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Process for the fuming treatment of metallurgical slag - Google Patents

05, Jun. 2025

Process for the fuming treatment of metallurgical slag - Google Patents

USA - Process for the fuming treatment of metallurgical slag - Google Patents

Process for the fuming treatment of metallurgical slag Download PDF

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Publication number: USA
Authority: US; United States
Prior art keywords: slag; furnace; zones; fuming; treatment
Prior art date: -08-25
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.): Expired - Lifetime

Application number

US06/714,506

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USA (en

Inventor

Eric O. A. Sundstrom

Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)

Boliden AB

Original Assignee

Boliden AB

Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)

-08-25

Filing date

-08-16

Publication date

-02-24

-08-16 Application filed by Boliden AB filed Critical Boliden AB

-02-24 Application granted granted Critical

-02-24 Publication of USA publication Critical patent/USA/en

-02-24 Anticipated expiration legal-status Critical

Status Expired - Lifetime legal-status Critical Current

Classifications

- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B7/00—Working up raw materials other than ores, e.g. scrap, to produce non-ferrous metals and compounds thereof; Methods of a general interest or applied to the winning of more than two metals
- C22B7/04—Working-up slag
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling

Definitions

the present invention relates to a continuous process for the fuming treatment of metallurgical slag, e.g. lead and copper smelting furnace slag, for the purpose of recovering valuable metals in the slag, such as zinc, lead, tin, cadmium and germanium.
metallurgical slag e.g. lead and copper smelting furnace slag
Metallurgical slags are most often of the silicate type, i.e. they contain silica as a substantial component. The less valuable constituents from the original charge are also usually collected in these slags, e.g. its content of gangue which may contain CaO, MgO, Al 2 O 3 and SiO 2 . In non-ferrous metallurgy the slags often contain a large amount of the iron of the original charge in the form of oxides, primarily FeO but also magnetite.
Certain sulphides may also be dissolved in slags, preferably at high slag temperatures. These sulphides dissolved in slag can in turn dissolve other sulphides secondarily, which latter sulphides primarily have lower solubility. It may be mentioned here that Cu 2 S and PbS, for example, can be dissolved secondarily in ZnS or FeS, which primarily has been dissolved in slag.
So-called mechanical metal losses to the slag can occur with low slag temperatures or with high slag viscosity, due to incomplete separation of slag and the product, e.g. mattes, striven for in the metallurgical process.
slag-fuming One of the most economical methods of recovering valuable constituents from slags is one generally known as slag-fuming.
Slag-fuming is generally carried out batch-wise in furnaces, each furnace cycle usually being divided into three periods, viz. charging, fuming and tapping.
Fuel usually in the form of pulverized coal, and air is blown into and through the slag during the charging period via tuyeres which are usually placed horizontally in the furnace walls, the relationship between fuel and air being selected so that the ingoing coal is substantially combusted to CO 2 .
Heat is hereby supplied to the furnace and the temperature of the charged slag rises.
the fuel supply is increased in relation to the air supply, usually so that only a part of the C present in the fuel is oxidised, and then primarily the CO only.
oxides of metals predominantly of metals more precious than iron are reduced to metals, which at the temperatures in question have a high partial pressure and therefore accompany outgoing furnace gases.
the metals are subsequently recovered from these gases by reoxidation and separation of solid oxides formed. Since the temperature of the slag decreases during this period of the furnace cycle, the relation between air and fuel supply must once again be changed on approaching and during the last period, i.e. the tapping period, so that the slag temperature increases for facilitating tapping and also an obtional subsequent separation process, in which remaining sulphides in the treated slag are separated.
slag containing i.a. about 13% by weight Zn is treated. After fuming the slag contains about 2-2,5% by weight Zn, which is equivalent to a zinc yield of only about 80-85%.
High yield of zinc is however an economical pre-requisite for slag fuming, which means that the discontinuous processes which easily achieve a zinc yield of 90% and more, have so far been superior with regard to yield. It has also been found that fuel economy for the continuous process described is considerably poorer than in discontinuous processes.
