This invention relates to a hot blast main mixer stage, in particular for a hot blast main from a hot blast stove supplying hot blast to a blast furnace.

A typical hot blast stove will supply hot blast to the blast furnace over a range of temperature over the course of its blast period. The minimum temperature at the end of the blast period coincides with the required hot blast temperature at the blast furnace. Due to the way hot blast stoves work, it is not possible to supply a constant hot blast temperature throughout the blast period, so the temperature at the start of the blast period is higher than required. This blast temperature then cools over the course of the blast period and eventually reaches the target temperature. While the stove is generating hot blast at a temperature higher than that which is required at the blast furnace, the temperature is balanced by mixing the hot blast with a supply of cold blast which has bypassed the stoves, in order to keep the hot blast temperature to the furnace constant.

Conventionally, the mixing has been carried out using either a mixer pot, or a radial multi-pipe mixer. One example is an eight pipe mixer, each of the pipes being arranged in such a way that all of them are perpendicular to the main. However, it is desirable to improve the degree of mixing of the hot and cold blast to get a more uniform temperature before it enters the blast furnace.

In accordance with a first aspect of the present invention, a hot blast main mixer stage comprises a section of hot blast main and a plurality of cold blast inputs to the section; wherein the inputs are spaced about a circumference of the main in an asymmetric arrangement; wherein one of the inputs is radial, converging at a common centre; and wherein at least one of the inputs is non-radial, converging, not at the common centre, or diverging from the common centre.

The combination of radial and non-radial inputs, converging at a common centre, or converging other than at a common centre, or diverging from a common centre, in an asymmetric arrangement about the circumference of the main, improves mixing of the hot and cold blast.

Preferably, the number of cold blast inputs is an odd number greater than one.

Typically, there are an even number of non-radial inputs and one radial input. Preferably, the non-radial inputs are closer to one another on the circumference of the main than they are to the radial input.

Preferably, for a converging non-radial input, the or each non-radial cold blast input converges outside the circumference of the hot blast main.

Preferably, more than half of the cold blast inputs are non-radial.

Preferably, more than half of the cold blast inputs are in one half of the circumference of the hot blast main and the others are in the other half of the circumference of the hot blast main.

Preferably, the non-radial inputs are in one half of the circumference of the hot blast main and the radial inputs are in the other half.

In accordance with a second aspect of the present invention, a method of mixing hot and cold blast in a hot blast main comprises arranging a plurality of cold blast inputs to the hot blast main in an asymmetric arrangement about the circumference of the hot blast main; arranging one of the cold blast inputs to be radial, converging at a common centre; arranging for at least two of the cold blast inputs to diverge from the common centre, or converge, not at the common centre; supplying hot blast through the hot blast main; and supplying cold blast through the plurality of cold blast inputs.

An example of a mixer stage according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates an example of a typical blast furnace arrangement with hot blast supplied by a hot blast stove, via a hot blast main;

Figure 2 is an example of a first embodiment of a mixer stage, for a hot blast main, according to the present invention;

Figure 3 illustrates how, in the embodiment of Fig.2, the hot and cold blast mixes along the pipe at different points downstream of the mixer stage; and,

Figure 4 illustrates a first alternative embodiment of a mixer stage according to the invention.

Figure 5 illustrates a second alternative embodiment of a mixer stage according to the invention.

Figure 6 illustrates a third alternative embodiment of a mixer stage according to the invention. For the purpose of this application, the term radial means converging at a common centre. The term non-radial here means does not converge at the common centre. The common centre is the central axis of the hot blast main.

In a typical blast furnace arrangement, as illustrated in Fig.1 a, a blast furnace 1 receives hot blast via a hot blast main 2. The hot blast is generated in a hot blast stove 3. The hot blast stove comprises a combustion chamber 4 and chequer chamber 5. A cold blast main 6 supplies cold blast to the combustion chamber 4 of the hot blast stove and also, via a cold blast bypass 7, supplies cold blast though a cold blast mixer 8 directly to the hot blast main 2. A combustion gas supply 25 and combustion air supply 26 enter the combustion chamber and a waste gas main 27 exits the chequer chamber 5 and feeds a waste gas stack 28. A conventional hot blast stove must generate hot blast at temperatures well above the required hot blast temperature and cool the hot blast to the required temperature by the addition of cold blast, except at the end of the cycle when the hot blast is at the target temperature without cooling.

The hot blast temperature varies by up to 100°C in the course of the blast period. The hot blast temperature may start at around 1300°C and come down to about 1200°C at the end of the blast period. Sometimes, the start temperature is higher and the range less, e.g. a start temperature of 1325°C dropping to 1250°C. The cold blast temperature is constant and typically in the region of 150°C to 200°C. The outlet temperature is thus maintained at a constant temperature.

Figs, lb, lc and Id illustrate how each of the hot blast stoves is on blast in series whilst the other stoves are going through a gassing cycle. In Fig. lb, stove C is on blast and stoves A and B are gassing. In Fig. lc, stove B is on blast and stoves A and C are gassing. In Fig. Id, stove A is on blast and stoves B and C are gassing.

