WO2021012043A1

WO2021012043A1 - Method and system for slag vitrification of toxic elements

Info

Publication number: WO2021012043A1
Application number: PCT/CA2020/051004
Authority: WO
Inventors: Jean-Marc Lalancette; Hubert Dumont; David Lemieux; Romain Barbaroux
Original assignee: Dundee Sustainable Technologies Inc.
Priority date: 2019-07-24
Filing date: 2020-07-21
Publication date: 2021-01-28
Also published as: CA3141668A1

Abstract

A method for slag vitrification of target elements, comprising forming target elements-containing pellets having a melting temperature lower than a melting temperature of a molten slag, mixing the target elements-containing pellets with the molten slag; and cooling. Vitrification of target elements using molten fayalite slag, comprising pelletizing the target elements with sodium oxide, mixing resulting pellets with the molten slag; and cooling is described.

Description

TITLE OF THE INVENTION

Method and system for slag vitrification of toxic elements FIELD OF THE INVENTION

[0001] The present invention relates to disposal of toxic elements. More specifically, the present invention is concerned with a method and system for slag vitrification of toxic elements.

BACKGROUND OF THE INVENTION

[0002] Base metals such as copper, zinc, nickel or cobalt for example are extracted by smelting from row ores that typically contain toxic elements such as arsenic (As), antimony (Sb), lead (Pb) and cadmium (Cd), sometimes in large amounts. During the smelting process, such toxic elements are typically volatilized as oxides and carried with flue dusts. Electrostatic precipitators or baghouses may be used to collect the flue dusts, which then need be disposed of safely due to their toxic content, which may be as high as 60% in the case of arsenic for instance. After the base metals have been smelted from the raw ores, a molten slag is left over. Arsenic for example is found in the smelter dust from copper, gold, and lead smelters, and is recovered primarily from copper refinement dust. In the case of copper smelting, the separation of the copper metal during smelting is incomplete and the final molten slag may contain a few per cents of entrained copper. As currently practiced, this final molten slag is cooled down and submitted to flotation after size reduction by crushing in order to recover metallic values remaining therein.

[0003] The safe disposal of flue dusts from smelting or roasting operation may be achieved by vitrification with recovery of the base or precious metals prior to glass formation. Sequestration of arsenic may be achieved with a glass composition comprising an iron oxide agent that prevents excessive volatilization of the arsenic during the fusion for glass formation.

[0004] There is still a need in the art for a method and a system for slag vitrification of toxic elements.

SUMMARY OF THE INVENTION

[0005] More specifically, in accordance with the present invention, there is provided a method for slag vitrification of target elements, comprising forming target elements-containing pellets having a melting temperature lower than a melting temperature of a molten slag, mixing the target elements-containing pellets with the molten slag; and cooling. [0006] There is further provided a method for the vitrification of target elements using molten fayalite slag, comprising pelletizing the target elements with sodium oxide, mixing resulting pellets with the molten slag; and cooling.

[0007] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] In the appended drawings:

[0009] FIG. 1 to FIG. 7 show pages of“Phase Diagram for Ceramists” by E.M Levine and al 1964, The American Ceramic Society.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0010] The present invention is illustrated in further details by the following non-limiting examples relating to disposal of arsenic resulting from copper smelting. All compositions herein are in weight percentage unless otherwise indicated.

[0011] The molten slag resulting from copper smelting is mostly an iron silicate (Fe²⁺)2Si04 with amounts of oxides of calcium and aluminum, lead, copper, zinc, arsenic and trace elements. Such fayalite (Fe2Si04) has a typical composition of: Si0₂: 25%; Fe: 46%; Al₂0₃: 2.8%; CaO: 7.0%; As: 0.55%; Cu: 2.5%; Zn: 2.6%; Pb: 0.6%, or : 2FeO.SiO2.0.3CaO on an approximate molar basis.

[0012] In a nutshell, the method as exemplified in the present disclosure comprises using the molten slag resulting from smelting of bases metals from arsenic-containing ores, as a glass forming material for vitrification of the arsenic in the flue dusts resulting from the smelting of the bases metals.

[0013] The method basically uses the molten slag resulting from smelting of bases metals from arsenic-containing ores, available on site, as a source of iron and silicate for the vitrification of arsenic. A challenge is the presence of amounts of calcium typically added in the flue dusts in order to abate their acidity.

[0014] Typically, when one ton of a copper ore comprising 5% arsenic is treated in a smelter, up to 50 Kg of arsenic is volatilized and carried in the flue dusts with about 50 Kg of other minerals. The amount of molten slag produced is around one ton per ton of ore as an order of magnitude. 250 Kg of this molten slag allows obtaining an arsenical glass containing 20% arsenic. If all the molten slag is used as a glass forming material, the total corresponding glass produced may contain about 5% arsenic.

