熔炼，煅烧和石墨化之简介

日期：2018-03-25 点击次数：1721

熔炼，煅烧和石墨化之简介

2018-03-25 

【TiangongKaiwu Tripod casting】

哈喽，小编尼克向诸位问好！欢迎光顾石墨化之友！今天就让我们一起探寻与冶炼相关的事吧！本期我们主要致力于熔炼，煅烧和石墨化等的区别，后期我们会逐渐转向石墨化技术探讨和研究。期待各位的继续关注，您的关注和订阅是小编*大的动力！

说到熔炼，我们*先听到的或许就是我国古代的“天工开物”了。“熔炼，是将金属材料及其它辅助材料投入加热炉溶化并调质，炉料在高温（1300～1600K）炉内物料发生一定的物理、化学变化，产出粗金属或金属富集物和炉渣的火法冶金过程。炉料除精矿、焙砂、烧结矿等外，有时还需添加为使炉料易于熔融的熔剂，以及为进行某种反应而加入还原剂。此外，为提供必须的温度，往往需加入燃料燃烧，并送入空气或富氧空气。粗金属或金属富集物由于与熔融炉渣互溶度很小和密度差分为两层而得以分离。富集物有锍、黄渣等，它们尚须经过吹炼或其他方法处理才能得到金属。”——来自百度百科

【Cowles furnace】

Smelting is a process of applying heat to ore in order to melt out a base metal. It is a form of extractivemetallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a chemical reducingagent todecompose the ore, driving off other elements as gases or slag and leaving the metal base behind.The reducing agent is commonly a source of carbon, such as coke—or, in earlier times, charcoal.

The carbon (or carbon monoxide derived from it)removes oxygen from the ore, leaving the elementalmetal. The carbon thus oxidizes in two stages, producing firstcarbonmonoxide andthen carbon dioxide. As most ores are impure, it isoften necessary to use flux, such as limestone, to remove the accompanying rockgangue as slag. Plants for the electrolytic reduction of aluminium are also generally referred to as aluminiumsmelters. Laborersworking in the smelting industry have reported respiratory illnesses inhibitingtheir ability to perform the physical tasks demanded by their jobs. Smeltinginvolves more than just melting the metal out of its ore. Most ores are thechemical compound of the metal and other elements, such as oxygen (as an oxide), sulfur (as a sulfide), or carbon and oxygen together (asa carbonate). To extract the metal, workersmust make these compounds undergo a chemical reaction. Smelting thereforeconsists of using suitablereducingsubstances that combine with those oxidizing elements to free the metal.

【Phosphate Smelting Furnace】

煅烧(calcining)是指缓和了物质分子结构的内在张力，从而使之能够适应在塑形过程中不断增强的力量，并于完工时更加结实。玻璃在窑中吹制后，借由在窑中煅烧及慢慢冷却，可使其强度和硬度增高。金属的延展性更大，因此它可以适于如锤打、扭转和弯曲等处理方式。由于这些操作均会增加金属坚固性能，因此为了达到好的效果，有时就必须重复煅烧。有些材料如玻璃和钢，煅烧后的冷却要徐徐进行，而在其他如铜或黄铜煅烧后就需急速冷却。

【Lime Shaft】

而我们后期会讨论无烟煤(Anthracites)煅烧获取煤质增碳剂的相关研究。煅煤增碳剂的生产设施与石灰窑的原理类似，如上图所示。无烟煤因为其价格低廉，含碳量高，结构致密，被用作多种炭素材料的原料。但是作为炭素材料原料，无烟煤在使用之前必须先要经过煅烧，去除其中的挥发分，并使其电阻率降低，导热率和真密度提高。在工业获取无烟煤增碳剂中，无烟煤的煅烧通常是采取两种方法完成。一种的在1300℃左右的罐式炉或回转窑中进行煅烧的气煅煤，另一种是在平均温度1700℃左右的电煅炉中煅烧的电煅煤。

【原煤的工业分析】

【气煤和电煅煤的性能指标】

【无烟煤中的显微结构类型】

Calcination also refers to as calcining. The IUPAC defines it as 'heating to hightemperatures in air or oxygen'. However, calcination is also used to mean athermaltreatmentprocess in the absence or limited supply of air or oxygen applied to ores and other solid materials to bringabout a thermaldecomposition. A calciner is a steel cylinder that rotates inside a heated furnace andperforms indirect high-temperature processing (550–1150 °C, or1000–2100 °F) within a controlled atmosphere.