Conventionally, hot and cold blast are mixed in the blast main, either by means of a mixer pot, or a multi-pipe arrangement as mentioned above. A mixer pot tends to be used when there is a change of elevation between hot blast branches and a furnace bustle main. The mixer pot comprises a vertical refractory lined vessel with two inlets and one outlet. The hot and cold blast flows both enter the mixer at either the top or the bottom of the pot, and then the mixed flow leaves the pot at the other end (top or bottom depending on the relative elevation of the hot blast branch and the bustle main). A multi-pipe radial inlet mixer may for example consist of eight small diameter pipes arranged in two banks of four pipes, symmetrically, on either side of the hot blast main, entering the main radially. However, there needs to be sufficient length of hot blast main before the blast furnace to allow for full mixing of the hot and cold blast.

The present invention improves on the prior art by means of an asymmetric arrangement of inlets for the cold blast, at least one of which is non-radial. Fig.2 illustrates a first example, with three cold blast inlet pipes 12a, 13a and 14 arranged asymmetrically around the hot blast main, supplied via a ring 10 from a cold blast supply 11. For ease of illustration, two ofthe pipes 12a, 13a are non-radial, entering the main parallel to one another. The other pipe 14 enters the main radially, an extension of this pipe would pass through the centre of the main. The asymmetric arrangement of pipes leads to the generation of a swirling flow pattern in the cross sectional plane the hot blast main.

The swirl pattern at various points downstream of the pipe entry, related to the diameter ofthe hot blast main for a typical 2.1m diameter main, is illustrated in Figs.3a to 3d. Fig.3a shows the swirl pattern at the point of entry ofthe cold blast 22a, 23a, 24a. Fig.3b shows mixing one diameter downstream, Fig.3c shows mixing two diameters downstream and Fig.3d shows the degree of mixing five diameters downstream. The three flows 22a, 23a, 24a from the pipes act to enhance each ofthe other inlet flows, leading to the generation of a relatively strong swirling flow pattern which carries well into the downstream flow. When the cold blast inlets are symmetrical and perpendicular to the main, converging on the centreline ofthe main, the flows act more to cancel each other out as they are approaching each other head on.

Fig.4 illustrates an alternative embodiment, where the non-radial inlets 12b, 13b positively diverge from one another, rather than running parallel, in order to improve the swirling flow pattern. This helps to redistribute the hot and cold flows, leading to better overall mixing and results in a more uniform blast temperature downstream of the mixer. In this example of an asymmetric arrangment, which if a section through the hot blast main perpendicular to the centreline is viewed end on, gives the appearance of the pipe having at least one radial inlet in one half and at least one non-radial inlet in the opposite half of the circumference, there is a single radial inlet, positioned in the opposite half of the circumference of the hot blast main to the pair of non-radial inlets. Although the examples shown are for two non-radial and one radial inlet, the invention is applicable to any asymmetric arrangement with at least one non-radial inlet. Although, it is preferred that the tangential inputs are symmetrical about an extension of a central axis of a single radial input, in the opposite half of the hot blast main, as illustrated in Figs.2 and 4, other embodiments are possible. The inputs may be equidistant from one another on the circumference of the hot blast main, but typically with the non-radial inputs closer to one another than they are to the radial input.

Another example is illustrated in Fig.5, which includes a single large radial inlet 14, with four smaller asymmetric inlets 29, 30, 31, 32 arranged opposite to this single radial inlet. The smaller asymmetric inlets consist in this case of two non-radial, tangential inlets 29, 30, converging to a point outside the hot blast main 2, and two non- radial, diverging inlets 31, 32 arranged inside the tangential inlets.

A further example, shown in Fig.6, includes two radial inlets 33, 34 and two non-radial, tangential inlets 35, 36 arranged on opposite sides of the main 2. The preferred arrangement is to have the radial and non-radial inlets in opposite halves of the circumference of the hot blast main, although other embodiments may be used. Usually, all of the inlets will be on the same circle on the circumference, but if there is a need, for manufacturing reasons, such as to fit into available space, to offset the inlets by a small amount relative to one another along the length of the hot blast main, this could be done. However, this is not so convenient in terms of the cold blast supply ring 10.

The examples of Figs. 2 and 4 position two non-radial pipes above and below a single radial pipe on the opposite side to causes the recirculation pattern which improves mixing. The best results are achieved where the two non-radial pipes are parallel to or angled away from the single radial pipe. Although a pair of non-radial pipes which converge at a point other than the centre, or central axis, of the hot blast main could be used, it is preferred that if the non-radial pipes converge, they converge at a point outside the hot blast main circumference.

The hot blast main is of a metal, typically steel, construction, with a refractory lining inside. The non-radial inlets enter the lining of the main at an angle, whereas the radial inlets enter substantially perpendicular to the lining. The reduced number of penetrations into the hot blast main of the examples of Figs.2 and 4, compared with a conventional eight pipe mixer has a beneficial effect in terms of retaining more of the strength of the main. The length of pipe required between the entry of the cold blast and the entry to the blast furnace depends on available space and may vary. In terms of getting good mixing, a longer pipe is better, for example somewhere in the region of 10 metres, or typically 15% of the overall hot blast main length. However, the use of the present invention enables better mixing without increasing the overall length.

The total flow area in the cold blast pipes depends on the volume of cold blast required to create the correct temperature mixture. This depends on a lot of factors, including the hot blast temperature, the cold blast temperature and the hot blast flow- rate. Typically, the total flow area of the inlets for the cold blast mixer is around 10 to 15% of the total flow area for the hot blast main. Reducing the overall flow area may further improve mixing by increasing the velocity of the cold blast as it enters the hot blast flow.