高炉的操作实践和使用寿命

高炉的操作实践和战役寿命

重建或更换高炉 (BF) 的成本非常高。因此，延长 BF 活动寿命的技术很重要，需要非常积极地进行。

大型 BF 通常每单位体积的活动产出略高。这种差异是因为较大的 BF 通常具有更现代的设计和良好的自动化。由于综合钢厂的生存能力取决于铁水 (HM) 的持续供应，因此在拥有少量大型熔炉的工厂中，这对于延长使用寿命非常重要。

BF运动寿命延长技术（图1）分为以下三类。

• 运营实践 - BF 流程的控制对广告系列的生命周期有重大影响。 BF 的运行不仅是为了满足生产需要，而且是为了最大限度地延长其使用寿命。因此，有必要随着活动的进展以及对问题领域的响应来修改运营实践，以最大限度地延长活动寿命。
• 补救措施 - 一旦影响 BF 寿命的磨损或损坏变得明显，将使用或开发工程维修技术以最大限度地延长活动寿命。
• 改进的设计 - 随着改进的材料和设备的开发，这些将被纳入未来的重建中，以延长高炉关键区域的使用寿命，这样做具有成本效益。

图1延长高炉使用寿命的技术

本文讨论了提高广告系列寿命的操作方法。影响BF广告系列寿命的操作方法如下所述。

生产力

高炉的生产率通常以每天每单位高炉体积 (cum) 的 HM 吨 (t) 表示。高生产率意味着在更高的负载下降率下增加材料的吞吐量，与增加炉膛活动以去除更多的液体产品。当高炉被用力驱动时，操作稳定性受到影响，炉料下降不太顺畅，熔化区较高。这些都会影响高炉壁磨损。液体产品会加速炉缸磨损，导致出钢口条件更加恶劣。

低生产率涉及长时间的低热风量，这会导致热风穿透减少和高炉壁上的气流增加，除非对炉料分布进行适当的修改。通常长时间停产对炉膛状况有不利影响。

当考虑到已实现较长活动寿命的 BF 的生产力水平时，很明显，这些 BF 在大部分活动中并未发挥最大潜力。常见的因素是稳定、一致的操作，并采用实践来监控和保护墙壁和炉膛。在低于最大产量的生产水平下，这种操作更容易实现。但是，很难定义生产率指数的普遍值（t/cum/天）来实现这一点，因为该指标还受到除高炉驱动率之外的几个因素的影响。这些因素包括高炉内部形状、耐火材料磨损状态、当地运行条件和维护周期等。

为了最大限度地延长活动寿命，需要一种策略来使 BF 以稳定、受控的方式运行。许多 BF 重建都涉及增加内部体积，而不是增加输出，而是能够以较低的生产力水平实现生产目标，从而提供更稳定的操作和更长的活动寿命的潜力。

BF的频繁停机会降低其生产力是一个事实，但由于停止/启动操作次数过多，广告系列的寿命也会降低。单位体积的广告系列产量减少与停机时间百分比不成比例。按此标准衡量的长时间活动最好通过 BF 连续运行而不长时间停机来实现。

还需要短期降低生产率，以解决高炉上发现的问题区域，以保护熔炉的完整性，从而避免活动过早结束。

负担

为了在合理的生产率水平下稳定运行高炉，优质焦炭是必要的。事实上，焦炭是导致运行不良的主要原因之一。运行不良通常会导致不稳定的，甚至是冷操作，可能对高炉衬里造成破坏，从而对活动寿命造成破坏。

可乐需要坚固和稳定，以支撑负载的重量，同时尽量减少机械故障。它要足够大、尺寸紧凑、细粉最少，以形成可渗透的床，通过它液体可以在不限制上升气体的情况下滴入炉膛。需要一致的尺寸以避免不希望的渗透率变化，并支持在 BF 半径上改变焦炭层厚度的概念，以控制径向气流。焦炭要足够对溶液损失不反应，在这种条件下保持其强度，并且碱含量低，以最大限度地减少滚道中的碱气化，这对焦炭分解和炉内耐火材料有不利影响。需要控制焦炭水分和碳含量的变化，以尽量减少它们对过程热状态的影响。

在高水平的风口烃注入下，焦炭的加入比例相应降低，因此焦炭质量变得更加重要。

一种通用的焦炭质量，用于稳定运行和长高炉寿命，很难指定，因为不同类型的操作不仅有不同的焦炭要求，而且物理性质也会根据焦炉与高炉之间的取样点。

在使用来自多个来源的焦炭的情况下，要么需要充分混合，要么单独装入不同的焦炭，这是必不可少的，因为不同性质的焦炭比例的波动会导致焦炭的不稳定状态。高炉。

高炉中心的焦炭逐渐取代了死人和炉膛中的焦炭，必须保持可渗透性，让液体通过炉膛中心排出。这样可以避免过多的外围流动炉膛中的 HM 会导致侧壁底部的严重耐火材料磨损。通常观察到炉膛垫中心温度的升高随着死人焦炭尺寸的增加，这表明炉膛中心活动增加。孔径尺寸焦炭筛网是维持炉缸渗透率的重要参数，通常有利于增加筛网尺寸，并将额外产生的与矿料混合的小焦炭远离高炉中心线装入。

使用优质焦炭的目的是确保大焦炭到达高炉的下部区域。为了监控这一点，最好不时在风口处对焦炭进行取样用于评估焦炭通过熔炉的分解。这通常在计划维护期间进行。从风口孔中耙出大量焦炭样品，并将其性质与相应的进料焦炭样品进行比较。通过这种方式，其他影响因素焦炭大小也可以识别。