a further inconvenience with such a procedure is that easy-volatilized components of the slag, e.g. certain sulphides, are to a large extent expelled out of the slag at the heating in the settling pool.
the slag is to be subjected to further separation processes after the fuming treatment, it can be subjected to further heating in a third furnace zone, and which has been found essential in connection with copper smelting furnace slag for a subsequent copper recovery from the slag is a fourth furnace zone be subjected to reduction of its magnetite content before it is tapped from the furnace.
the heating operations can thus be performed without any inconvenience at all, since the total amount of furnace gas, which gas is substantially inert in relation to sulphides, can be used as a carrier gas for transporting out of the furnace volatilized sulphides as well as products volatilized at reduction treatments. Furthermore, the collected furnace gas can be treated in one single gas cleaning and metal recovery system. The outlet for the furnace gases is consequently preferably placed at or adjacent said first zone of the furnace, whereby furthermore any significant unintensional oxidation of the volatilized slag components is avoided in the furnace.
the furnace zones can merge directly into each other without any mechanical boundary between them, although the oxygen and fuel supply to each furnace zone differs from the oxygen and fuel supply to adjacent furnace zone or zones. Since, especially in comparatively short furnaces, troublesome longitudinal mixing of untreated, partially treated and completely treated slag is difficult to avoid, the furnace can suitably be provided with intermediate walls or thresholds transverse to the through-flow direction of the slag to partially separate the slag volumes in the different treatment zones, thereby preventing or inhibiting longitudinal slag flow of an undesirable nature.
These intermediate walls can be made in a plurality of different ways, and they can be cooled or uncooled. They can furthermore be impermeable or provided with through-flow openings of different sizes and shapes and can, for example, be made as gratings.
the fuming period itself can be divided into several sub-periods, during which the air/fuel ratios are optimated with regard to the variation of the slag composition during the fuming period. For instance, in certain cases it can be advantageous to use more strongly reducing conditions the further the reduction treatment continues.
Such subdivision of the fuming period and possibly, in as far as is desirable, other treatment periods as well is enabled in the process according to the invention by dividing up the said second furnace zone and other furnace zones into at least two partial zones, which can thereby also be separated by individual temperature adjusting zones. Intermediate walls of the kind described above separating the partial zones and also separating these and the temperature adjusting zones can thereby further improve the efficiency.
the use of oxygen gas or an air/oxygen gas mixture in a suitable zone can improve the possibilities of smelting cold slag charged into the zone in question.
solid, liquid or gaseous fuel together with oxygen containing gas is introduced, preferably adjacent the furnace bottom, into the slag in each of the different zones and partial zones, respectively.
Pulverized coal is preferably used as the solid fuel, and can be produced by grinding slack or other lowgrade coal qualities in a milling plant adjoining the furnace plant.
the furnace may also be supplied with liquid fuels, preferably liquid petroleum products. It is extremely advantageous to use fuel oil, since it is easy to transport to the furnace and is furthermore easy to introduce therein at a desired constant flow.
pulverized coal can preferably be supplied to the reduction zones, while fuel oil is used solely as fuel in the process and is suitably supplied to the heating zones.
a furnace for continuous slag fuming in accordance with a preferred embodiment of the invention is continuously supplied with 38 tons of molten slag per hour.
the slag which is of the fayalite type contains, inter alia, 11% by weight Zn.
threshold-like intermediate walls controlling the slag flow through the furnace the latter is partially divided up into three zones, respectively comprising 27.8%, 55.5% and 16.7%. of the furnace volume.
mm Hg and 20° C. preheated to about 300° C.
the combustion gas will essentially comprise carbon dioxide and nitrogen
the same amount of coal and air is supplied to the third furnace zone as to the first furnace zone, whereby the slag temperature, which has dropped when passing the second furnace zone, is once again raised.
the outgoing slag has a zinc content of 1.2% by weight.
the throughput of slag during the furnace cycle will only be about 90% of that with continuous fuming.

