CA2930762A1

CA2930762A1 - Roasting of sulfur-poor metal oxide concentrates

Info

Publication number: CA2930762A1
Application number: CA2930762A
Authority: CA
Inventors: Maija-Leena Metsarinta; Jussi Liipo; Marcus Runkel
Original assignee: Outotec Finland Oy
Current assignee: Metso Metals Oy
Priority date: 2013-11-27
Filing date: 2013-11-27
Publication date: 2015-06-04
Anticipated expiration: 2033-11-27
Also published as: CA2930762C; WO2015078493A1; AP2016009226A0

Abstract

The present invention provides a method of pre-treating sulfur-poor metal oxide concentrate, comprising the steps of: providing sulfur-poor metal oxide concentrate having sulfur content below 10 % w/w; (a) heating the sulfur-poor metal oxide concentrate at an elevated temperature and in presence of oxygen and an external fuel source to oxidize sulfur compounds contained in the sulfur-poor concentrate to corresponding oxides and/or to thermally decompose the said sulfur compounds to obtain a sulfur-depleted oxide concentrate and a sulfur-containing off-gas; (b) separating the sulfur-depleted metal oxide calcine from the sulfur-containing off-gas; (c) cooling the obtained sulfur-depleted oxide calcine; (d) cooling the off-gas by quenching; and (e) scrubbing the cooled off-gas to recover sulfur.

Description

ROASTING OF SULFUR-POOR METAL OXIDE CONCENTRATES
FIELD OF THE INVENTION
The present invention relates to pre-treatment of metal oxide con-centrates, in particular concentrates with low sulfur content, and provides a method of pre-treating of sulfur-poor metal oxide concentrate for removing most or all the sulfur contained in the oxide concentrate.
BACKGROUND OF THE INVENTION
Roasting is typically applied to concentrates such as sphalerite or pyrite having a high heat value which provides enough energy for the heating of the roasting step and even extra energy is produced. However, this type of roasting is not possible for material with limited sulfur and carbon content.
However, there is an existing need for a method of processing con-centrates with low sulfur and possibly also low carbon content as these con-centrates may bear valuable metals, such as niobium (Nb), which are scarcely available.
In order to recover scarce valuable metals from concentrates with low sulfur content the concentrate must be pre-treated to remove most or all the sulfur contained in the concentrate. This allows further processing of the concentrates in hydrometallurgical processes which do not tolerate the pres-ence of sulfur.
To our knowledge there are no existing methods for the pre-treatment of oxide concentrates having a low sulfur content.
BRIEF DESCRIPTION OF THE INVENTION
It is thus an object to provide a method and an apparatus for imple-menting the method so as to overcome the above problems. The objects of the invention are achieved by a method and an arrangement, which are character-ized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea of heating of oxide concentrates with limited sulfur content in presence of oxygen and an external fuel source which provides the energy required for oxidation and/or thermal composition of the sulfur. This is advantageously accomplished in a fluidized bed furnace, whereby the external fuel source provided by fuel lances.

2 BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figure 1 shows a first predominance diagram for Fe-O-S systems;
Figure 2 shows a second predominance diagram for Fe-O-S sys-tems; and Figure 3 illustrates a flow diagram of a first example of the method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method of pre-treating sulfur-poor metal oxide concentrate, comprising the steps of: (o) providing sulfur-poor metal oxide concentrate having sulfur content below 10 (:)/0 w/w; (a) heating the sulfur-poor metal oxide concentrate at an elevated temperature and in pres-ence of oxygen and an external fuel source to oxidize sulfur compounds con-tained in the sulfur-poor metal oxide concentrate to corresponding oxides and/or to thermally decompose the said sulfur compounds to obtain a sulfur-depleted metal oxide calcine and a sulfur-containing off-gas; and (b) separat-ing the sulfur-depleted metal oxide calcine from the sulfur-containing off-gas;
(c) cooling the obtained sulfur-depleted oxide calcine; (d) cooling the off-gas by quenching; and (e) scrubbing the cooled off-gas to recover sulfur.
The heating step (a) can be performed in any roaster found suitable by a person skilled in the art. The roasting step (a) can for example be carried out in a roaster selected from the group consisting of a rotary kiln, a fluidized bed reactor, such as a bubbling fluidized bed roaster, a circulating fluidized bed roaster, an annular fluidized bed roaster, and a flash reactor. In accord-ance with an advantageous example of the present invention the heating step (a) is accomplished in a fluidized bed furnace. This ensures a very good mass and heat transfer. In a preferred example of the invention the fluidized bed fur-nace is a bubbling fluidized bed furnace or a circulating fluidized bed furnace.
The term "sulfur-poor metal oxide concentrate" refers to concen-trates having sulfur content below 10 % w/w and comprising more than (:)/0 w/w metal oxide, in particular from 50 to 90 "Yo w/w metal oxide, more particularly from 60 to 70 (:)/0 w/w metal oxide.