优质、一致的焦炭质量以及对库存线和波什焦炭的监控显然是延长活动寿命的重要策略。

矿石配料混合

高炉使用多种矿石配料组件，如烧结矿、球团矿和分级铁矿石 (SIO) 等。各种助熔剂也用于矿石配料。

少量其他材料，例如回收废料、铁粉、轧钢氧化皮、转炉渣、钛铁矿、回收废料甚至直接还原铁或粒状铁有时也用于矿石装载。这些材料的使用通常取决于当地因素。

综合钢厂通常有烧结厂，因此这些工厂中的高炉在炉料中使用很大比例的烧结矿，其余的炉料主要由 SIO 和/或球团组成。首选球团由于其优越的性能，在一些植物中超过 SIO 以平衡负担。

在全球范围内，高炉负荷中的球团百分比从 0 % 到 100 % 不等。不同工厂的经验表明，使用高比例球团的高炉在较低的热负荷变化中会受到较大的影响。烟囱和炉膛，导致烟囱和炉膛过度磨损，缩短活动寿命。其中一个原因是对负荷分布的控制不足。球团的休止角比烧结矿或焦炭低得多，当降落在料线倾斜，容易滚动。这导致靠近高炉中心的矿层相对较厚，这会促使高炉壁上的气流过多。

这种情况正在通过在下部竖井中增加高密度冷却和改进的物料分配设备来解决。通过颗粒装料可以观察到较低的冷却壁温度波动、滑动增加和 HM 温度波动，这采用配料分配控制，中心焦加料，球团加入坚果焦。

单个炉料成分的一个重要方面是软化和熔化特性。高炉压降的主要部分是在矿石炉料软化、熔化和滴落的区域气体上升通过的焦炭床。较宽的熔化和软化范围会导致压降增加，并且较大的粘合区根部会撞击下部竖井砖砌体，从而使耐火材料在比所需范围更广的范围内暴露于高温。较低的壁温和/或较少的热波动有助于延长竖井砖的使用寿命。

多组分炉料的熔化和软化特性与单个组分的不同。因此，软化和熔化测试数据不仅要考虑单个炉料成分，还要考虑建议的矿石组合，以帮助选矿。

为了最大限度地减少热和化学变化，需要均匀的负载。负载组件应尽可能紧密混合。这取决于负载组件的数量和单独的充电系统，但它通常可以通过选择储料仓和卸料顺序来达到合理的程度。

如果材料质量一致并且有足够的壁冷能力和适当的分布控制，则可以使用不同的负载实现稳定的高炉运行和较长的活动寿命。 

矿石质量

一个可渗透的高炉是稳定运行所必需的。重要的是矿料要坚固、尺寸紧密并有效筛选以去除细粉。它不能在堆垛中过度分解并产生额外的细粉. 它必须有足够的多孔性、可还原性和尺寸，使其在到达软化区时能够被有效地还原。这样，内聚区的限制较小，富含 FeO 的熔渣较少，并且较低的热负荷BF区较低，利于平稳运行。

矿石成分的软化和熔融特性对高炉的运行有重要影响。粘性区的限制和熔融特性差会导致炉料下降不稳定、运行不稳定和热波动。这些条件可能会缩短 BF 壁的寿命。

没有标准化的软化和熔化测试，并且引用了许多指标来表示软化和熔化温度，例如直接还原开始、熔化过程中的压降和滴料量等。

负担分布

负荷分布是影响高炉运动寿命的主要因素之一，它不仅会影响运行的稳定性，而且通过确定高炉内的径向气体流量，它是高炉壁磨损率的主要控制因素之一。

通常径向气流由炉料中的矿石与焦炭的比例控制，因为焦炭尺寸通常较大。这通常通过将材料装入离散层并改变层厚度来实现高炉半径。因此，高炉壁的保护是通过增加壁上矿层的比例来实现的，这会导致壁冷却系统带走的热量减少。但是，比例是有限制的靠近高炉壁的矿石材料，以避免形成非活性层，这可能会促进壁积层的形成，并使未准备好的炉料进入高炉的下部区域并增加风口损失。高炉必须足以让高炉在所需的生产水平上稳定运行。大部分焦炭形成了一个相对渗透性较低的区域，下降的液体较少，从而允许使用最大的鼓风量。爆破压力波动大，负荷下降不稳定。

高炉中心的焦炭取代了炉膛中的焦炭，富含焦炭的可渗透中心促进了可渗透的炉膛，这与流经炉膛的液体流动有关。中央焦炭烟囱不是过宽。在这种情况下，由于上升气体的热容量过高，可能会导致效率低下并损坏炉顶的某些部分。

分体式充电

更复杂的分配系统允许通过使用超过一个尺寸范围的给定材料来额外控制负载分配。最常用的做法之一是加入细矿石材料，通常来自对主要矿石的筛选。在靠近高炉壁的地方单独加入少量细粉，以局部降低渗透性，从而保护壁。单独装入小批量较细的材料通常会降低高炉的装料容量。

坚果可乐

灵活的装料系统允许使用坚果焦炭螺母（典型尺寸在 10 毫米至 30 毫米范围内）。将混合在矿石配料中并沿半径中部放置的坚果焦的装料通过提高粘性区矿石层的还原效率和渗透性来改善操作。装填坚果焦可提高渗透性并降低机腹温度。装在壁上的坚果焦，夹在两个矿石装料之间，防止了当细矿石装在壁上时出现不活跃的壁区。球团中加入坚果焦，增加了球团的休止角，从而降低了高炉中心的矿石负担比例。