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Engineering & Computer Science (AREA)
Chemical & Material Sciences (AREA)
Life Sciences & Earth Sciences (AREA)
Environmental & Geological Engineering (AREA)
General Life Sciences & Earth Sciences (AREA)
Geology (AREA)
Manufacturing & Machinery (AREA)
Materials Engineering (AREA)
Mechanical Engineering (AREA)
Metallurgy (AREA)
Organic Chemistry (AREA)
Manufacture And Refinement Of Metals (AREA)

Abstract

A continuous process for the fuming treatment of metallurgical slag, e.g. lead and copper smelting furnace slag. Passing the slag continuously through consecutive slag treatment zones of a fuming furnace, subjecting it in a first one of said zones to heating to fume off comparatively volatile, preferably sulphide-bound constituents thereof, subjecting it in a second one of said zones to reduction to reduce preferably oxide-bound constituents and fuming off said last mentioned constituents in an elementary form and adjusting the temperature of the slag in said first zone such that the reduction treatment in said second zone is accomplishable substantially without further heating the slag.

Description

The present invention relates to a continuous process for the fuming treatment of metallurgical slag, e.g. lead and copper smelting furnace slag, for the purpose of recovering valuable metals in the slag, such as zinc, lead, tin, cadmium and germanium. Metallurgical slags are most often of the silicate type, i.e. they contain silica as a substantial component. The less valuable constituents from the original charge are also usually collected in these slags, e.g. its content of gangue which may contain CaO, MgO, Al2 O3 and SiO2. In non-ferrous metallurgy the slags often contain a large amount of the iron of the original charge in the form of oxides, primarily FeO but also magnetite. Apart from the slag components mentioned above there are often valuable and recoverable constituents in metallurgical slags, this being at least partly due to the fact that primarily the silicates in certain acidic or neutral slags have the ability of taking up metals present in oxidic form. Larger amounts of zinc can thus be dissolved oxidically in iron silicate slags, for example. Certain sulphides may also be dissolved in slags, preferably at high slag temperatures. These sulphides dissolved in slag can in turn dissolve other sulphides secondarily, which latter sulphides primarily have lower solubility. It may be mentioned here that Cu2 S and PbS, for example, can be dissolved secondarily in ZnS or FeS, which primarily has been dissolved in slag. So-called mechanical metal losses to the slag can occur with low slag temperatures or with high slag viscosity, due to incomplete separation of slag and the product, e.g. mattes, striven for in the metallurgical process. One of the most economical methods of recovering valuable constituents from slags is one generally known as slag-fuming. Slag-fuming is generally carried out batch-wise in furnaces, each furnace cycle usually being divided into three periods, viz. charging, fuming and tapping. Fuel, usually in the form of pulverized coal, and air is blown into and through the slag during the charging period via tuyeres which are usually placed horizontally in the furnace walls, the relationship between fuel and air being selected so that the ingoing coal is substantially combusted to CO2. Heat is hereby supplied to the furnace and the temperature of the charged slag rises. During the fuming period the fuel supply is increased in relation to the air supply, usually so that only a part of the C present in the fuel is oxidised, and then primarily the CO only. Under the reducing action of the atmosphere in the slag caused by C and CO, oxides of metals predominantly of metals more precious than iron, are reduced to metals, which at the temperatures in question have a high partial pressure and therefore accompany outgoing furnace gases. The metals are subsequently recovered from these gases by reoxidation and separation of solid oxides formed. Since the temperature of the slag decreases during this period of the furnace cycle, the relation between air and fuel supply must once again be changed on approaching and during the last period, i.e. the tapping period, so that the slag temperature increases for facilitating tapping and also an obtional subsequent separation process, in which remaining sulphides in the treated slag are separated. A plant for batch-wise or discontinuous slag fuming is described in the "Journal of Metals," June , pages 1-8, for example. In modern metallurgy there are attempts to turn from discontinuous to continuous processes, if possible. There are many reasons for these attempts, but they are primarily motivated by the increased demands for integrated process systems, automation, productivity and working hygiene. These conditions also apply to the slag fuming process, where a discontinuous method has many drawbacks in these respects. For