3 PCT/EP2013/074831 Sulfur-poor metal oxide concentrate pre-treated with the method of the invention is preferably a concentrate having sulfur content from 0.2 to 8 % w/w, more preferably from 3 to 7 % w/w. In a suitable example of the pre-sent invention sulfur-poor metal oxide concentrate comprises niobium oxide (e.g. Nb205), in particular from 30 to 60 % w/w, more particularly from 50 to 60 % w/w. In a further example of the present invention sulfur comprised in the sulfur-poor concentrate is mostly pyrite. In particularly suitable example of the present invention the sulfur-poor metal oxide concentrate comprises from 30 to 40 % w/w Nb and 5 to 8 % w/w Fe.
Further in accordance with an example of the present invention the carbon content of the sulfur-poor metal oxide concentrate is so low, that it to-gether with sulfide it does not produce enough heat for heating material to re-quired temperature. The carbon content of the oxide concentrate affects the heat value of the concentrate and thus oxide concentrate with particularly low carbon content cannot be treated with conventional pre-treatment methods.
The method of the invention advantageously provides sulfur-depleted metal oxide calcine having sulfur content below 0.2 % w/w, more preferably below 0.15 % w/w. Such pre-treated oxide calcine can be processed in by suitable hydrometallurgical methods known by person skilled in the art for recovering the valuable metals comprised in the said pre-treated oxide calcine The term "calcine" as used herein and hereafter refers to a product obtained by heating a concentrate to a high temperature but below the melting or fusing point, causing loss of moisture, reduction or oxidation, and at least partial the decomposition of carbonates and other compounds.
The method of the invention is applicable for removal of sulfur from a sulfur-poor metal oxide concentrate regardless of whether the sulfur is pre-sent as sulfate(s), sulfide(s), or both. The conversion of sulfides and/or sulfates to corresponding oxides with oxygen is dependent on the local oxygen concen-tration. Oxygen can be introduced into the heating step (a) for example as air, oxygen enriched air, or other oxygen containing gas, preferably as air or oxy-gen enriched air. The required amount of oxygen in the heating step (a) de-pends on the sulfur content of the oxide concentrate and the desired sulfur content of the pre-treated oxide concentrate and the mineralogical nature of the sulfides and/or sulfates as well as carbon and/or carbonates comprised in the sulfur-poor metal oxide concentrate.

4 PCT/EP2013/074831 The external fuel source is preferably liquid and/or gaseous fuel, such as methane, propane, butane, natural gas or heavy oil. Use of solid fuel sources, such as sulfur bearing carbonaceous materials, is not desirable as they are easily retained in the solid concentrate and thus would not allow ob-taming pre-treated material with a very low sulfur and carbon content. The fuel source is preferably proved to the roasting stage through one or more fuel lances. Fuel lance supplies the liquid and/or gaseous fuel into the furnace by a liquid-fuel passage that extends into the furnace chamber. Fuel is oxidized with oxygen containing fluidizing media. The required number of fuel lances de-w pends on the need of additional energy and the capacity of the plant. Fuel lances can be provided on one or more, preferably one or two, levels depend-ing on the size of the furnace. The required amount of fuel is depended on the desired temperature of the heating step (a) the reaction kinetics, and thermo-dynamic stability.
The temperature of the heating step (a) must be kept within oxide stability range. This is dependent of the nature of the sulfur-poor metal oxide concentrate, but varies typically from 500 to 1000 C depending on the miner-als. Temperature must also be minimized to minimize the need of external fuel source. The retention time of step (a) is depended on the mineralogical nature of the sulfur-poor ore. For example sulfur in present as a pyrite can be easily removed in minutes. Figure 1 and Figure 2 show calculations relating to suita-ble conditions regarding temperature and oxygen pressure of the heating step (a) for oxidation of sulfur comprised in a sulfur-poor metal oxide concentrate comprising sulfur mainly as pyrite (iron sulfide) in such manner that sulfide oxi-dizes to iron oxides (FeO, Fe304 and/or Fe203) and no iron sulfates (FeSO4) are formed.
As the preferable process conditions of the heating step (a) are di-rectly affected by the nature of the pre-treated sulfur-poor metal oxide concen-trate the nature of the said oxide concentrate is preferably determined before-hand and the process conditions are selected accordingly. The accordance with one example of the present invention the process conditions of the heating step (a) are determined by the steps of: (i) analyzing the nature of the sulfur com-pound(s) and the nature of the metal oxide(s) of the sulfur-poor metal oxide con-centrate; and (ii) selecting the process conditions in such manner that the metal oxide retains it stability while the sulfur compounds are oxidized and/or thermally decomposed.

