Background

The ferro alloy that is used in by far the largest tonnage is standard grade or high-carbon ferro-manganese. It is the principal deoxidising agent in steelmaking and an important alloying element used for the production of ordinary mild steel as well as sophisticated low and high alloy steels. It has also the property of controlling the effect of sulphur, which when manganese is present, forms manganese sulphide and tends to float out of the liquid steel. Manganese has the effect of stabilising austenite. Steel with 12 to 14% Mn is fully austenitic, and is used on a large scale for its wear and abrasion-resisting characteristics. Bulk of the alloy these days is produced in electric furnaces, especially in those countries which have considerable manganese ore resources, such as India, South Africa and lately Australia. The process is quite flexible in that slags can be reprocessed to ferro-silico-manganese and then to refined low-carbon ferro-manganese or even to manganese metal.

The other high-carbon alloy of manganese is spiegel. This is an 18 to 20% Mn alloy with 6 to 8 % carbon. It is commonly used as a recarburizing agent, particularly if manganese addition is also desirable. Ferro-silico-manganese also called silico-manganese, is generally made in two grades, both containing 65 to 70% Mn, but with either 15 to 20% Si or 20 to 25% Si. These alloys are preferred by steelmakers needing to add manganese and silicon simultaneously for deoxidation purposes. The product of deoxidisation is a fluid manganese silicate slag. Silico-manganese is also the starting point for the manufacture of refined or medium to low-carbon ferro-manganese.The carbon content of silico-manganese is in inverse proportion to its silicon content. When the liquid alloy is reacted with manganese ore or a slag high in MnO, silicon is oxidised, thereby increasing the Mn content without an increase in carbon.

The different grades of refined ferro-manganese available are in the range 75 to 80% Mn, with varying carbon contents from 0.1 to 2% in 0.5% steps, the price varying inversely with carbon content. Taking into consideration the wide utility potential of the alloy and high cost of production, it is essential to formulate suitable strategies for improving the economic viability of the production process.

Raw Materials

Manganese ores

Manganese ores usually contain iron oxides, and in these ores the manganese can occur as MnO₂, MnO₃, Mn₃O₄ and also as MnSiO₃. Occasionally MnO₂ can occur in a relatively pure form, known as pyrolusite. For making standard ferro-manganese the Mn:Fe ratio should be 6 to 1. If it is higher than this, it can either be blended with a low-ratio ore, or scrap iron can be added to the furnace burden like in the smelting of ferro-silicon. Production of high grade Fe-Mn (85 to 90% Mn) is not advisable as the high-carbon alloy will disintegrate to powder on cooling or during storage.

The various manganese minerals found in these ores are: manganite (Mn₂O₃), hausmanite (Mn₃O₄), beaunite (3Mn₂O₃. MnSiO₃), rhodonite (MnSiO₃) and psilomelane, a hydrated oxide.

There are ores containing 32 to 40% Mn, but with a Mn:Fe ratio of 4:1 to 2:1, which are only suitable for blending with higher ratio ores, or for making spiegeleisen.

One of the major impurities present in manganese ore is phosphorus, most of which enters the alloy on smelting. For that reason, ores with less then 0.1% P₂O₅ are at a premium. Sulphur is no problem because it forms manganese sulphide, which is removed in the slag as well as in the fume. Ores with more than about 10% SiO₂ are used for silico-manganese production.

Ores containing 45 to 48% manganese are considered medium grade, whereas a high-grade ore will contain 50 to 56 % Mn.

As for sizing, the bulk of the ore should be in the size range of 40-60 mm with fines below 6 mm screened out.

Flux

Some ores are self-fluxing, depending upon the method of furnace operation, but to reduce the MnO content of the slag, limestone or dolomite is added to the burden.

Reductant

Coke is the usual reductant, and this is used in the size range of 6-20mm. If available, bituminous coal char is a desirable ingredient because it increases the charge resistance, permitting a higher operating voltage - the use of partially burnt coal (Jhama) as a partial dubstitute to coke needs to be looked into.

To make silico-manganese, silicious manganese ores and by-product slags (predominantly manganese silicates) from the ferro-manganese operation are used.

Alloy Specifications

There are a host of specifications for manganese alloys which vary in manganese, carbon, silicon and even nitrogen content. Some large suppliers list as many as twenty-four different analyses. As regards ferro-manganese, the ASTM specification No. A 99-61splits the alloys into the standard (high-carbon) grade, medium and low-carbon grades, as shown in Table 1. In the UK, the standard grade in reckoned as 78% Mn, and there is a premium or a discount for every 1% Mn above or below this figure. In some countries, the high-carbon alloy is specified into grades, namely 70 to 75% Mn and 75 to 80% Mn.

Because of the scarcity of low-phosphorus manganese ores, many users, especially alloy steelmakers, will pay a premium for low-phosphorus ferro-manganese below, say 0.15 % P.

Silico-manganese is generally sold in the UK in two principal grades, namely 14 to 20% Si and 20 to 25% Si, with Mn in both cases being 65 to 75%. In some countries, however, the 14 to 20% Si range is further subdivided. Some suppliers also list a higher silicon alloy with 28 to 32% Si and 0.1% carbon.

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