instance, it can be mentioned that charging and tapping times reach about 30% of the total time for treating a charge, i.e. of the whole furnace cycle. Furthermore, the heat load on the furnace will be very uneven with alternating warmer and colder periods, in turn negatively affecting the life of the fuming furnace. It may also be mentioned that steam boilers connected to discontinuous slag fuming formers will have a periodically varying steam production, which naturally creates certain complications in an integrated steam system. Processes for continuous treatment of slag have been previously proposed. Attempts have thereby been made of continuously treating slags containing non-ferrous metals in special shaft furnaces. It has been found impossible to operate such processes on an industrial scale. In the British patent specification 1 169 904 (Jordanov et al), however, there is described a slag fuming process in which molten slag containing recoverable non-ferrous metals is continuously tapped from a shaft furnace into a separate settler where the slag temperature is raised by about 100° C., the slag being continuously tapped off from the settler to a fuming furnace, and air and carbonaceous material being continuously passed to the fuming furnace and blown through the slag, whereafter the slag is continuously tapped from the fuming furnace. The above described method has been used in a plant at Plovdiv in Bulgaria since . In the last-mentioned plant, slag containing i.a. about 13% by weight Zn is treated. After fuming the slag contains about 2-2,5% by weight Zn, which is equivalent to a zinc yield of only about 80-85%. High yield of zinc is however an economical pre-requisite for slag fuming, which means that the discontinuous processes which easily achieve a zinc yield of 90% and more, have so far been superior with regard to yield. It has also been found that fuel economy for the continuous process described is considerably poorer than in discontinuous processes. A further inconvenience with such a procedure is that easy-volatilized components of the slag, e.g. certain sulphides, are to a large extent expelled out of the slag at the heating in the settling pool. This means that--if special measures are not taken for transportation of the volatilized sulphides out of the furnace, for instance by a powerful through-flow of an inert carrier gas in the furnace in a manner such as not to disturb the settling process, such through-flow, in its turn, requiring the arrangement of suitable inlet and outlet means for the carrier gas and possibly a separate gas cleaning system--the sulphides expelled out of the slag, possibly oxidized to sulphates, basic sulphates, or even oxides, will cause severe incrustions on the furnace walls and in the gas conducting means and furthermore reduced yields of the process. It has now been surprisingly found that the said drawbacks in continuous slag fuming can be circumvented and furthermore larger throughout can be obtained than in a corresponding discontinuous furnace, if the slag is fumed according to a process which is characterized in that the slag, during continuous passage through a fuming furnace having consecutive slag treatment zones, in a first furnace zone is subjected to heat treatment for fuming off comparatively volatile, preferably sulphide-bound constituents thereof, and in a subsequent second furnace zone is subjected to reduction treatment for reducing preferably oxide-bound constituents, and fuming off these in elementary form, the temperature of the slag in the first furnace zone being adjusted so that the reduction treatment in the second furnace zone can be carried out substantially without further heating of the slag. If the slag is to be subjected to further separation processes after the fuming treatment, it can be subjected to further heating in a third furnace zone, and which has been found essential in connection with copper smelting furnace slag for a subsequent copper recovery from the slag is a fourth furnace zone be subjected to reduction of its magnetite content before it is tapped from the furnace. In the process in accordance with the invention, the heating operations, for instance, can thus be performed without any inconvenience at all, since the total amount of furnace gas, which gas is substantially inert in relation to sulphides, can be used as a carrier gas for transporting out of the furnace volatilized sulphides as well as products volatilized at reduction treatments. Furthermore, the collected furnace gas can be treated in one single gas cleaning and metal recovery system. The outlet for the furnace gases is consequently preferably placed at or adjacent said first zone of the furnace, whereby furthermore any significant unintensional oxidation of the volatilized slag components is avoided in the furnace. The furnace zones can merge directly into each other without any mechanical boundary between them, although the oxygen and fuel supply to each furnace