尺寸隔离

许多装料系统在输入材料中产生一定程度的尺寸分离。如果要排出的初始材料较细而最终材料较粗，则可以利用该特性来有利于径向尺寸分布，从而有利于径向气流分布。这种类型的偏析通常发生在带式炉上而不是跳过式炉上，并且在无罩顶的情况下更容易控制。还可以对充电系统进行适当的修改，以增强所需的分离特性。

向下滚动倾斜的原料线也可能发生额外的径向尺寸偏析。当一种组分具有不同的尺寸范围和化学性质时，尺寸偏析还可以改变炉料沿高炉半径的熔化和软化特性。

一些充电系统导致负载分布的周向变化。这些变化应通过设计或操作尽量减少。

中心焦炭装料

高炉中心通常需要大量焦炭，以鼓励足够的中心工作以稳定运行。在较高的生产率和以高水平的风口烃注入操作时尤其如此。然而，在炉子中心完全使用焦炭进行操作的燃料效率较低，并且已经开发了技术来最小化中心焦炭装料区域的宽度。在没有钟罩的顶部，这是通过在旋转溜槽完全降低的情况下装入一小批焦炭来实现的。

炉膛中需要一个可渗透的焦炭床，以促进液体流过炉膛中心并减少外围流动，这会导致侧壁过度磨损。死人和炉膛中的焦炭逐渐被炉中心的焦炭取代。中心焦炭装料降低了高炉中心矿石材料的百分比，并提高了炉膛渗透率。通过在中心增加稳定焦炭装料，可以进一步提高炉缸渗透率。

护甲寿命

为了延长运动寿命，重要的是尽量减少因负载材料的直接影响而对固定喉部装甲造成的磨损。虽然可以修复喉部装甲或加入保护板，但这可能涉及长时间的维护停机，这本身可能对熔炉寿命有害。因此，应选择负载分布和使用的库存线高度，以避免这种负载影响。

铁水质量

当在炉膛中没有保护壳的情况下进行操作时，通常通过铁和炉渣的溶液侵蚀来去除炉膛碳。铁在接触炉缸耐火材料之前的早期渗碳可最大限度地减少这种炉缸磨损。

对于早期渗碳，液体和焦炭之间需要延长接触时间。在给定的生产率下，较高的滴水区和死人以及较高的内聚区可能会促进这一点。这通常会导致 HM 硅 (Si) 的增加。通常，碳饱和水平随着 Si 含量的增加而降低。因此，对于给定的 BF 尺寸和 HM 温度，HM 在更高的 Si 水平下更接近饱和。

此外，HM Si 的增加会增加 HM 液相线温度，从而降低其流动性。这往往会降低炉膛内的流速，并促使炉膛耐火材料上形成凝固层。

在较低的 HM 温度下，铁的碳饱和水平较低并且较早达到。低 HM 温度具有增加铁粘度的额外好处，从而减少外围流动，降低溶解保护性颅骨和穿透细裂缝和孔隙的趋势。

较高的 HM Si 和较低的 HM 温度很难同时实现，因为较高的内聚区通常会导致炉温升高，但总体效果是使进入炉膛的 HM 变得更接近碳饱和。在不影响高炉热态的情况下，降低高顶压力可能会导致 Si 略有增加。 Si 含量越高，炉膛碳溶解的可能性就越低。

风口直径

选择风口直径以确保在给定的操作条件下有足够的爆破穿透力，并防止过多的气体上升到高炉壁。风口尺寸的选择影响高炉的对中工作程度以及对炉腹和下竖井壁的保护程度。通常需要改变高炉周围的风口直径来保证气流的周向平衡。

尽管风口尺寸经过精心选择，但在更换风口时，通常会观察到直径显着增加，特别是在实现长寿命时。这会影响上述两个因素，并且在给定时间后更换风口对于活动寿命而言是有利的，不仅可以最大限度地减少风口磨损的影响，而且可以减少漏水到高炉的可能性和计划外关闭的次数更换故障风口的爆破期。

出钢口正上方的风口直径往往会减小，甚至关闭风口，以促进铸造的顺利进行，并减少出铁口上方的炼铁量。

由于炉膛侧壁温度高，风口直径通常会局部减小，以减少问题区域的滴液和炉膛活动。这是通过添加风口插件或更换风口来完成的。在严重的情况下，或作为短期应急措施，可以用粘土堵住有问题的风口。这通常可以快速降低相应的炉膛侧壁温度。

铸造厂实践

铸造车间实践在控制炉膛内的液体流动和避免可能撞击滚道、影响鼓风分布甚至导致风口或吹管损坏的高液位方面发挥着重要作用。这些因素会影响运行的稳定性，导致停爆期，并可能影响活动寿命。

螺纹孔长度

由于出钢口较长，熔融产品不仅从炉膛下部抽取，而且还从更靠近炉膛中心的位置抽取。这减少了出铁口附近的周边流动，从而减少了炉缸侧壁的磨损。为了延长出铁口长度，需要在一段时间内增加出铁口质量的注入量，逐步增大高炉内部的蘑菇状尺寸，这也保护了出铁口下方的耐火材料。短出铁口长度和从大间距出铁口交替浇铸时，侧壁温度波动增加，可能会增加耐火材料的侵蚀。

