Molten silicate slag

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

In the beginning it was well known to people that most metals, as for instance tin or lead, melt at the reduction temperature. If so they are easily separated from the gangue. On the other hand, a metal that is reduced at a temperature below its melting point, wiU be mixed up with gangue and molten silicate slag. The latter was the case for the early direct method of iron manufacture. In antiquity, iron was manufactured with direct reduction from ore to forgeable iron, see figure 8.4. 

Lighter mixed silicate slag floats over the denser, molten copper(I) sulfide. Slag is drained off from time to time. Molten CU2S is transferred to a Bessmer converter where it is air oxidized at elevated temperatures producing metallic copper and sulfur dioxide ... 

The molten matte layer is transferred to a converter, where air is blown through it and sand is added so that the FeS forms a silicate slag... 

Native copper ore is crushed, concentrated by washing with water, smelted, and cast into bars. Oxide and carbonate ores are treated with carhon in a smeller. Sulfide ore treatment is complex, hut. in brief, consists of smelling to a matte of cuprous sulfide, ferrous sulfide, and silica, which molten matte is treated in a converter by the addition of lime and air is forced under pressure through the mass. The products are blister copper, ferrous calcium silicate slag, and SO . Refining is conducted by electrolysis, and the anode mud is treated to obtain the gold and silver. See Fig. I. 

The last part of ionic electrochemistry, ionics, is about pure electrolytes. A few decades back this electrochemistry would have been all about high-temperature liquids (liquid common salt at 850 °C was the role model). However, this has changed, and the temperatures for eliminating the solvent have deaeased considerably. Some molten salts are now room temperature liquids. At the other end of the temperature scale are the molten silicates, where large polyanions predominate. These are important not only in the steel industry, where molten silicate mixtures form blast furnace slags, but also in the corresponding frozen liquids, the glasses. 

Raw material addition and phosphorus vapor removal from the furnace occur on a more-or-less continuous basis. Removal of the molten calcium silicate slag, more than 7 tonnes of which is formed for each tonne of phos-... 

In order to model the flow behavior of molten silicate suspensions such as magmas and slags, the rheological behavior must be known as a function of the concentration of suspended crystals, melt composition, and external conditions. We have determined the viscosity and crystallization sequence for a Kilauea Iki basalt between 1250°C and 1149°C at 100 kPa total pressure and f02 corresponding to the quartz-fayalite-magnetite buffer in an iron-saturated Pt30Rh rotating cup. viscometer of the Couette type. The apparent viscosity varies from 9 to 879 Pa.s. The concentration of suspended crystals varies from 18 volume percent at 1250°C to 59 volume percent at 1149 C. The molten silicate suspension shows power-law behavior ... 

The molten ferroalloy is regularly tapped by tilting the arc furnace and pouring it into ladles while the silicate slag is also tapped but on the other side and disposed of in landfill. The sili-cothermic process yields a low-carbon ferrochrome or LC ferrochrome (0.05 to 0.50 wt.% C) but with a silicon content of 8 to 12 wt.% Si. Further purification of ferrochrome can be performed by the Simplex process, which consists in reacting, in the solid state, high-carbon with oxidized ferrochromium to produce the extra-low-carbon grade (0.01 wt% C). 

Slag consists mainly of molten silicate minerals, with aluminates, phosphates, and other ionic compormds as constituents. A slag is formed when a basic metal oxide such as CaO reacts at high temperatures with molten silica (Si02) ... 

Concentration of copper is done by flotation, and roasting converts iron sulfides to iron oxides. The copper remains as the sulfide if the temperature is kept below 800 °C. Reduction of the roasted ore in a furnace at 1400 °C causes the material to melt and separate into two layers. The bottom layer, called copper matte, consists chiefly of the molten sulfides of copper and iron. The top layer is a silicate slag formed by the reaction of oxides of Fe, Ca, and A1 with Si02 (which typically is present in the ore or can be added). For example,... 

Carbides are binary compounds in which carbon is the more electronegative partner. We shall come across silicon carbide, SiC, again later when we look at the chemistry of silicon this is a very hard material, manufactured in large amounts as an abrasive known as carborundum. Its hardness can be accounted for by the fact that its crystal structure is the same as that of diamond, with alternate carbon atoms replaced by silicon (Figure 10.13). A major industrial use of SiC is in steel refining where its addition to the molten metal removes metal oxide impurities with production of CO and a silicate slag, which is skimmed off. 

Reaction (13.4) is exothermic and reversible, and begins at about 700 K by Le Chatelier s Principle, more iron is produced higher up the furnace (cooler) than below (hotter). In the hotter region (around 900 K), reaction (13.5) occurs irreversibly, and the iron(II) oxide formed is reduced by the coke [reaction (13.6)] further down. The limestone forms calcium oxide which fuses with earthy material in the ore to give a slag of calcium silicate this floats on the molten iron (which falls to the bottom of the furnace) and can bo run off at intervals. The iron is run off and solidified as pigs —boat-shaped pieces about 40 cm long. 

As in die case of die diffusion properties, die viscous properties of die molten salts and slags, which play an important role in die movement of bulk phases, are also very stiiicture-seiisitive, and will be refeiTed to in specific examples. For example, die viscosity of liquid silicates are in die range 1-100 poise. The viscosities of molten metals are very similar from one metal to anodier, but die numerical value is usually in die range 1-10 centipoise. This range should be compared widi die familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for die temperature dependence of die viscosity of liquids as an AiTlienius expression is... 

Since the reactions (15), (16), and (17) require successively higher temperatures, the blast furnace temperature is kept highest near the bottom of the furnace. Near the bottom, the temperature, is sufficiently high that the impure iron—saturated with carbon—collects there as a molten liquid. The slag, which is mainly calcium silicate, CaSi03, removes any sand in the ore through reaction with limestone, CaC03. 

At blast furnace temperatures, calcium silicate is a liquid, called slag. Being less dense than iron, slag pools on the surface of the molten metal. Both products are drained periodically through openings in the bottom of the furnace. 

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