zone differs from the oxygen and fuel supply to adjacent furnace zone or zones. Since, especially in comparatively short furnaces, troublesome longitudinal mixing of untreated, partially treated and completely treated slag is difficult to avoid, the furnace can suitably be provided with intermediate walls or thresholds transverse to the through-flow direction of the slag to partially separate the slag volumes in the different treatment zones, thereby preventing or inhibiting longitudinal slag flow of an undesirable nature. These intermediate walls can be made in a plurality of different ways, and they can be cooled or uncooled. They can furthermore be impermeable or provided with through-flow openings of different sizes and shapes and can, for example, be made as gratings. To enable better utilization of reduction agents and fuel in slag fuming, the fuming period itself can be divided into several sub-periods, during which the air/fuel ratios are optimated with regard to the variation of the slag composition during the fuming period. For instance, in certain cases it can be advantageous to use more strongly reducing conditions the further the reduction treatment continues. Such subdivision of the fuming period and possibly, in as far as is desirable, other treatment periods as well is enabled in the process according to the invention by dividing up the said second furnace zone and other furnace zones into at least two partial zones, which can thereby also be separated by individual temperature adjusting zones. Intermediate walls of the kind described above separating the partial zones and also separating these and the temperature adjusting zones can thereby further improve the efficiency. Furthermore, the use of oxygen gas or an air/oxygen gas mixture in a suitable zone can improve the possibilities of smelting cold slag charged into the zone in question. To carry out the heat treatment and the reduction treatment of the slag, solid, liquid or gaseous fuel together with oxygen containing gas is introduced, preferably adjacent the furnace bottom, into the slag in each of the different zones and partial zones, respectively. Pulverized coal is preferably used as the solid fuel, and can be produced by grinding slack or other lowgrade coal qualities in a milling plant adjoining the furnace plant. To advantage, the furnace may also be supplied with liquid fuels, preferably liquid petroleum products. It is extremely advantageous to use fuel oil, since it is easy to transport to the furnace and is furthermore easy to introduce therein at a desired constant flow. It is also possible to use both fuel oil and pulverized coal, which can be an advantage, since in certain cases fuel oil has been found to be less suitable from the point of view of fuming efficiency. In such cases pulverized coal can preferably be supplied to the reduction zones, while fuel oil is used solely as fuel in the process and is suitably supplied to the heating zones. The advantages of the process according to the invention are illustrated by the following examples: A furnace for continuous slag fuming in accordance with a preferred embodiment of the invention is continuously supplied with 38 tons of molten slag per hour. The slag which is of the fayalite type contains, inter alia, 11% by weight Zn. By means of threshold-like intermediate walls controlling the slag flow through the furnace, the latter is partially divided up into three zones, respectively comprising 27.8%, 55.5% and 16.7%. of the furnace volume. About kg pulverized coal together with about m3 air (calculated at normal conditions, i.e. mm Hg and 20° C.), preheated to about 300° C., are supplied to the first zone per hour, whereat the combustion gas will essentially comprise carbon dioxide and nitrogen, and the slag temperature increase by about 100° C. kg of pulverized coal and about m3 of air, preheated to about 300° C., are supplied per hour to the second zone. The same amount of coal and air is supplied to the third furnace zone as to the first furnace zone, whereby the slag temperature, which has dropped when passing the second furnace zone, is once again raised. The outgoing slag has a zinc content of 1.2% by weight. For discontinuous slag fuming carried out on the same type of slag in the same furnace, although without any division into zones and intended to reach the same outgoing zine content as in the continuous slag fuming just described as an example, the throughput of slag during the furnace cycle will only be about 90% of that with continuous fuming. Obtaining the same zinc yield in the discontinuous process as in the continuous one thus requires longer times per furnace cycle, which in turn means that the throughput in the discontinuous process will be lower. In actual fact, the throughput in a discontinuous process will be further reduced because for practical reasons the furnace cycles cannot follow immediately after each other without a certain amount of intermediate rigging time.

