US3118757A - Method and means for preheating ferrous oxide - Google Patents

Description

Jan. 21, 1964 L. PERAS 3,118,757

METHOD AND MEANS FOR PREHEATING FERROUS OXIDE Filed March 27, 1961 2 sheets-sheet 1 Y 24 Luc l-EN -'RAs INVENT OR ATTORNEYj METHOD AND MEANS FOR PREHEATING FERROUS OXIDE Filed March 27, 1961 L. PERAS Jan. 21, 1964 2 Sheets-Sheet 2 Tilt-L2 INVENTOR Lucvau PERAS ATTORNEY 5 United States Patent 3,118,757 METHGD AND MEANS FOR PREHEATING FERRGUS OXIDE Lucien Pras, Billancourt, Seine, France, assignor to Begin Nationals des Usines Renault, Biiiancourt, France Filed Mar. 27, 1961, Ser. No. 98,436 Ciaims priority, application France Mar. 31, 1960 3 Claims. (Cl. 75-1) This invention relates to methods and means for reheating ferrous oxide, also called protoxide of iron, and it is its chief object to provide an improved method of reheating ferrous oxide applicable in a procedure for preparing pure iron in powder form, which consists in reducin the hot ferrous oxide to a fluidized state by using a hydrogen flow.

Whereas in general iron is produced by reducing directly the natural or calcined iron ores, this method is completely new since the starting point is the ferrous oxide FeO. It consists essentially in using as a starting material the ferrous oxide FeO obtained beforehand through a separate operation, for example by applying the method disclosed in the French patent filed on March 11, 1960, for "Method and Means for Preparing Ferrous Oxide, by the same applicant, corresponding to United States patent application Serial No. 94,878, filed March 10, 1961. In this case the ferrous oxide is obtained in the tempered state and is therefore highly suitable for a crushing step whereby, as disclosed in another French patent filed on March 15, 1960, for Method of Preparing Pure iron Powder, particularly suitable for its reduction by means of hydrogen H into a fluidized layer or bed at a temperature in the range of 600 C. to 750 C. and corresponding to United States patent application Serial N0. 94,671, filed March 10, 1961.

The present invention contemplates reheating ferrous oxide before reducing same, this procedure being attended by two difliculties. Firstly, it is not possible to reheat ferrous oxide in a strongly oxidizing or reducing atmosphere without deteriorating it. Secondly, it is difficult to transfer thereto the heat necessary for increasing its temperature to 700 C. Furthermore, it is contemplated to reduce the hydrogen consumption during the subsequent reduction step.

The method constituting the subject-matter of this invention is characterized in that the problem of reheating the ferrous oxide powder is considered separately from that of its reduction alone. To this end, the ferrous oxide is reheated up to a relatively high temperature, preferably in the range of 700 to 740 C. into a layer or bed fluidized by the passage of a hot gas containing primarily C0, C0 and N secondarily H and H 0, which gas is in a state of equilibrium with FeO at temperature above 550 C. Thus, although a slow incipient dissociation occurs at the beginning of the reheating step no sooner it has begun it recedes as the reheating is continued (at a relatively fast rate) so as to yield integrally FeO when the temperature reaches about 550 C. Thus, it is above this temperature that the gas is exactly in equilibrium.

This gas is derived from the incomplete combustion of high-methane natural gas with a suitably metered quantity of air. Of course, according to local facilities, a gaseous or liquid hydrocarbon, a generator gas or coke gas may be sub tituted for natural gas. This combustion yields a hot gas and it is the heat of this gas that is used for reheating the ferrous oxide. The gas issuing from the upper portion of the apparatus used for reheating the fluidized ferrous oxide is divided into two fractions: the first or smaller fraction is burned in the hearth of a regen- Patented Jan. 21, E954 ice erator to which secondary air is supplied, the combustion products thus produced flowing along the metal tubular nest of the regenerator or in an equivalent apparatus, before escaping through a funnel; or other, more important fraction is reheated in the tubular nest of the regenerator or in the equivalent apparatus and recycled in the hearth of the primary burner. Therefore, the suitably heatinsulated reheater for the fluidized product is not equipped with means for heating the outer surface of its walls. Thus, except for the losses, the whole of the heat de veloped by the combustion of the natural gas is used for internally reheating the ferrous oxide in fluidized bed form, so that the most economical heating conditions are achieved.