[0015] In case when the recovery of copper remining in the molten slag is done by decantation or flotation, after reduction of size by crushing and grounding, vitrification of the arsenic being effective even in presence of a few per cents of entrained copper remaining in the slag prior to the vitrification, the arsenic-containing flue dusts are pelletized into arsenic-containing pellets, for example with lime in order to produce arsenite or arsenate of calcium, and the arsenic-containing pellets are dissolved in the molten slag to produce a glass containing up to 20% arsenic, a number of issues occur. First, due to the size reduction by crushing and grounding prior to the vitrification performed for recovery of copper remaining in the molten slag by decantation or flotation as mentioned hereinabove, any still remaining copper becomes integrated within the structure of the glass and thus cannot be recovered therefrom. Moreover, the sequestration of arsenic in the glass phase is poor. A second problem originates from the arsenic present as a salt of calcium in the molten slag.

[0016] Indeed, as shown in Fig. 80 of FIG. 1, the melting point of fayalite 2Fe0.Si02 is in the range between about 1000°C and about 1150°C. The presence of amounts of other oxides may change the melting temperature of the fayalite slag. For instance, alkali oxides such as K2O and Na20, and lead oxide result in a decrease of the melting temperature as shown by Fig. 397 (FIG. 2), Fig. 487 (FIG. 3) and Fig. 496 (FIG. 4) respectively. In contrast, in presence of CaO (Fig. 586 in FIG. 5), MgO (Fig 682 in FIG. 6) and AI2O3 (Fig 872 in FIG. 7) the melting point is raised.

[0017] Thus, the molten slag resulting from copper smelting has a melting temperature comprised, depending on minor components, in a range between about 950°C and about 1250°C.

[0018] Upon mixing the molten slag and the arsenic oxides-containing pellets, the calcium oxide at the surface of the pellets increases. This local increase in calcium, from 2FeO.SiO2.0.3CaO in the slag to values much richer in CaO on the surface of the pellets, results in a local increase of the melting temperature, to above 1250°C, at the surface of the pellets, thus resulting in a surface crust, which in turn makes the dissolution of the pellets in the molten slag difficult and/or incomplete. [0019] In order to circumvent these difficulties, in an embodiment of a method of the present disclosure, the arsenic-containing flue dusts are pelletized prior to their mixing with the molten slag. In an experiment, the following glass forming composition was selected (w/w): flue dusts: 35%; silica 26.4%; fayalite: 23.6%, sodium carbonate (soda ash Na2C03): 15.5%. The silica, fayalite and sodium carbonate were ground to 100 microns before being wet mixed with the flue dusts that contain arsenious oxide in a range between about 50 and about 60% (w/w). The resulting mixture was then pelletized and submitted to thermal curing at 200°C.

[0020] The molar composition of the resulting pellets was about: 5.5. SiO¾ 1.59 FeO; 1.0 Na20; and 0.2 CaO, beside arsenic oxide. Fig. 487 in FIG. 3 indicates that the melting temperature of such composition is below 1000°C, which is typically much lower than the melting point of fayalite, i.e. of the slag. As a result, fusion of the resulting pellets efficiently occurs in contact with the molten slag, resulting in efficient mixing of phases and incorporation of the arsenic in the glass composition independently of the mixing sequence, namely either when the slag is added to the pellets or when the pellets are added in the slag. Adding the pellets to the slag may be preferred so as to avoid introducing pelletized arsenic in an empty hot reactor.

[0021] To obtain an homogeneous and inert arsenic glass, the viscosity of the molten slag is to be taken into account. In case the relatively cold pellets, of a typical temperature in a range between about 100°C and about 200°C, are added to the molten slag, which is at a temperature in the range between about 1000°C and about 1150°C, part of the molten slag is expected to freeze around the pellets unless some form of stirring is used; even with the presently discussed low-melting pellets (melting temperature below 1000°C), and achieving homogeneity may be a lengthy process. In an embodiment of an aspect of the present disclosure, to overcome this issue, a rotating tilting furnace or a hot gas circulation through the reaction mass may be used to control the temperature of the mixture. Moreover, depending on the composition and temperature of the molten slag and on the composition and temperature of the pellets, direct heating of the mixture may be used to maintain the fluidity of the mixture for a period of time in order to obtain the target homogeneity of the glass.

