What Is Manganese?
Manganese is a silvery-white solid metal, harder than iron, with an unusually complex structure, which is the reason for its embrittlement.
There are four known modifications of manganese. Metal alloys can stabilize any of them and obtain solid solutions with very different properties.
Manganese is a heavy metal with an atomic weight of more than 40. It is passivated in the air and covered with a dense oxide film that prevents further reaction with oxygen. Due to this film, manganese is inactive under normal conditions.
Manganese reacts with many simple substances, acids, and bases when heated, forming compounds with very different oxidation states: -1, -6, +2, +3, +4, +7. The metal belongs to the transition elements, exhibiting both reducing and oxidizing properties. Metals, such as iron, form solid solutions without reacting.
Where Is Manganese Mined?
Manganese is a fairly common element and the second most abundant metal after iron, making up about 0.1% of the weight of the earth’s crust. The most common mineral among manganese-containing compounds is pyrolusite, which is manganese dioxide (MnO2). The minerals hausmannite (Mn3O4) and braunite (Mn2O3) are also of great importance.
Manganese mainly accompanies iron in its ores, but independent deposits are also found. Manganese ores with high metal content deposits are found in Ukraine, South Africa, India, and Brazil. Up to 40% of the world’s reserves of manganese ores are concentrated in the territory of the Chiatura deposit (Transcaucasia).
Manganese dispersed in rocks is washed out by water and carried into the world’s oceans. Its content in seawater is low, but on the bottom of the oceans, it forms concretions together with iron, in which the element’s content reaches 45%. These deposits are considered promising for further development.
Where Is Manganese Used?
The main use of manganese (up to 90%) is in the metallurgical industry, mainly in producing alloy steels. Here, the metal is used as an alloying component.
1% of manganese turns steel into stainless steel. Manganese steel, containing up to 15% Mn, has high hardness, strength, and wear resistance. Manganese improves steel’s weldability because it lowers the melting point of oxides, making the welding process easier. Adding manganese to steel improves its deformability, producing complex shapes and parts without material cracking or breaking.
Adding manganese to cast iron when processing it into steel helps remove the sulfur from the cast iron, which is then transferred into slag.
A copper/ nickel alloy containing 13% manganese has a high electrical resistance and is “indifferent” to temperature fluctuations.
Manganese alloys with carbon and silicon are in high demand. Based on them, goods for various sectors of the economy are created—from armor, excavating, and crushing machines to equipment for the food industry.
Fig. End consumers of manganese in different branches of industry.
Manganese is a constituent of a number of magnesium-based alloys; it increases their resistance to corrosion.
Metal dioxide oxidizes ammonia and participates in organic and inorganic salts’ decomposition reactions. In this case, manganese dioxide acts as a catalyst. MnO2 is also a black and dark brown colorant for enamels and glazes in the ceramic industry.
Some metal compounds are used in fine organic synthesis and industrial organic synthesis.
Manganese arsenide has a gigantic magnetocaloric effect that becomes significantly stronger if exposed to high pressure. Manganese telluride is a promising thermoelectric material.
Potassium permanganate is used as an oxidizing and bleaching agent and disinfectant.
Manganese chloride, sulfate, and some of its other salts are used to produce colorants, fertilizers, seed mordants (growth stimulants), and siccatives.
Importance of Manganese
The physical and chemical properties of manganese are those that, in practice, we do not deal with the metal itself. Still, with its numerous compounds and alloys, its advantages and disadvantages should be considered from this point of view.
Manganese Advantages
- Manganese forms various alloys with almost all metals, which is a definite plus.
- Iron and manganese are completely mutually soluble, i.e., they form solid solutions with any ratio of elements and are homogeneous in properties. In this case, the alloy will have a much lower boiling point than manganese.
- Manganese alloys with carbon and silicon are of great practical importance. Both of these alloys are widely used in the steel industry.
- Numerous and varied manganese compounds are used in the chemical, textile, and glass industries, in producing fertilizers, and so on. Manganese’s chemical activity is the basis for this diversity.
Manganese Disadvantages
The disadvantages of this metal are associated with its peculiar structural characteristics, which prevent it from being used as an engineering structural material.
- The main one is fragility by high hardness.
- Relatively high melting +1244оC) and boiling points (+2095оС). Working with metal with such high indicators is difficult.
- The electrical conductivity of manganese is very low, so its use in electrical engineering is also limited.
Manganese Mining Process
How is manganese mined? Commercial production of manganese begins with ore mining and concentration.
Mining and Concentrating
There are several types of manganese ores: easy ore, which can be concentrated by washing and precipitation; refractory ore (carbonate and oxidized ores); and nonconcentrated high-quality ores.