To avoid any settling of cold ferrous oxide in the handling and loading equipment the presence of water vapour is carefully avoided until the ore temperature exceeds C. To this end a first preheater stage is produced in the fluidized ferrous oxide bed by using an inactive A gas heated beforehand to a sufficient temperature. This inactive gas may consist of the nitrogen of carbon dioxide gas, or still better, of a mixture of nitrogen and carbon dioxide (for example CO +4N derived from combustion products from which any water content has been condensed by preliminary cooling. The gas issuing from the fluidized preheater chamber is drawn by a circulation fan, reheated and recycled. Thus, it circulates under closed-circuit conditions, except for normal current losses. The advantage of this procedure lies not only in the fact that it prevents the moisture in the reheater gas B from causing a water condensation in the cold ferrous om'de the powder to settle, but also in that it isolates the B gas from any possible contact with air, even if the ferrous oxide were momentarily missing or the pressure reduced. Thus, any risk of explosion is definitely avoided.

This method uses adva geously powder feed means whereby the three gaseous circuits A, B and H are safe ly isolated from one another. For instance the circula tion of the pulverulent product between two successive apparatus may be effected through a metering worm conveyor, with a circulation channel filled with powder having a high apparent density since it is not fluidized. if desired, a regulator of the differential pressure-gauge type may be connected to either end of the worm conveyor to indicate the difierence in pressure between the nearest points of two consecutive gas circuits, so that the pressures may be adjusted with a view to preserve the independence of the A, B and H gas circuits.

The reheater apparatus proper consists of a plurality of stages of fluidized beds or layers disposed in cascade and supplied continuously with powdered ferrous oxide circulating rom top to bottom, while the reheater gas B circulates in counter-current relationship, that is, from bottom to top of the cascade apparatus. Thus, in the case of an installation producing 1 ton per hour of reduced iron powder, that is, consuming 1.3 tons of ferrous oxide, the consumption of natural CH gas is 1,130 cu. ft./ hour, the recycling rate being about .75; thus, approximately one-fourth of the B gas issuing from the reheater apparatus is burned in the regenerator whereas the other three-fourths are reheated in the regenerator to about 650 C. The primary combustion of the methane gas produces combustion products which, after mixing with the recycled gas, yield a B gas at about 850 C. The reheater tower in the example given hereinabove comprises four stages and receives in its upper portion preheated ferrous oxide at 100 C. The temperatures obtained in the four stages are about 250, 400, 550 and 700 C. respectively. The temperature of the B gas issuing from the upper portion is about 250 C. and under these conditions the temperature of the reheated ferrous oxide issuing from the lower portion is about 790 C.

The thus reheated P60 in the form of a calibrated powder is led through a suitable handling device such as a Worm conveyor to the reactor in which the reduction by hydrogen into pulverulent pure iron is to take place later on. This Worm conveyor or like device isolates the circuits of the B gas of the reducing hydrogen.

With the foregoing and other objects in view, the invention resides in the novel method and apparatus, in the arrangement and combination of parts and steps, and in the details of construction hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention.

Other objects and advantages will become apparent from the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a diagrammatical section showing a typical installation for carrying out the method of this invention, and

FIGURE 2 is a diagrammatic illustration of a metering worm conveyor for transporting and feeding the powder in the installation of FIG. 1, which is equipped with a differential pressure-gauge.

Referring first to FIG. 1, it will be seen that the installation comprises in general a system 1 for loading the ferrous oxide, a device 4 for preheating the ferrous oxide in the form of fluidized bed or layer by means of an inactive gas such as a mixture of CO and N then the reheater apparatus proper 1%) consisting of a plurality of stages of fluidized beds or layers wherein the ferrous oxide and the reheater gas circulate in counter-current relation: ship, the primary hearth 12 being located at the bottom of the rcheater, and finally a regenerator l7 equipped with a secondary hearth 13 and adapted to supply recycled hot :gas to the primary hearth. V

More particularly, the loading system 1 comprises a loading hopper 30 from which the ferrous oxide drawn by an air current produced by a fan 31 is conveyed to the cyclone separator 2 from which the ferrous oxide falls into the hopper 3, Whereafter it is taken by the metering worm conveyor 32 and fed to the reheater device 4- where it falls onto a grid overlying a chamber supplied with a stream of hot inert gas adapted to heat the ferrous oxide at about 100 C. This gas consists preferably of a CO +4N mixture derived through the duct 33 from combustion products from which any water has been condensed beforehand. The gas recycled by the circulation fan 6 is fed to a steam reheater 7 and then directed from beneath against the aforesaid grid of reheater 4. This gas is taken from the upper portion of the reheater by the fan 6 after passing through a suitable precipitator 5.

