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Slag

Slag is a partially vitreous by-product of smelting ore to purify metals. It can be considered to be a mixture of metal oxides; however, slags can contain metal sulfides (see also matte) and metal atoms in the elemental form. While slags are generally used as a waste removal mechanism in metal smelting, they can also serve other purposes, such as assisting in smelt temperature control and minimizing re-oxidation of the final liquid metal product before casting.

Smelting of ores
In nature, the ores of metals such as iron, copper, lead, aluminium, and other metals are found in impure states, often oxidized and mixed in with silicates of other metals. During smelting, when the ore is exposed to high temperatures, these impurities are separated from the molten metal and can be removed. The collection of compounds that is removed is the slag. In many smelting processes, oxides are introduced to control the slag chemistry, assisting in the removal of impurities and protecting the furnace refractory lining from excessive wear. In this case, the slag is termed synthetic. A good example is steelmaking slag: quicklime and magnesite are introduced for refractory protection, neutralising the alumina and silica separated from the metal, and assist in the removal of sulfur and phosphorus from the steel.

Ferrous and non-ferrous smelting processes produce different slags. The smelting of copper and lead in non-ferrous smelting, for instance, is designed to remove the iron and silica that often occurs with those ores and separates it as an iron silicate based slag. Slag from steel mills in ferrous smelting, on the other hand, is designed to minimize iron loss and so mainly contains oxides of calcium, magnesium, and aluminium.

Ancient slag use
The Bronze Age of the Mediterranean was synonymous with vast quantities of differential metallurgical processes. A slag by-product of such workings was a colorful, glassy, vitreous material found on the surfaces of slag from ancient copper foundries, primarily blue or green, which was formerly chipped away and melted down to make glassware products and jewelry, or was ground into powder to add to glazes for use in ceramics. Some of the earliest such uses for the by-products of slag have been found in ancient Egypt.

Modern slag uses
Slag has many commercial uses, and is rarely discarded. It is often reprocessed to separate any other metals that it may contain. The remnants of this recovery can be used in railroad track ballast, and as fertilizer. It has been used as a road base material and as a cheap and durable means of roughening sloping faces of seawalls to progressively arrest the movement of waves. Blocks of slag have been used in the construction of retaining walls and foundations.

Ground granulated slag is often used in concrete in combination with Portland cement as part of a blended cement. Ground granulated slag reacts with water to produce cementitious properties. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability. Since the unit volume of Portland cement is reduced, this concrete is less vulnerable to alkali-silica and sulfate attack.[citation needed]

What was once an unwanted by-product of the steel making process, can now be recycled and used in the manufacture of high performance concretes.

When iron ore is heated in a blast furnace, the impurities or ‘slag’, which include large quantities of calcium and silica, become molten and are separated from the raw iron.

As the slag is channeled out of the furnace, thousands of gallons of water are poured over it. This rapid cooling, often from a temperature of around 2,600°C, is the start of the granulating process. This process causes several chemical reactions to take place within the material, and gives the slag its cementitious properties.

The water carries the slag in its slurry format to a large agitation tank, from where it is pumped along a piping system into a number of gravel based filter beds. The filter beds then retain the slag granules, while the water filters away and is returned to the system.

When the filtering process is complete, the remaining slag granules, which now give the appearance of coarse beach sand, can be scooped out of the filter bed and transferred to the grinding facility where they are ground into particles that are finer than Portland cement

This previously unwanted recycled product is used in the manufacture of high performance concretes, especially those used in the construction of bridges and coastal features, where its low permeability and greater resistance to chlorides and sulfates can help to reduce corrosive action and deterioration of the structure.

Basic slag
Basic slag is a byproduct of steelmaking by the basic version of the Bessemer process or the Linz-Donawitz process. It is largely limestone or dolomite which has absorbed phosphate from the iron ore being smelted. Because of the slowly-released phosphate content, as well as for its liming effect, it is valued as fertilizer in gardens and farms in steelmaking areas. According to the American Association of Plant Food Control Officials, basic slag must contain at least 12% total phosphoric acid (P2O5) or be labeled "low phosphate".

炉渣

   又称溶渣。火法冶金过程中生成的浮在金属等液态物质表面的熔体，其组成以氧化物（二氧化硅，氧化铝，氧化钙，氧化镁）为主，还常含有硫化物并夹带少量金属。
　　炉渣的组分靠加入适量的熔剂（石灰、石英石、萤石等）进行调整。在冶炼过程中通过对炉渣组分和性质的控制，能使脉石和氧化杂质的产物与熔融金属或硫顺利分离，脱除金属中的害杂质，吸收液态金属中的非金属夹杂物不直接受炉气污染，富集有用的金属氧化物；在电炉冶炼中还是电阻发热体。炉渣在保证冶炼操作顺利进行、冶炼产品质量、金属回收率等各方面起着决定性作用，例如炼钢作业中有“炼好渣，才能炼好钢”的说法。
　　根据冶金过程的不同，炉渣可分为熔炼渣、精炼渣、合成渣；根据炉渣性质，有碱性渣、酸性渣和中性渣之分。许多炉渣有重要用处。例如高炉渣可作水泥原料；高磷渣可作肥料；含钒、钛渣分别可作为提炼钒、钛的原料等。 有些炉渣可用来制炉渣水泥、炉渣砖、炉渣玻璃等。
　　cinder
　　煤在锅炉燃烧室中产生的熔融物，由煤灰组成。可作砖、瓦等原料。