5 PCT/EP2013/074831 The nature of the sulfur compound(s) and the oxide(s) of the sulfur-poor metal oxide concentrate in step (i) can be analyzed by methods known suitable by a person skilled in the art. The main elements of the sulfur-poor metal oxide concentrate can for example be analyzed by inductively coupled plasma (ICP) mass spectrometry. The sulfur content of the oxide concentrate can be determined for example by an elemental analyzer utilizing combustion techniques, such as Eltra CS2000. The chemical composition of the main components of the oxide concentrate can be analyzed for example by field emission scanning electron microscope. The main mineralogy of the oxide concentrate can be determined for example by X-ray diffraction. Preferably the nature of the sulfur compound(s) and the nature of the oxide(s) of the sulfur-poor concentrate is analyzed at least by X-ray diffraction.
The process conditions in step (ii) are then selected by performing stability calculations and utilizing thermochemical estimations, such as those shown in Figure 1 and/or Figure 2. The temperature of the heating step (a) can be further determined by testing the reaction temperature of the sulfur-poor metal oxide concentrate using a thermogravimetric/differential scanning calo-rimetric (TG/DSC) method which indicates the temperature(s) where the sulfur compound(s) are oxidized and/or thermally decomposed.
In accordance with a further example of the present invention the separation step (b) is accomplished by a cyclone. In accordance with a still further example of the present invention cooling of calcine in step (c) is ac-complished by a fluidized bed cooler.
The method of the invention is particularly suitable for the sulfur-poor metal oxide concentrate comprising small amounts chlorides and/or fluo-rides. Thus in an advantageous example of the invention the sulfur-poor metal oxide concentrate comprises chlorides and/or fluorides. The sulfur-poor metal oxide concentrate can comprise from 0 to 4 % w/w, in particular from 1 to 2 % w/w, chlorides and/or fluorides. In accordance with the method of the in-vention these compounds are volatilized and when present as gaseous HF and HCI in the off-gas also removed in the scrubbing step (e). It is not possible to recycle off-gas comprising HCI and/or HF. The presence of HCI and/or HF also prevents utilization of the off-gas in an acid plant.
When the off-gas contains no or only minor amounts of HCI and/or HF scrubbing step (e) can be accomplished for example by regenerative scrubbing methods to recover sulfur. The wash chemicals can be recycled.

6 PCT/EP2013/074831 When SO2 and/or HCI and/or HF are present scrubbing in step (e) can be done for example with an aqueous alkaline solution, such as NaOH (aq) and Ca0H2 (aq).
Figure 3 illustrates an example of the method of the present inven-tion. With reference to Figure 3, sulfur-poor metal oxide concentrate (1) and an external fuel source (2) are introduced into a roaster (A) and heated in pres-ence of preheated air (3) to oxidize sulfur compounds contained in the sulfur-poor metal oxide concentrate to corresponding oxides and/or to thermally de-compose the said sulfur compounds, to obtain a gas/calcine mixture (4) com-1() prising sulfur-depleted metal oxide calcine and sulfur-containing off-gas. The gas/calcine mixture is then introduced into a cyclone (B) and separated to ob-tain a sulfur-depleted metal oxide calcine (5) and a sulfur-containing off-gas (6). The sulfur-containing gas (6) can analyzed with a gas analyzer (G) and the process conditions of the roaster (A) can be adjusted accordingly.
With further reference to Figure 3, the obtained sulfur-depleted met-al oxide calcine (5) is then cooled in a fluidized bed cooler (C) where the heat of the said calcine is utilized to heat the air (3') entering the heating step (a).
Further the air (3') used and in the fluidized be utilized as a warm gas (13) in drying of the concentrate (1). The sulfur-containing off-gas (6) is cooled by quenching with water (8) in a quencher (D) and the thus obtained cooled off-gas (7) is scrubbed with an aqueous alkaline solution (9) to produce cleaned off-gas (10). The wash waters (11) and (12) from the quencher (D) and the scrubber (E) can be further treated (F) to recover sulfur removed from the off-gas.