The ferrous oxide is' taken by the metering worm conveyor 8 and fed into the upper portion of the fourstage reheater 16 where it is heated successivel for example at 250, 400, 550 and 706 C.; at the lower portion, the ferrous oxide is taken by the metering worm conveyor 22 and fed to the hydrogen reducing apparatus or reactor 24; at the lower portion of the reheater 110 is the primary hearth 12 wherea suitable mixture of. highmethane gas (for instance occurring as a natural gas) and .air is fed to the burner 11.

The combustion products, after mixing 'with the recycled gas from duct 23, deliver rehea r gas at a temperature of about 858 (3., gas COilel 3 of a mixture 0f C0, C02, H2, H20 and P12.

This gas is taken from the upper portion of the reheater through a precipitator 13 by the circulation fan 14- and fed in a ratio adjusted by valves 15 of about three-fourths to the tubular nest 19 of regenerator 17 and about onefourth to the secondary burner 16 disposed at the bottom of this apparatus connected to a supply of secondary air.

The gas having passed through the tubular nest recycled at a temperature of about 650 C. through the duct 21.

As already stated hereinabove, the ferrous oxide powder is transferred from one apparatus to another adjacent apparatus preferably by means of a metering worm conveyor to avoid any direct communication between the gases present in these apparatus. It is also advantageous to provide a differential pressure-gauge connected as shown in FIG. 2 across the two ends of the worm conveyor to avoid any undesired and untimely transfer of gas therewith. This pressure gauge comprising for example two diaphragm members M1, M2 controls by means of its index regulating devices (not shown) associated with each one of the aforesaid two apparatus, respectively.

T his method is characterized inter alia by the folio. ing advantages:

1:; it aifords substantial savings in fuel, and

quent hydrogen reduction apparatus,

it ensures a minimum hyd gen consumption, nmximum rate reduction as cons ent with a given size of the ion apparatus, and a high degree of purity of the reduc.

tron prodr ct.

though t. e present invention has been described in con" inction with preferred embodiments, it is to be unerstood that modifications and variati ns may be resorted to witnout departing from the spirit and scope of the invention, as those skilled in the art will readily unden stand. 5 ch modifications and variations are considered to be w hin the purview scope of the invention and appended ciairns.

I claim:

1. The method for preheating ferrous oxide as preparation for the direct reduction of said ferrous oxide by means of hydrogen in a fluidized bed apparatus which comprises continuously feeding ferrous ()Xldf, in powder form to a first preheating zone, fluidizing and preheating said ferrous oxide first preheating zone by passing rethrough a current of moisture-free gas the C6 and N at a temperature of about continuously withdrawing ferrous oxide at a temperature of about 309 C, from said first preheating zone and the same to a second preheating zone, continuousiy flowing said ferrous oxide through said second zone a plurality of successive stages, fluidizing and preheating the ferrous oxide in each stage by a of a second comprising primarily C0, C0 N and secondarily H and H 0, which second gas an overall composition which is in a state of equilibrth FeG at temperatures above 550 C., said second gas passing through said second zone counter-current to the flow of said ferrous oxide therethrough, the initial temperature of Sal uecond gas being at about 850 C, continuously discharging ferrous oxide at a term erature of from about 6GS8% C. from said second accordance with claim 1 in which the second gas s r-Mani is in chemic cc, brium with ferrous at temperatures ranging from 650 to accordance w 11 obtained References @ited in the file of this patent UNlTED STATES PATENTS

Claims (1)

1. THE METHOD FOR PREHEATING FERROUS OXIDE AS PREPARATION FOR THE DIRECT REDUCTION OF SAID FERROUS OXIDE BY MEANS OF HYDROGEN IN A FLUIDIZED BED APPARATUS WHICH COMPRISES CONTINUOUSLY FEEDING FERROUS OXIDE IN POWDER FORM TO A FIRST PREHEATING ZONE, FLUIDIZIG AND PREHEATING SAID FERROUS OXIDE IN SAID FIRST PREHEATING ZONE BY PASSING UPWARDLY THERETHROUGH A CURRENT A MOISTURE-FREE GAS COMPRISING CO2 AND N2 AT A TEMPERATURE OF ABOUT 100* C., CONTINUOUSLY WITHDRAWING FERROUS OXIDE AT A TEMPERATURE OF ABOUT 100*C., FROM SAID FIRST PREHEATING ZONE AND PASSING THE SAME TO A SECOND PREHEATING ZONE, CONTINUOUSLY FLOWING SAID FERROUS OXIDE THROUGH SAID SECOND ZONE IN A PLURALITY OF SUCCESSIVE STAGES, FLUIDIZING AND FURTHER PREHEATING THE FERROUS OXIDE IN EACH STAGE BY A

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