由于冻结层的损失和/或炉床碳溶解，可能会经历高炉床垫温度，而炉床侧壁温度是令人满意的。在这种情况下，可能需要缩短出铁口，方法是减少注入的出铁口质量，并可能通过减少出铁口倾斜度。这有助于减少高炉中心附近的 HM 流量并增加保留在炉底垫上的液体。 

丝锥直径

维持给定生产率所需的出铁口直径取决于高炉参数，例如浇注时间的比例、顶部压力、渣量、炉膛焦炭尺寸、液体粘度和出铁口质量的性质。如果出铁口对于给定的生产率来说太小，那么就不可能干铸熔炉。如果出钢口太大，在浇铸过程中从熔炉中排出的熔融产品可能会减少，因为出钢口会过早吹出，因为出铁口上方的液体在炉膛另一侧的液体下降通过之前就被排出了可乐床。在这两种情况下，炉膛内的液位都居高不下，最终影响稳定运行。因此，需要根据经验得出最佳出铁口尺寸。

当使用单个出铁口时，必须选择尺寸以使高炉能够干浇，并为出铁口质量在两次浇注之间固化提供足够的时间。在使用备用出铁口的高炉上，在某些操作条件下可能需要不同尺寸的出铁口，以确保整个炉膛排水。

在炉缸垫磨损的多出铁口 BF 上，可能需要增加出铁口直径。这与出铁口长度的缩短一起，减少了流经炉垫的铁流量，增加了铸造结束时炉膛中的残余铁，从而促进了炉垫上冻结层的形成。

攻丝孔质量

出铁口质量特性对于高炉操作很重要。质量必须快速凝固并在铸件之间完全固化，以形成坚固耐用的出铁口。出铁口质量必须具有良好的粘附性能，以建立一个坚固、永久的结构，阻止液体流动并保护出铁口下方的炉缸耐火材料。

数量、位置和业绩

在中型单出铁口高炉上可以获得高生产率。然而，当有多个出铁口可用时，这是有优势的，这在更高的生产水平上是必要的。从炉膛相对两侧的出铁口交替浇铸可以更有效地排出炉缸，也可以延长出铁口物质完全固化的时间，从而形成更耐用的出铁口。两个出铁口的存在允许对主铁流道进行重大翻新，而无需经过一段时间的喷砂处理。如果在多出铁口 BF 的炉膛壁上出现热点，则可以使用不会促进侵蚀区域周边流动的替代出铁口。在多出铁口炉上，由周边流动造成的侧壁磨损会更均匀地分布在周边。

对于大型、高生产率的 BF，最好有四个出铁孔，这样可以在一个流道修复而另一个流道待机的同时操作相对的一对。为了平衡侧壁磨损并促进炉膛完全排水，理想情况下，这些位置的间隔为 90 度。

施放频率和速率

浇注率取决于所用出铁口钻头的尺寸、出铁口质量的磨损特性、顶压、液体的粘度和使用的出铁口数量。随着现代高性能出铁口质量，有减少铸件数量的趋势，从而降低出铁口运营成本。通过降低浇注速度，炉膛内的液体速度会降低，但它们会持续更长的时间。在多铸厂 BF 上，可以同时从相对的出铁口浇铸（搭接浇铸），使出铁口质量在比浇铸持续时间更短的时间内完全固化，并且人力和物流使之成为可能。这种技术降低了炉膛内的流速，尽管它通常只用于高液位时或在高炉停止鼓风之前。

必须不惜一切代价避免长时间的铸造延误，以尽量减少对高炉运营的干扰。这需要铸造厂设备的良好设计和可靠运行、良好的铸造厂实践以及 HM 钢包的良好协调运输。

碱和锌

碱金属和锌对高炉工艺和耐火材料有有害影响。负担是将碱金属和锌的含量保持在最低经济水平。通常，碱和锌的含量控制在 5 kg/tHM 以下（最佳实践为 2 kg/tHM），但由于碱蒸汽在下降的炉料上冷凝，高炉中会积聚大量的再循环负荷。这会导致烧结矿的降解和焦炭分解增加，并促进炉壁积炭的形成，所有这些都会导致炉料下降不规则和高炉运行不稳定。

碱金属和锌以气态形式渗入高炉壁耐火材料的裂缝和孔隙中。由此产生的化学侵蚀和热循环削弱了耐火材料的表层，最终被下降的负荷去除，允许重复该过程。

活动结束后的炉膛解剖表明，在侧壁底部发生过度磨损，并且通常在外壳和碳的热面之间形成脆性区域。在这个脆性区域中经常发现高含量的碱和锌。已经提出了涉及这些化合物的各种分解机制。侧壁中的应力和热裂允许气态碱和锌渗透并沉积在孔隙中。这导致砖膨胀、脆化、进一步膨胀并最终破坏耐火材料。如果在耐火材料的热面上冻结了附着物或结壳，则可以实现对碱和锌的显着保护，从而保护耐火材料免受化学侵蚀。

大部分碱在炉渣中除去，其余在炉顶气中除去。然而，渣实践、热状态和炉料分布在碱去除中起主要作用。炉渣碱度的降低通过扩大或加强中心工作的程度，随着高炉的热水平或顶部温度的增加而增加了在炉渣中去除的碱量。此外，对于给定的碱负载，由于增加的负载停留时间，焦炭降解可能在风口碳氢化合物注入率高的操作中更大。重要的是要监测碱金属和锌的输入和输出的平衡，并且高炉在与这些元素的输入水平相适应的热和化学状态下运行，以促进它们在炉渣和炉顶气中的去除。