Manganese ore is concentrated using multiple methods: gravity, magnetic, and flotation in various combinations. These methods are preceded by the so-called raw ore washing stage, which is of great importance in the general technological scheme of concentration of all types of manganese ore (oxide, mixed, carbonate).
The most well-known industrial method for clotting of manganese-containing raw materials is sintering.
If manganese carbonate ore is used, it is first roasted. In some cases, the ore is further subjected to sulfuric acid leaching. Then, manganese in the resulting concentrate is reduced by utilizing coke (carbothermic reduction). The same processes occur when producing manganese sintering as when sintering iron ores:
- Dehydration;
- Dissociation of oxides and carbonates;
- Redox reactions;
- Interaction in complex oxide systems.
Manganese Extraction Process and Refining
Manganese production process is directly related to its use. Its main consumer is the steel industry, and its needs require not the metal itself but its compound with iron – -ferromanganese. Therefore, we often mean the compound needed for ferrous metallurgy when discussing obtaining manganese.
Previously, ferromanganese was produced in blast furnaces. However, due to a shortage of coke and the need to use poor manganese ores, manufacturers switched to smelting in electric furnaces.
Open- and closed-type furnaces lined with coal are used for smelting to obtain high-carbon ferromanganese. Melting is carried out at a 110–160 V voltage by flux and flux-free methods. The second method is more economical as it allows more complete extraction of the element; however, with a high silica content in the ore, only the flux method is possible.
The flux-free method is a continuous process. A burden consisting of manganese ore, coke, and iron turnings is charged as long as it is melted. It is important to ensure that there are enough reducing agents. Ferromanganese and slag are tapped simultaneously 5–6 times per shift.
Manganese Ore Concentration Process
Let’s see how manganese is processed.
Manganese of technical purity (95–99.8% Mn) is called metallic and is obtained by electrosilicothermic and electrolytic methods.
Carbonate manganese ores, containing 19.0-19.4% of Mn, or carbonate concentrates, containing 24-25% of Mn, are the starting raw materials for producing of metallic manganese.
By-products of this technology are suitable for use in the following industries and agriculture:
1) gypsum-containing lixiviated sludge – as a raw material additive for the production of cement, mortars, slag concrete, building structural elements, road surfaces, bricks, etc.;
2) solid salt of ammonia sulfate, sodium, and potassium in combination with nitrogen fertilizers – when growing industrial crops;
3) magnesium and calcium fluorides when pelletizing with limestone – as a flux in ferrous and non-ferrous metallurgy;
4) alloy of heavy non-ferrous metals – for mixing into concentrates for smelting nickel and copper;
5) anode sludge (MnO2 base) – as an additive to the main raw material for ferromanganese production.
The process flow diagram for the electrolytic production of manganese includes seven process stages:
Leaching Ore or Concentrate with Sulfuric Acid
After crushing to a fraction of 0-20 mm and averaging carbonate manganese ore, a belt conveyor feeds it into an intermediate bin with a batch-weighing feeder for grinding in a ball mill.
Grinding manganese-containing raw materials to a particle size of less than 0.1 mm is required for leaching manganese. Wet grinding of raw materials eliminates their drying and does not harm the environmental situation, but is often unacceptable due to the disruption of the water balance of the process. An advanced technological solution is the wet carbonate ore grinding in a spent electrolyte (anolyte) containing sulfuric acid. In this case, a sulfuric acid reaction with carbonates and oxides of metals in the ore takes place simultaneously with grinding.
Silicon, aluminum, manganese (4+), titanium, phosphorus, and vanadium oxides contained in carbonate ore are practically insoluble in weak sulfuric acid solutions (3-30 g/l H2SO4). These compounds form the basis of the insoluble residue during acid leaching at the limit of wet ore grinding.
When wet ore grinding is combined with acid, the leaching transition of manganese into the solution during this operation is about 80%.
However, this technological process requires lining the ball mill’s working surfaces with acid-resistant rubber.
Parameters of the technological mode of “wet” grinding (leaching) of carbonate ore are:
- Ore supply into the mill – 3-6 t/h;
- Anolyte supply into the mill – 30-40 t/h;
- Sulfuric acid supply (92.5%) into the mill – 0.2-0.5 t/h;
- The circulation load of return pulp to the mill is experimentally 10-12 t/h (selected when working with a hydrocyclone).
After “wet” grinding (leaching), the pulp flows by gravity into a cascade of interconnected six agitators (2 backups) with a capacity of 50 m3 each.