7 PCT/EP2013/074831 EXAMPLES
Example 1 Nb205 concentrate was analyzed chemically and its mineralogy and particle size distribution where determined. The particle size of the oxide con-centrate was 88 pm and d90 207 pm. The oxide concentrate consisted mainly of zoned to homogeneous pyrochlore-columbite and pyrite grains. The concen-trate comprised Nb 36.4 % w/w, S 7.61 % w/w, Cl 0.83 % w/w and F 1.8 %
w/w. A 150g sample of the oxide concentrate was roasted in a laboratory-scale fluidized bed furnace that comprised a vertical trans temp 21/2" x 20" furnace and a fused quartz tube. A porous quartz sinter was used as a grate and aided in distributing the gas flow. Roasting was performed as a batch operation and at temperature of 750 C under an air flow. The sulfur and carbon contents of the sample were analyzed periodically.
Sulfur and carbon content of the thus obtained product was 0.13 and 0.06 % w/w, respectively, after two minutes roasting at temperature of 750 C.
It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The in-vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method of pre-treating sulfur-poor metal oxide concentrate, comprising the steps of:
(o) providing sulfur-poor metal oxide concentrate having sulfur con-tent below 10 % w/w;
(a) heating the sulfur-poor metal oxide concentrate at an elevated temperature and in presence of oxygen and an external fuel source to oxidize sulfur compounds contained in the sulfur-poor concentrate to corresponding oxides and/or to thermally decompose the said sulfur compounds to obtain a sulfur-depleted metal oxide calcine and a sulfur-containing off-gas;
(b) separating the sulfur-depleted metal oxide calcine from the sul-fur-containing off-gas;
(c) cooling the obtained sulfur-depleted oxide calcine;
(d) cooling the off-gas by quenching; and (e) scrubbing the cooled off-gas to recover sulfur.

2. The method as claimed in claim 1, wherein the sulfur-poor metal oxide concentrate comprises more than 40 % w/w metal oxide, in particular from 50 to 90 % w/w metal oxide, more particularly from 60 to 70 % w/w metal oxide.

3. The method as claimed in claim 1 or 2, wherein the sulfur-poor metal oxide concentrate comprises niobium oxide.

4. The method as claimed in any one of claims 1 to 3, wherein the sulfur content of the sulfur-poor metal oxide concentrate is from 0.2 to 8 %
w/w, more preferably from 1 to 7 % w/w.

5. The method as claimed in any one of claims 1 to 4, wherein step (a) is accomplished in a fluidized bed furnace.

6. The method as claimed in claim 5, wherein the fluidized bed fur-nace is a bubbling fluidized bed furnace or a circulating fluidized bed furnace.

7. The method as claimed in any one of claims 1 to 6, wherein the sulfur-poor metal oxide concentrate comprises chlorides and/or fluorides.

8. The method as claimed in claim 7, wherein the sulfur-poor metal oxide concentrate comprises 0 to 4 % w/w, preferably from 1 to 2 % w/w, chlo-rides and/or fluorides.

9. The method as claimed in any one of claims 1 to 8, wherein the sulfur content of the sulfur-depleted metal oxide calcine is below 0.2 % w/w, more preferably below 0.15 % w/w.

10. The method as claimed in any one of claims 1 to 9, wherein the elevated temperature is from 500 to 1000 °C, more preferably from 700 to 900 °C.

11. The method as claimed in any one of claims 1 to 10, wherein the process conditions of step (a) are determined by the steps of:
(i) analyzing the nature of the sulfur compound(s) and the nature of the metal oxide(s) of the sulfur-poor metal oxide concentrate; and (ii) selecting the process conditions in such manner that the metal oxide retains it stability while the sulfur compounds are oxidized and/or ther-mally decomposed.

12. The method as claimed in any one of claims 1 to 11, wherein the separation step (b) is accomplished in a cyclone.

13. The method as claimed in any one of claims 1 to 12, wherein the cooling of calcine in step (c) is accomplished by a fluidized bed cooler.

14. The method as claimed in any one of claims 1 to 13, wherein an aqueous alkaline solution, such as NaOH (aq) or CaOH2 (aq), is utilized in the scrubbing step (e).

15. The method as claimed in any one of claims 1 to 14, wherein the external fuel source is liquid and/or gaseous fuel.

16. The method as claimed in claim 15, wherein the fuel source is selected from the group consisting of methane, propane, butane, natural gas and heavy oil.