二氧化钛添加

高炉作业结束时的炉膛内衬样品通常含有含钛沉积物。这些在炉膛侧壁的侵蚀区域、蝾螈中以及砖孔隙和接缝中形成保护层。钛通常以碳氮化物Ti(C,N)、碳化钛(TiC)和氮化钛(TiN)的固溶体形式存在。因此，目前的实践涉及将二氧化钛 (TiO2) 引入 BF 以促进这些保护层。通常使用三种方法来引入 TiO2。这些是 (i) 添加到炉料中，(ii) 在风口处注入，(iii) 通过出铁口质量添加。

最常见的技术是将钛铁矿石（通常是钛铁矿）添加到炉料中。或者，可以通过烧结添加 TiO2，但含量较低。

Two strategies are generally adopted for TiO2 addition. The first one is remedial, commencing TiO2 additions only when high hearth temperatures are observed, indicating hearth wear. The other takes a preventive approach and adds a small quantity of TiO2 continuously, increasing the addition level if high temperatures are observed. The TiO2 intake for the preventive approach is generally 3 to5 kg/tHM, which usually results in up to 0.1 % Ti in the HM and 1 % to 1.5 % TiO2 in the slag. For remedial action, the TiO2 dosage can be up to 20 kg/tHM, at which level the HM may contain up to 0.3 % Ti and the slag up to 3.5 % TiO2. This creates operating problems due to high slag viscosity and scaffolding in the runner, and hence such high TiO2 levels are only used for short periods.

For promoting the precipitation of Ti(C,N), sometimes the TiO2 addition is increased before a shutdown so that the HM remaining in the hearth get saturated in Ti. As the hearth cools during the shutdown, this promotes precipitation. However the resumption of production is more difficult at high Ti levels as it  creates operational problems.

TiO2 can also be added by injecting TiO2 fines through the tuyeres. The advantages of the technique are (i) application at localized positions, (ii) reduced cost due to lower TiO2 rate, and (iii) good results from short time injection, and (iv) unchanged burden properties.

The third method of TiO2 addition is by the use of tap hole mass containing TiO2. One such mass which had been tried was tar bonded with approximately 10 % TiO2. Clearly, the titania is bound in the tap hole mass in an unreduced form, and is injected in relatively small quantities. However there are doubts whether it gets reduced and dissolves in HM in sufficient quantities to be precipitated or whether it is reduced and bonded adequately to the hearth sidewall to be of benefit.

TiO2 is normally partially reduced in BF and is dissolved in the HM. The solubility is greater at higher temperatures. If the Ti in the HM is nearing saturation and the refractory hot face temperature in eroded regions, cracks and pores temperature is lower than the HM temperature, then Ti is precipitated, as Ti(C,N). The technique is more likely to succeed at higher addition rates, but there are other factors which can  interfere with this basic mechanism, including thermal state of the hearth, metal/slag chemistry and liquid flow characteristics.

TiO2 additions is usually carried out in conjunction with other remedial actions such as reducing productivity, closing tuyeres and improving hearth cooling intensity. The direct effect of TiO2 addition is therefore often difficult to determine. It is essential to carry out regular, accurate Ti balances to assess the technique and modify operation to encourage Ti retention. The effect of high rate additions can even have a detrimental effect on furnace operation, negating any benefits.

The addition of TiO2 for hearth protection is normally to be considered as part of a hearth protection plan rather than in isolation.

Monitoring

Burden distribution is to be monitored regularly for ensuring the wall protection and a stable and driving BF. Changes in the operating parameters, e.g. changes in tuyere hydrocarbon injectant rate or blast volume, may need adjustments to burden distribution. The effect of burden distribution is usually monitored with various probes and instruments.

For maximizing the campaign life, it is necessary that the charging equipment is capable of controlling accurately  the burden distribution. Also necessary instrumentations are to be fitted to comprehensively monitor the BF operation so that the burden distribution is changed and assessed in a controlled and technical manner. 

Instrumentation and control

Early warning of hearth problem areas is vital to maximize campaign life, and thermocouples located in the hearth sidewall and in the hearth pad are absolutely necessary to monitor hearth wear. Revised operating practices and actions to protect the hearth are to be taken as a result of increasing hearth temperatures. Hearth pad and sidewall temperatures can also give an indication of liquid flow in the hearth, an important factor in hearth wear.

Temperatures recorded by thermocouples are influenced by only a small area round the thermocouple. It is therefore vitally important to locate the thermocouples in the critical wear areas. Important areas are below the tap holes and around the base of the sidewalls where the so called ‘elephant’s foot’ wear pattern is normally found. An adequate number of thermocouples are to be installed, in the best layout to give as complete coverage as far as practical. At several locations, thermocouples can be positioned at two or three different depths to allow calculation of the thermal profile in the refractory and hence the thickness of residual refractory. 

Movement of carbon blocks can nip hearth pad thermocouples, causing false hot junctions or total failure. These problems can be overcome by fitting the thermocouples in sheaths. Thermocouples are also to be positioned around the tap holes, to monitor tap hole conditions and operation.

Additional thermocouples are often added part way through a campaign in areas of known refractory wear, to give a more localized picture of developing problems. Similarly, thermocouples are often added to repaired areas to monitor the repair.

Monitor hearth cooling

Heat flux in the hearth pad or stave cooling water can be determined from the water flow rates and the difference between inlet and outlet water temperature, using resistance thermometers. It can be used only to give an indication of the average hearth wear. It is particularly applicable in the later stages of a campaign, following thermocouple deterioration. Monitoring long term trends in hearth cooling water temperature may give an indication of the efficiency of the cooling system.