Sulfuric acid (92.5%) is continuously supplied to the main agitator to maintain pH = 2 and “acid pulp” in the reductive leaching of anode sludge. To intensify the leaching process, the temperature of the pulp in the agitator is maintained within 50-550оC by heating the reactor with steam. At this stage, additional carbonate leaching occurs.
From the main leaching reactor, the pulp is fed by gravity into the agitator to oxidize iron and reduce manganese 4+. The operation is carried out by adding a calculated amount of hydrogen peroxide in an acidic environment to the solution at a pulp temperature of 50-55оC.
The technological purpose of this operation is as follows: manganese dioxide (MnO2) is practically insoluble in weak sulfuric acid solutions. At the same time, its content in the source ore is more than 1%. Therefore, to transfer it into a solution, it is necessary to reduce it to manganese oxide (MnO), which is carried out with hydrogen peroxide in an acidic environment.
Since the previously introduced sulfuric acid is consumed at the “redox leaching” stage, the acidity value at the agitator’s exit is pH = 3, corresponding to the concentration of H2SO4 in the pulp—2-3 g/l.
Neutralization of Leaching Slurry with Ammonia Water
After redox leaching, gravity feeds the pulp into the next agitator for neutralization. The agitator is continuously supplied with an aqueous 25% solution of ammonia NH4OH, the pulp of manganese carbonate isolated during the preparation of a part of the sulfate solution for evaporation (to remove sodium and potassium sulfates from the technological cycle), and with ammonium sulfate (NH4)2SO4 solution to increase its content in the sulfate solution up to 150-155 g/l.
The main purpose of this operation is to neutralize the acidic pulp to pH = 6.0-6.5 and hydrolytically clear the sulfate solution of dissolved impurities (iron, aluminum, copper, cobalt, silicon, etc.).
Manganese processing parameters:
- рН = 5,5-6,0;
- temperature 65-80оС;
- time – 1 hour.
After neutralization, the pulp is directed by gravity to a settler-thickener with a capacity of 250-300 m3. At a pulp inflow rate of up to 45 t/h, solid suspensions of the pulp are compacted to a “solid” content (lower discharge of the thickener) of 100-150 g/l. The “solid” content in the clarified part of the pulp (the upper discharge of the thickener) is no more than 1-2 g/l.
If necessary, a 1% polyacrylamide solution is introduced into the pulp flow, entering the thickener to speed up the clarification process.
Processing mode parameters of pulp clarification:
- pulp supply speed – up to 45 t/h;
- clarification time – 5-6 h;
- pulp temperature– 60-70о С;
- solid content in the clarified solution – no more than 2 g/l;
- solid content in the condensed part – 100-150 g/l;
- pH value in the thickener is 5.5-6.0.
After the thickener, the top drain (the clarified part) is sent for control filtration on a disk filter-thickener, and the bottom drain (the thickened part of the pulp) is sent to a disk filter for filtration.
Cleaning of Sulfate Solutions from Alkali and Alkaline Earth Metals
When carbonate manganese ore is leached with sulfuric acid, calcium, and magnesium carbonates are transferred into the solution. It is estimated that in one leaching cycle, the total concentration of CaSO4 and MgSO4 can be up to 110 g/l. Meanwhile, practical standards for the concentration of calcium and magnesium sulfates should be at most 4 g/l. Exceeding the norm leads to rapid “scaling” of pipelines, diaphragms, and heat exchangers – from leaching to electrolysis.
At the same time, the acceptable concentration of alkali metal sulfates (Na + K) in the feeding solution should not exceed 100 g/l because exceeding the content of these metals by more than 100 g/l leads to “salting” of the electrolyte and a decrease of its electrical conductivity.
To remove alkaline and alkali earth metals from the process cycle and to maintain optimal concentrations, three sequential operations are used:
- Sedimentation of calcium and magnesium fluorides through ammonium fluoride;
- Ammonium carbonate sedimentation;
- Evaporation of solutions containing ammonium, potassium, and sodium sulfates.
The final product, “dry” ammonium sulfate salt, is used as a fertilizer in agriculture.
Sulfide Purification of Solutions from Heavy Metals
To reduce the nickel, copper, and cobalt content, the solution is subjected to sulfide purification using a heavy metal precipitant—a hydrogen sulfide solution in ammonia water, H2S concentration 80-95 g/l.
In practice, this operation can be divided into two stages:
- The solution to be cleared is fed into an absorption column, where hydrogen sulfide released during the dissolution of sulfide sludge obtained from the previous sulfide purification is captured;
- A solution enriched with hydrogen sulfide is fed into a cascade of agitators, where a heavy metal precipitant is dosed into the first working apparatus.
Sulfide purification mode:
- рН = 6,5;
- temperature – 50-60оС;
- precipitant consumption – 3 l/m3 of the solution.