The process of calcination derives its name fromthe Latin calcinare (to burn lime) due to its most commonapplication, the decomposition of calcium carbonate (limestone) to calcium oxide (lime) and carbon dioxide, in order to create cement. The product of calcination isusually referred to in general as "calcine", regardless of the actualminerals undergoing thermal treatment. Calcination is carried out infurnaces or reactors (sometimes referred toas kilns or calciners) of various designsincluding shaftfurnaces, rotarykilns, multiplehearth furnaces, andfluidizedbed reactors.

Examplesof calcination processes include the following:

•      decomposition of carbonate minerals, as in the calcination of limestoneto drive off carbondioxide;

•      decomposition of hydrated minerals, as in the calcination of bauxite andgypsum, to remove crystalline water as water vapor;

•      decomposition of volatile matter contained in raw petroleum coke;

•      heat treatment to effect phase transformations, as in conversion ofanatase to rutile or devitrification of glass materials

•      removal of ammoniumions in the synthesis of zeolites.

Calcination reactions usually take place at orabove the thermal decomposition temperature (for decomposition andvolatilization reactions) or the transition temperature (for phasetransitions). This temperature is usually defined as the temperature at whichthe standard Gibbs freeenergy for aparticular calcination reaction is equal to zero. For example, in limestonecalcination, a decomposition process, the chemical reaction is CaCO₃→ CaO + CO₂(g)The standard Gibbs free energy of reaction is approximated as ΔG°_r= 177,100 − 158 T (J/mol). The standard free energy ofreaction is 0 in this case when the temperature, T, is equal to 1121 K,or 848 °C.

碳的微观结构示意图：1易石墨化碳 2难石墨化碳 3介于之间

石墨化是把焙烧制品置于石墨化炉内保护介质中加热到高温，使六角碳原子平面网格从二维空间的无序重叠转变为三维空间的有序重叠，且具有石墨结构的高温热处理过程。

Some applications, such as graphite electrodes forthe electric arc furnace require a higher thermal and electrical conductivitythan that of baked carbon materials. These synthetic graphites normally followa production process similar to that of baked carbon (forming, impregnation,rebaking) but require an additional process step, that of graphitization wheretemperatures of around 3000°C are achieved.

The final step in graphite manufacture is aconversion of baked carbon to graphite, called graphitizing, i.e. heat-treatingthe material at temperatures in the region of 2600 °C to 3300 °C. During thegraphitizing process, the more or less pre-ordered carbon (turbostratic carbon)is converted into a three-dimensionally ordered graphite structure. Depending onthe raw materials and the processing parameters, various degrees of convergenceto the ideal structure of a graphite single crystal are achieved.

Since graphitization increases the lattice orderand produces smaller layer distances, it simultaneously leads to a considerablegrowth of ordered domains. However, the degree of order that can be reacheddepends largely on the crystalline pre-order of the solid used. These reducedlattice layer distances are macroscopically noted as a contraction in volume. Thisgraphitization-shrinkage is approximately 3 to 5%. Due to this shrinkage,density of the graphite increases.

In general, the graphitization process includes thestages as given below:

•Amorphousor baked carbon is converted to electro graphite by a thermal treatment atapproximately 3000°C.

•Essentiallyany amorphous carbon material can be graphitized. The potential crystallitegrowth and ordering are latent within the baked carbon structure.

•Underthe influence of temperature the crystallites grow and rearrange in an orderedpattern of stacked parallel planes. This transformation is accompanied by achange in the physical properties of the material.

•Thegreater the degree of crystallite growth during heating up, the better thegraphitability (graphitization degree), which effects the final resistivityachieved.

•Thereis a variation among different needle cokes concerning the graphitability.

•Thegraphitization degree depends on the structure of the basic material(graphitability) and the applied graphitization temperature. It is determinedby x-ray measurements.

The graphitizing process is also accompanied by apurification of the material treated, normally reducing the content ofimpurities to considerably less than 1000ppm. For many applications, this purityis insufficient, so that a thermal purification at higher temperatures up to3100°C with longer residence times is carried out to reduce the impurities to aconcentration of less than 200ppm. If still lower ash values are required, athermo-chemical purification is necessary.

Even after graphitization at around 3000°C mostgraphites contain small amounts of metallic impurities. If the ash values inthe material have to be below 200ppm, thermal purification is applied. Byadding gaseous halogens or halogen compounds, all hetero-atoms forming stablecarbides are transferred into volatile halogen compounds and thus removed. Bymeans of this procedure, impurities may be lowered to less than 1ppm.

石油焦一般在1700℃就开始石墨化，而沥青焦则要在2000℃左右才能进入石墨化的转化阶段。（信息来源：石墨化之友）

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