Furnace wall conditions

The process conditions at the furnace wall are vital to campaign life. The walls is not to be subjected to high heat loads from an excessive quantity of gas ascending at the wall or impingement of the melting zone on the wall, which results in rapid deterioration of the refractory and wear of the cooling members. On the other hand the walls must not be so inactive that large accretions are permitted to form on them, which prevents smooth burden descent, control of burden distribution and stable blast furnace operation. To monitor wall conditions a variety of methods are used.

The common method of monitoring the walls is using in-wall thermocouples, positioned in the brick work, with the tips a short distance back from the hot face to give a good thermal response. Wall activity is monitored from the temperature level and fluctuations.

There must be a good coverage of thermocouples both vertically and circumferentially to monitor the walls adequately. Typically seven levels of thermocouples, each with eight circumferential positions are used. With a large number of thermocouples, it is difficult for the operator to monitor the variation of them all. By using the temperatures at many points, an isothermal map is normally generated, identifying regions of high or low temperatures which relates to refractory wear, asymmetrical operation or accretion formation. The dynamic temperature behaviour is also be utilized to predict the formation or loss and extent of an accretion.

Throat or skin thermocouples are often installed around the periphery, just below the fixed throat armour. The thermocouple tips are installed level with the hot face of the refractory, to record gas temperature. These give a direct measure of the gas flow at the wall and are usually unaffected by deposition of material, unlike in-wall thermocouples lower in the stack.

Radial measuring probes

The use of retractable probes is one of the important techniques to monitor and optimize burden distribution, and hence campaign life. Such probes are the only method of measuring the variation in operating characteristics along the furnace radius, as opposed to relying  solely on wall measurements. They are essentially of two types namely (i) overburden,  and (ii) underburden.

Overburden probes have several functions. The simplest type is usually fixed, water cooled and measures the radial or diametrical top gas temperature profile and, in some instances, the gas analysis. Most retractable probes measure the stock line layer profile and can be of a mechanical type, where a weight is lowered to the stock line or a non-contact type, using radar, microwaves, lasers, etc.

Top gas velocity can also be physically determined to measure the quantity of gas flow, and top gas analysis and temperature measurement is frequently carried out in conjunction with the other functions. Probes are also used to determine the trajectory of material off the rotating chute or movable throat armour, for calibration of burden distribution predictive models and to determine the effect of charging chute wear.

Underburden, or in-burden, probes sample gas and measure temperature at a number of radial positions. They are generally positioned in the upper stack, typically 3 m to 6 m below the stock line. These probes are generally of two types. The consumable type, is typically 50 mm in diameter, bends with the descending burden and is straightened on withdrawal for subsequent re-use.

Since the top gas has to pass from the stock line up one of the four off takes, the gas flow pattern begins to distort near the stock line. A large degree of gas mixing then occurs above the burden, and overburden probes must be positioned close to the stock line, and preferably inclined, to give acceptable temperature and gas profiles. The upper stack underburden probes are more sensitive and give superior results to overburden probes. In addition, fixed overburden probes can be quite big in size and, depending on the stock line height, can create a ‘shadow’ and distort the burden distribution below them, which can give unrepresentative results.

Probes, especially underburden probes, are essential tools for prolonging BF campaign life.

Hearth models

In recent years, with increasing computing power available, many mathematical and numerical techniques have been developed to predict blast furnace hearth erosion and liquid flow in the hearth.

Hearth lining wear may be calculated by mathematical model, using temperature measurements from embedded thermocouples in the hearth bottom and sidewall. For this technique to be accurate, a good coverage of thermocouples is required and their depth of insertion needs to be known precisely, together with the thermal properties and geometry of the lining. The accuracy may also be affected by parameters that may change with time, such as the conductivity of ramming, thermal contact between courses of brickwork and the development of a brittle zone in the refractory, which can significantly change its conductivity.

Although hearth temperatures alone give a direct indication of hearth wear, this type of modelcombines information from the thermocouples, at differing distances from the hot face, to predict the extent of wear and solidified layers more accurately.

Direct measurement of hearth lining wear is difficult and undesirable since this requires test borings and embedded sensors through the full refractory thickness.

Artificial Intelligence

The blast furnace process is a complex one, with a large number of process variables. Modern, well instrumented furnaces have hundreds of sensors which require to be monitored by a decreasing number of operators. Consequently, computerized systems are being developed to process the primary information available and give secondary advice to the operators. This is based on a set of operating rules, statistical analysis of data, identifying trends that compare with historical data and use of intelligent techniques such as fuzzy logic and neural networks. The aim of these systems is to predict deviation from steady operation and to quantify the change in control parameters required to minimize the deviations in production and quality. This results in more stable BF operation, avoiding major operating problems such as erratic burden descent and chilled conditions, which is a primary requirement for long campaign life.

Furnace top sensors

Since the late 1970s, many BFs have been equipped with infra-red cameras viewing through windows in the top cone, to measure stock line temperature profile. This technique overcomes some of the disadvantages of fixed overburden temperature probes. The falling burden is not scattered as with probes, leading to a more symmetrical burden distribution, and by measuring material temperature the effects of stock line to probe distance, which can result in gas mixing and desensitizes the temperature profile, are avoided. A further benefit is that the rotation of the distribution chute in the furnace can be observed. However, these systems are expensive, difficult to maintain and experience problems in keeping the viewing window clean, due to the moist, dusty top gas. Problems have been experienced with the dust in the top gas also affecting the temperature distribution. Hence these cameras are not a standard fitment and many operators have abandoned them in favour of radial probes.