[0022] In a first experiment, arsenic-containing flue dusts pelletization was done using calcium oxide as taught in L.G. Twidwell, (Journal of Hazardous Materials, 8 (1983) 85-90). A fayalite having the following composition (w/w): As: 0.32%; Si: 22.44%; Fe: 18.94%; Ca: 2.10%; Na: 10.33% and Cu: 0.25% was melted at 1200°C. Arsenic- containing pellets (60g) prepared from the arsenic-containing flue dusts and having the following composition (w/w): As: 33.43%; Ca: 25.79%; Fe: 0.13%; Si: 0.09% and Cu: 0.95%, were added to the melted fayalite (160g). After two hours, the mixture was cooled. The resulting glass was lumpy and heterogenous and the EPA acetic acid test (TCLP 131) indicated a high leaching value of 277 mg of arsenic, a maximum standardly admitted being 5 mg of As. The arsenic content in the glass was 14% w/w.

[0023] In a second experiment, pelletization of the flue dust was done using sodium oxide. Arsenic-containing pellets (160g) prepared with the arsenic-containing flue dusts and having the following composition (w/w): As: 47.9; Ca: 0.77%; Fe: 0.15%; Na: 17.71%; Si: 0.07% and Cu: 1.42% were added to the molten fayalite of the same composition (134g) as in the first experiment. After two hours at 1200°C, the mixture was cooled. The resulting glass was homogeneous without lumps or pits and the EPA acetic acid test (TCLP 131) indicated a leaching of 2.66 mg of As, below the 5.0 mg As maximum standardly admitted limit. The arsenic content in the glass was 15% w/w.

[0024] In a third experiment, pelletization of the flue dust was done using recycled glass as a source of sodium, with the following composition: Fe: 0.2%; Mg: 0.9%; Ca: 7.8%; Si:35.2%; Al: 1.0%; Na: 9.35%; K: 0.5%; and B:0.03%. Pellets were prepared containing 70% w/w of this recycled glass composition and 30% w/w of the flue dusts containing 55% w/w of AS2O3. The pellets (60g) were mixed with the molten fayalite (140g) as in the second experiment described hereinabove. A homogenous glass containing 5% As w/w was then obtained with a TCLP 131 value of 1.05 mg of As, which is well below the 5.0 mg As maximum standardly admitted limit of 5.0 mg.

[0025] In a fourth experiment, pelletization of the flue dust was done using sodium/iron oxide. Starting from a molten slag having the same composition as in the second experiment described hereinabove, pellets (60g) having the following composition: Na2C03: 21. Og; flue dust: 39. Og and fayalite: 3.2g were added to the molten slag (134g) at 1200°C. After heating the resulting mixture for two hours at 1200°C, the obtained glass was casted and cooled. The glass had the following composition: As: 10.56%; Si: 18.96%; Fe: 10.45%; Ca:1.46%; Cu: 0.60%; Zn: 0.57%; Na: 12.19% and Al: 0.47%. The TCLP 131 test indicated 3.64ppm As, below the 5.0 mg As maximum standardly admitted. The weight of casted glass was 176.12g, indicating a volatilization of arsenic at a level of 5.84%.

[0026] Thus, a composition for the pelletized arsenic-containing flue dusts that comprises sodium/iron oxides as stabilizing agent, is found to decrease the refractoriness of the molten slag. Using such sodium-rich pellets in a mixture with the molten slag, a very homogeneous glass was obtained containing 15% As (w/w), and the EPA acetic acid test (TCLP 131 ) below 5ppm indicated the arsenic was efficiently sequestrated. A similar result was obtained with sodium oxide/iron oxide pellets: the molten slag, mostly fayalite, was used very efficiently for the encapsulation of arsenic. [0027] The method thus comprises vitrification of arsenic oxides by pelletization of the flue dusts with fusible oxides rather, as opposed to refractory oxides, and control of the fluidity of the mixture of the molten slag and the pelletized flue dusts by heating until obtaining a homogeneous glass. An effective sequestration of arsenic is obtained with a significant economy in energy and raw materials as a result of using the slag in fusion, compared with industrial methods for the vitrification of arsenic that consist in mixing pelletized arsenic-loaded flue dusts with glass forming elements.

[0028] Thus, the molten slag is used to vitrify arsenic in arsenic-containing pellets of flue dusts having a lower melting point than the melting point of the slag. The lower melting point of the arsenic-containing pellets of flue dusts is achieved by pelletizing the flue dusts with sodium as opposed to with calcium.

[0029] Thus leveraging the energy and a number of elements already present in the molten slag and that are needed for the vitrification, an efficient and economical vitrification of arsenic is achieved.

[0030] There is thus provided a method comprising using the molten slag from base metals smelting, which is mostly an iron silicate and already in a molten phase, as vitrification agent of arsenic. The method comprises selecting a combination of sodium oxide from sodium carbonate, sodium silicate or recycled glass or iron oxides as such or as iron silicate or a mixture of both sodium oxide and iron oxides, to prepare flue dusts pellets of a melting temperature below 1000°C, from the arsenic-containing flue dusts, and mixing these arsenic-containing pellets with the molten slag.

