Some myths about DC arc furnaces - Mintek

Some myths about DC arc furnacesthrough a new one in case of an electrode breakage, suchthat practically no DC voltage increase can be seen. Althoughone second might appear as an instant to our eyes,electrically speaking this is a lot of time, representing 50 or60 entire cycles, in which control systems can take about 500corrective actions.In any case, the DC power supply should also be equippedwith over-voltage protection that shorts high-voltagetransients (e.g. from transformer energizing) to groundimmediately for protecting the thyristors.There has also been much concern about flashovers (alsoknown as stray arcing) to the furnace shell, particularly to thefurnace roof. This is somewhat critical where the slag is veryconductive, for example TiO 2 slag, but less important in thecase of the highly resistive slags such as those found inferronickel smelting or cobalt recovery from slags. With aproper design of the furnace, correct isolation of the roof holefrom the electrode, and an optimized feeding system, aflashover does not occur.Keep in mind that, under normal circumstances, the liquidbath in the furnace is the arc’s lowest resistance (‘shortestpath’) to the rectifier’s 0 V potential through to the anode inthe furnace bottom, and hence the arc has no ‘motivation’ totake a longer route via the furnace roof.And should it still happen, for whatever reason, by ahigh-speed potential monitoring of the furnace roof and/orshell, the rectifier control could detect a flashover within20 ms and immediately take corrective action in order toprevent any damage to the furnace.However, stray arcing is also a well-known phenomenonon AC arc furnaces, where the liquid bath needs to be keptisolated from ground, but with the difference that one cannottake very fast corrective actions without tripping thefurnace’s breaker.Myth 9: The bottom electrode has a short life and iscostly to maintainSince the invention of DC furnace technology, the bottomanode of the furnace has been considered one of the mostcritical parts. Different equipment manufacturers came outwith a variety of different anode designs. At the beginning,equipment manufacturers concentrated mainly on scrapmeltingapplications, always with a focus on the maintenancecosts and the metallurgically required lifetime. The threemain designs finally accepted on the market were the fin-typeelectrode from VAI Fuchs, the pin-type electrode from theSMS Group, and the conductive bottom electrode from ABB(which, after ABB divested its arc furnace business toConcast, was also claimed by the SMS Group). The first twoelectrode types are, without doubt, the best solution for steelmeltingfurnaces, taking into account the required lifetimeand cost of maintenance. For small smelting furnaces, themulti-pin type anode has proved to be a reliable solution.For industrial DC smelting processes, it can be said thatthe conductive bottom electrode provides a number ofadvantages to the process and furnace operation, and it is nolonger the ‘headache’ of DC technology as many people onceregarded it when comparing DC with AC technology. Becausefurnaces require very expensive refractory materials, thelifetime of the furnace is often of paramount importance. Asthe smelting process is a nearly continuous process, andthere is generally no direct contact between slag and hearth,this type of bottom electrode provides the longest lifetime forthe DC smelting operation. Depending on the use of thefurnace, today’s bottom electrodes can have a lifetime of 5 to10 years, without requiring any maintenance. Until recently,the conductive bottom hearth was protected by a patent, butis now available for public use.The conductive bottom electrode consists of a copperplatedsteel plate covered by metallized bricks. This ensuresconductivity even if local spots are covered by nonconductivematerial, and uses bricks to withstand themetallurgical impact. Another advantage of using bricks isthat the metal part of the bottom electrode requires only aircooling.Compared to other water-cooled systems, thispresents an important safety advantage.Myth 10: The DC arc furnace is a solution to allmetallurgical problemsDC arc furnaces have some wonderful features. They are verygood at accommodating finely sized feed materials (becauseof the open bath), and they do not require the use of coke orchar. They allow for the treatment of feed materials with awide variety of composition (because of the extra degree offreedom coming from the relative independence of the powersupplied by the open plasma arc). They are geometricallysimple and elegant, which reduces costs associated withcomplex roof designs and uneven wear on sidewalls.However, it is important to take cognisance of the effect ofthe hot off-gas on thermal efficiency, and of the absence of aburden that could otherwise assist in capturing volatilespecies that might fume off the molten bath.DC arc furnaces have been found to be well suited toreductive smelting processes, such as those used for treatingchromite, ilmenite, and nickel laterite ores, oxidized concentrates,and slags. On the other hand, it is not so clear that DCFigure 15—Principle of the conductive bottom electrode▲672 OCTOBER 2011 VOLUME 111 The Journal of The Southern African Institute of Mining and Metallurgy