Some furnaces are equipped with non-contact stock line profile measurement systems installed in the furnace top cone. These systems effectively replace a retractable overburden probe and, although expensive, have the advantage that they measure over a larger proportion of the stock line than the single radius of a probe.

Thermography

The use of thermal imaging cameras to detect hot spots, on the furnace shell, top gas system, tuyere stocks, stoves, hot blast and bustle mains and other ancillary plant, is important. Not only does it enable early detection of problem areas and permit their systematic rectification, but it also helps prevent catastrophic failures, in which the BF has to be taken off-blast in a sudden uncontrolled manner followed by an often difficult recovery, which would have a detrimental effect on campaign life.

  Leak detection

An efficient system of detecting water leaks into the BF from tuyeres and other cooling members is essential. Undetected water leaks may chill the furnace, resulting in erratic operation and difficult recovery from chilled conditions. Water leakage directly affects BF campaign life if it damages the refractories. Water leaks in lower, hotter regions of the BF, which are lined with carbonaceous materials, inevitably results in oxidation of the refractories. Rat holes in the hearth refractories can result, which can lead to breakouts. Water leakage can also result in tap hole problems which may disrupt operations.

Tuyere leak detection systems are often used. One leak detection system incorporates a system of magnetic flow meters with computer analysis of the differential flows. Another system of leak detection uses a pressurized closed circuit water system incorporating make up tanks with  the makeup frequency indicating the severity of a leak. Other systems involve observation of gas bubbles or dissolved CO content in the water, differential pressure measurements etc.

A good leak detection system often warns the operator of a water leak in its early stages, before an immediate off blast is required. This gives the opportunity for the leaking member to be isolated prior to the furnace being taken off in a controlled manner, with reduction in tuyere hydrocarbon injection and ore/coke ratio adjustments, thereby minimizing detrimental effects resulting from the subsequent stoppage.

Plant maintenance

All maintenance work possible are to be carried out during production, thereby reducing the off blast time necessary. To minimize the duration of a planned stoppage, good planning and advance preparation are necessary. Although these factors are obvious for economics and to maximize plant output, their long term effect on furnace life is not always considered.

Preparations should always be in hand for maintenance to be carried out if the furnace comes off blast unplanned for other reasons. For instance, if the furnace is off for a tuyere change, it may be possible for work to be carried out on the charging system. If the furnace is off blast for problems at the steel melting shop, then it may be possible for more extensive maintenance to be performed. In this way, the total number of stoppages during a BF campaign can be reduced and their duration minimized.

Effective maintenance reduces the number of breakdowns which result in unplanned stoppages. This  involve routine maintenance, regular inspections, periodic  checking of important instrumentation, and condition monitoring, e.g. vibration and thermal monitoring. This is most important at later stages of a campaign, as ancillary equipment gets older and less reliable.

Similarly, improved cast house maintenance techniques can reduce off blast time, e.g. extension of the life of the main iron runner on a single tap hole furnace reduces downtime.

Off blast periods

The number of off blast periods, mainly unplanned ones, has a major effect on campaign life in terms of output per unit volume, which is reduced disproportionally to the percentage downtime. Wall damage can result from an increased degree of wall working at the lower blast volumes encountered whilst coming off and on blast, cooling and reheating of the refractories or erratic operation during recovery from the stoppage.

Some BF operators indicate that off blast periods ‘rest’ the hearth and allow a protective skull to form or thicken. In fact, taking the BF off blast is often an emergency procedure, at later stages of the campaign, when high temperatures are detected within the hearth refractory.

Short stoppages

For planned stoppages, additional coke can be charged several hours in advance, to compensate for the reduced blast conditions and the heat losses during the stoppage period. This extra coke in the lower regions of the BF assists smooth recovery from the stoppage. It is usual to decrease or remove tuyere hydrocarbon injection for a stoppage.

At high injection rates, there is a much lower proportion of coke in the BF, which is consequently less permeable and this may hinder recovery from the stoppage. In addition, at high injection rates, the BF is markedly fuel deficient during the recovery until the injection is resumed. This may not happen until the blast volume has reached about half of its full rate, when an adequate raceway is formed and the injectant can be consumed safely. In case there are operating problems in establishing raceway conditions and returning to the level of blast at which injection is possible, it can result into cold conditions or tuyere blockages with slag and the BF is fuel deficient at a time when additional fuel is needed.

In addition to ore/coke ratio compensation, a burden change is generally desirable for a stoppage period. Smaller material components of the burden is to be removed from the burden to promote permeability following the stoppage. High levels of titaniferrous ores is also to be reduced to avoid problems at lower HM temperatures after the stoppage. The proportion of burden components that deteriorate when at high temperatures over a long period, such as ores prone to decrepitation, are to be reduced in a stoppage burden. In addition, a more acid burden may be charged to compensate for higher Si content in the HM during recovery from the shut down.

During a stoppage, other deleterious factors can occur which affects the return to full blast operations. For example, this may include (i) extended periods at reduced blast volume to cast the furnace dry before the off blast, (ii) an extended stoppage period for a variety of reasons, (iii) water leakage into the BF during the shutdown, and (iv) problems during the recovery that may require  several off blasts (may be to rectify blast leaks or charging faults etc.). Under such circumstances, the undesirable operating conditions are extended and the additional coke charged may not be adequate, leading to a less smooth recovery from the stoppage.