[0031] In an embodiment of the present disclosure, the method uses flue dust pellets comprising (w/w) Fe0/Fe203 in a range between about 7 and about 15%; Na20 in a range between about 10 and about 25%; S1O2 in a range between about 15 and about 30%; and CaO in a range between about 1 and about 4%, and having a melting temperature below 1000 °C, and a molten slag as defined by the CaO-FeO-SiC phase diagrams for a melting temperature in a range between about 950 and about 1250°C, typically between about 1000°C and about 1150°C. Such use of the molten slag as the vitrification medium for arsenic yields economy of energy and raw materials, in an operational way.

[0032] Arsenic (As), as well as other toxic elements such a antimony (Sb), lead (Pb) and cadmium (Cd), may thus be sequestered by vitrification using the molten slag. [0033] The scope of the claims should not be limited by the embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

1. A method for slag vitrification of target elements, comprising forming target elements- containing pellets having a melting temperature lower than a melting temperature of a molten slag, mixing the target elements-containing pellets with the molten slag; and cooling.

2. The method as in Claim 1 , wherein said forming target elements-containing pellets comprises pelletization with fusible oxides.

3. The method as in Claim 1 , comprising collecting molten slag and flue dusts from base metal smelting operation, forming a composition of the flue dusts, pelletizing the composition of the flue dusts with fusible oxides, and mixing the molten slag and the pelletized flue dusts.

4. The method as in any one of Claims 1 to 3, wherein the molten slag is a fayalite comprising one or more of oxides of calcium and aluminum, lead, copper, zinc, arsenic and trace elements, and the molten slag has a melting temperature in a range between 950 and 1250°C.

5. The method as any one of Claims 1 to 4, wherein the target elements-containing pellets comprise non-target elements that decrease the refractoriness of the molten slag.

6. The method of any one of claims 1 to 5, wherein the target elements-containing pellets comprises non-target sodium or sodium/iron oxides.

7. The method of any one of claims 1 to 6, wherein the target elements-containing pellets comprise at least one of: non-target sodium oxide, non-target sodium carbonate, non-target sodium silicate, non-target recycled glass, non-target iron oxides; and non-target iron silicate.

8. The method as any one of Claims 1 to 7, comprising at least one of : heating and stirring of the mixture of the target elements-containing pellets and the molten slag.

9. The method as any one of Claims 1 to 8, comprising controlling the viscosity of the mixture of the target elements-containing pellets with the molten slag.

10. The method as any one of Claims 1 to 9, comprising collecting a fayalite molten slag and flue dusts from smelting of base metals from ores containing the base metals and at least one of: As, Sb, Cd and Pb; forming a composition of the flue dusts comprising fusible oxides; pelletizing the composition of the flue dusts; and mixing the molten slag and the pelletized composition of the flue dusts.

11. The method as any one of Claims 1 to 10, wherein the pellets comprise (w/w): Fe0/Fe203 in a range between 7 and 15%; Na20 in a range between 10 and 25%; S1O2 in a range between 15 and 30%; and CaO in a range between 1 and 4%; and the molten slag is CaO-FeO-SiC with a melting temperature in a range between 950°C and 1250°C.

12. The method as any one of Claims 1 to 11 , wherein the pellets comprise (w/w): Fe0/Fe203 in a range between 7 and 15%; Na20 in a range between 10 and 25%; S1O2 in a range between 15 and 30%; and CaO in a range between 1 and 4%; and the molten slag is CaO-FeO-Si02 with a melting temperature in a range between 1000°C and 1150°C.

13. The method as any one of Claims 1 to 12, wherein the target elements are at least one of As,

Sb, Cd and Pb.

14. A method for the vitrification of target elements using molten fayalite slag, comprising pelletizing the target elements with sodium oxide, mixing resulting pellets with the molten slag; and cooling.

15. The method as in claim 14, comprising controlling the viscosity of the mixture during mixing.

16. The method as in any one of claims 14 and 15, comprising stirring the mixture during mixing.

17. The method as in any one of claims 14 to 16, comprising heating the mixture during mixing.

18. The method as in any one of claims 14 to 17, wherein the molten fayalite slag has a melting temperature in a range between 950°C and 1250°C and the pellets have a melting temperature lower than the melting temperature of the molten fayalite slag.

19. The method as in any one of claims 14 to 18, wherein the molten fayalite slag has a melting temperature in a range between 1000°C and 1150°C and the pellets have a melting temperature lower than the melting temperature of the molten fayalite slag.

20. The method as in any one of claims 14 to 19, wherein the target elements are at least one of As, Sb, Cd and Pb.