Absorption Purification of Solutions
Hydrolytic and sulfide purification of solutions from impurities in combination with filtration does not provide a high-quality solution that meets the requirements for obtaining branded electrolytic manganese because solutions contain silicon, colloidal sulfur, fine particles in the form of suspensions of heavy metal compounds (oxides and sulfides). Therefore, absorption purification is used to remove these impurities.
The process is carried out by introducing a solution of iron sulfate Fe2(SO4)3 at 50-100 ml per 1 liter of purified solution. Iron sulfate hydrolyzes in an environment pH = 6.0-6.5 and at a temperature of 50-550 C. Iron hydroxide particles absorb suspended particles of impurities, becoming larger over time when in contact.
The absorption purification sludge, containing more than 90% of Fe(OH)3, is returned for leaching carbonate ore after filtration and concentration in cartridge filters, and the solution is sent for saturation with sulfur dioxide (SO2).
Saturation of the Feed Solution with Sulfurous-Acid Anhydride to Obtain a Solution of Manganese Sulfate
To increase the manganese cathode current output and obtain a compact metal deposit on the cathode, the feed solution is saturated with sulfurous acid anhydride by passing gaseous SO2 through the lower part of the absorption device.
The off-gases from the absorption apparatus contain traces of SO2 and SO3, so they are sent to gas cleaning with ammonia water, and the feed solution is fed for electrolysis.
Electrolysis with Obtaining Metallic Manganese and Insoluble Sediments (Leaching Sludge)
Manganese is one of the most electronegative metals obtained by electrolysis of aqueous solutions. The standard potential for the main cathode reaction is φ = -1.10 V. Therefore, the reduction of hydrogen ions is the parallel reaction occurring at the cathode. This reaction is undesirable because its occurrence reduces manganese’s current efficiency.
The necessary conditions for obtaining manganese with high current efficiency are:
- The low acid intensity of the solution;
- The minimum content of impurities in the solution, at which the overvoltage of hydrogen evolution is small (iron, nickel, copper, cobalt, etc.);
- Low electrolyte temperature.
Technological parameters of the electrolysis process:
- Electric current intensity – 17 000 А;
- Temperature of electrolysis – 35–38оС;
- Current efficiency – 55%;
- Bath potential – 5 V;
- Number of anodes – 25 pcs.;
- Number of cathodes – 24 pcs.;
- Cathodic current density – 450 А/mm2;
The cathodes are made of titanium sheet (ВТ 1-0) 3 mm thick. The anodes are made of lead-silver alloy (1% silver). They are perforated cast platinum 10 mm thick, with round holes.
The manganese sedimentations on the cathode are removed as manganese pigs and smelted in induction furnaces.
Anolyte is used in hydrometallurgical processing for ore leaching.
During the electrolysis process, waste off-gases and sludge are formed in addition to the main products (cathode deposit and anolyte).
The amount of off-gases is 0.952 t/t manganese. The main components of off-gases are water vapor and oxygen formed at the anode.
The amount of sludge is 0.377 t/t manganese (solid equivalent). An iron oxidizer leaches iron sulfate and prepares an iron sulfate solution.
Electrolysis baths are cleaned once a month. During this operation, defective diaphragms and anodes are replaced.
Implementing electrolytic technology results in the commercial production of electrolytic manganese Mn997 (Mn content 99.7%) or Mn998 (Mn content 99.85%).
This metal is widely used as an alloying component for producing alloy steels and manganese alloys with various metals, primarily silicon. Metallic manganese is also used to produce various compounds in demand in the chemical, textile, and glass industries, such as fertilizer and other industries.
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Conclusion
Manganese is an essential metal in the modern world and has many applications. The main use (up to 90%) of manganese is in the metallurgical industry, mainly in producing alloy steels.
The principal raw material sources of manganese are natural minerals such as pyrolusite, which is manganese dioxide MnO2, hausmannite Mn3O4, and braunite Mn2O3.
Waste slag from ferroalloy production and other man-made waste from metallurgical production can be considered a secondary raw material base for manganese production. Promising raw material resources are соncretions with iron at the bottom of the world ocean, where manganese content reaches 45%.
The concentration of manganese-containing ores precedes the industrial production of manganese. Before processing, the concentrates are also roasted and sintered.
The main advantages of the electrolytic method of producing metallic manganese are producing rather pure metal (up to 99.5% Mn) and possibly processing low-grade manganese ores.
As a result of the implementation of electrolytic technology, electrolytic manganese Mn997 (Mn content 99.7%) or Mn998 (Mn content 99.85%) is obtained as a commercial product.