To ensure smooth operation and minimize the effect of a stoppage on the life of BF, some operators believe a slow start after a planned stoppage. A typical of this is to control output to 90 % on the day before a stoppage and resume at 80 %, then 90 % output on the two days following the stoppage. However, this may not be acceptable to other operators, under conditions where high output is needed.

Unplanned stoppages are undesirable and, if possible, many BF operators try to delay taking the BF off blast for long enough to allow a compensated burden to descend to bosh level.  Attempt is usually made to cast the BF as dry as possible, to avoid getting slag back into the tuyeres and blowpipes, which may freeze and further prolong the stoppage. This also gives time to prepare for the repair work due to be carried out and to minimize the time of off blast. To compensate for the heat lost due to an unplanned stoppage, the tuyere hydrocarbon injection is generally increased after coming back on blast, providing it is not already at its maximum level.

Production stoppages can also occur due to the problems in the steel melting shop or during periods of low demand. These occurrences are to be coordinated so as to get advanced warning wherever  possible, and to give the option of a compensated burden. The pig casting machine (PCM), torpedo ladle fleet or steel melting shop  mixers are to be used as a buffer for short stoppages. In certain circumstances, when there is minimal advance warning of a shutdown, the BF is not dry and there is little empty ladle capacity, and there is no PCM available, it is preferable to dump the HM.

Stack spray techniques for the repair of wall refractories have advanced, enabling the walls to be gunned in a relatively short stoppage, by blowing the burden down to a low level. Although this allows a large quantity of coke to be charged at the lower levels of the furnace to aid start up, there is often difficulty due to the quantity of rebounded refractory falling into the furnace. Start up is easier if a low rebound material is used and the BF is blown down to tuyere level, enabling the rebound material to be raked from the furnace. This can be achieved more effectively by the use of T shaped sheets of corrugated sheeting, inserted rolled up through the tuyere cooler apertures. The blow-in burden chemistry is also to  be adjusted to give a slag chemistry that enables the residual rebound material to be melted.

There is a difference of opinion as to whether or not cooling water flows should be decreased for stoppages of greater than a given duration. Some operators prefer reduced flows to maintain refractory temperatures. The majority prefer the hearth cooling water on full flow to promote a thicker protective skull, whilst others who reduce the water flow suggest that by removing less heat it assists a smooth start up.

Another factor which affects the recovery from a stoppage is the removal of an accretion from the BF walls, resulting from the additional wall working and erratic burden descent. This can results into chilled conditions at a time when they are least desirable. If an accretion is known to have formed, it is desirable to try and remove it before a long stoppage. A good system of accretion monitoring provide immediate warning in case of its occurrence, to enable thermal compensation as soon as possible.

Long stoppages

Depending on the duration of the stoppage, the BF may be filled with a coke blank and a low ore/coke burden, or the burden may be blown down to tuyere level. For stoppages of several weeks or longer, the salamander is to be tapped. If this is not done, not only will a considerable amount of process heat be needed, during the recovery, to melt it, but it will expand whilst still solid and create undue stresses on the hearth refractories and shell, shortening their life. This is even more important with those BFs, where the sump depth has been increased to reduce peripheral iron flow in the hearth. It is desirable to monitor these stresses with strain gauges attached to the hearth jacket, and to develop procedures to minimize such stresses.

To recover from longer stoppages, when the BF is in a cold condition, it is necessary to’ warm the hearth and establish an early link between the tap hole and the tuyeres to allow liquids to be removed. This may be done by the use of a blast pipe at the tap hole or the use of an oxy-fuel lance. It is important to prevent the oxygen, entering at the tap hole, damaging the hearth carbon, which can directly shorten the campaign life. Recovery from chilled hearth conditions, following major water ingress during a routine maintenance stop, has been reported to have resulted in severe hearth erosion. Recovery from long shutdowns, with a large quantity of solid metal in the hearth and an impermeable dead man, may result in excessive peripheral flow in the hearth with accelerated hearth sidewall erosion.

Production rules

Being a continuous process, the BF is operated by a number of different operators who, without a set of rules to operate to, would react differently to a given situation. The individual actions taken may not be the correct one and, as a result, the process can be more variable than if the ideal action was taken. The majority of BFs are therefore operated according to set procedures that have been developed and improved from experience. These rules cover a wide area, including practical procedures and process control.

To maximize BF stability, it is necessary to control accurately both the thermal state and the aero-dynamics of the furnace. Steel plants usually devise their own rules to control thermal state, which generally involve the HM Si and temperature as indicators, with the use of top gas analysis and calculation of the quantity of heat available in the BF for silica reduction and to superheat the liquid products. Control of thermal state is usually by adjustment of conditions at the tuyere or by small changes in the quantity of coke charged. Furnace aero-dynamics are monitored by rules relating to furnace pressure drops and burden descent rates, with adjustment to blast volume, burden distribution or burden properties to achieve stability.

Operating rules are also necessary for non-routine operations, where damage to the BF may result from incorrect procedures, for instance in the recovery from chilled hearth conditions, where damage to refractories can happen. 

Specific rules for prolongation of BF life

Many operators have a specific set of operating practices for the prolongation of campaign life, which are in place to minimize damage to or prevent further deterioration of the BF. As the hearth is the critical region of the BF which cannot be repaired without a long shutdown, these rules or action plans often relate to hearth conditions. Typically, the actions are defined according to hearth temperatures or refractory thickness.