd Edition. Harz-Metall GmbH, Goslar 1995.
• Franz Pawlek: Metallurgy . Walter de Gruyter, Berlin 1982, ISBN 3-11-007458-3 .

Web links

• Internet presence of Recylex Germany: Harz-Metall, Norzinco and Weser-Metall GmbH
• Homepage of the Zinc Initiative - interesting facts about zinc extraction, processing and use

Individual evidence

1. ^ Establishment of a zinc smelter in Billwerder by the Pape, Henneberg & Co. company  in the German Digital Library