Claims (12)

I claim: 1. A continuous process for the fuming treatment of metallurgical slag, comprising as slag components one or more oxides selected from a first group consisting of CaO, MgO, Al2 O3, SiO2, FeO and Fe3 O4 and further comprising one or more recoverable metals selected from a second group consisting of zinc, lead, tin, cadmium and germanium in sulfide, oxide or sulfide and oxide bound form, the process comprising the steps of continuously introducing the slag in a molten state to a fuming furnace having consecutive slag treatment zones, causing the slag to continuously pass through said furnace zones and out of the furnace, subjecting the slag, in a first furnace zone to a heat treatment in an atmosphere substantially inert to sulfides by means of supplying fuel and oxygen to the slag so that sulfide-bound recoverable metals from said second group are fumed off in substantially sulfidic state and recovered, subjecting the slag in a subsequent second furnace zone to a reduction treatment by means of supplying fuel and oxygen to the slag so as to reduce the oxide bound recoverable metals of said second group and to fume off these in elementary form and recover them by reoxidation and separation of solid oxides formed thereby, the temperature of the slag in the first furnace zone being adjusted by the heat treatment so that the reduction treatment in the second furnace zone can be carried out substantially without further heating the slag. 2. Process as claimed in claim 1, wherein the slag is once again subjected to heat treatment in a third furnace zone following the second furnace zone. 3. Process as claimed in claim 2, wherein the treated slag is subjected to reduction treatment in a fourth furnace zone before it is tapped from the furnace. 4. Process as claimed in claim 1, wherein the individual zones are partly separated by intermediate walls for controlling the longitudinal slag flow in the furnace. 5. Process as claimed in claim 1, wherein the second furnace zone is subdivided into at least two partial zones. 6. Process as claimed in claim 5, wherein the partial zones are separated by temperature adjusting zones. 7. Process as claimed in claim 5, wherein the partial zones are separated from each other or from the temperature adjusting zones by means of intermediate walls. 8. Process as claimed in claim 1, wherein the fuming treatment is carried out by introducing solid, liquid or gaseous fuel and oxygen-containing gas to the slag in the different zones. 9. Process as claimed in claim 8, wherein pulverized coal is used as fuel. 10. Process as claimed in claim 8, wherein a liquid petroleum product is used as fuel. 11. Process as claimed in claim 10, wherein the liquid petroleum product is fuel oil. 12. Process as claimed in claim 8, wherein both fuel oil and pulverized coal are used as fuel. US06/714,506 -08-25 -08-16 Process for the fuming treatment of metallurgical slag Expired - Lifetime USA (en)

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Application Number Priority Date Filing Date Title SE -08-25 SEA SEB (en) -08-25 -08-25 PROCEDURE FOR CONTINUOUS SMOKING TREATMENT OF METALLURGIC SLAGS

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Cited By (8)

* Cited by examiner, † Cited by third party Publication number Priority date Publication date Assignee Title WOA1 (en) * -08-27 -03-10 Commonwealth Scientific And Industrial Research Or Process for the treatment of lead-zinc ores, concentrates or residues USA (en) * -08-18 -04-11 Cominco Ltd Method for slag fuming and reduction USA (en) * -04-21 -01-02 Asarco Incorporated Process for treating speiss USA (en) * -11-10 -07-21 A. Ahlstrom Corporation Method and apparatus for treating zinc concentrates USA (en) * -05-21 -04-25 Gnb Technologies, Inc. Method for treating rotary slag USA1 (en) * -09-27 -03-29 Takafumi Sasaki Dry processing method and system for converter slag in copper smelting WOA1 (en) -09-21 -03-27 Valeas Recycling Ab Plasma induced fuming CNA (en) * -01-12 -04-08 昆明有色冶金设计研究院股份公司 Process for recovering valuable metals through smelting by blowing oxygen-enriched air into traditional fuming furnace

Citations (2)

* Cited by examiner, † Cited by third party Publication number Priority date Publication date Assignee Title USA (en) * -03-24 -06-24 Za Zvetni Metali Dimiter Blago Method for treating non-ferrous metal slag USA (en) * -09-18 -07-01 Bechtel Int Corp Submerged smelting

- -08-25 SE SEA patent/SEB/en unknown
- -08-16 US US06/714,506 patent/USA/en not_active Expired - Lifetime
- -08-19 AU AU/76A patent/AUB2/en not_active Expired
- -08-19 DE DE patent/DEA1/en not_active Withdrawn
- -08-23 CA CA259,660A patent/CAA/en not_active Expired

Patent Citations (2)

Cited By (10)

* Cited by examiner, † Cited by third party Publication number Priority date Publication date Assignee Title USA (en) * -04-21 -01-02 Asarco Incorporated Process for treating speiss USA (en) * -08-18 -04-11 Cominco Ltd Method for slag fuming and reduction WOA1 (en) * -08-27 -03-10 Commonwealth Scientific And Industrial Research Or Process for the treatment of lead-zinc ores, concentrates or residues USA (en) * -11-10 -07-21 A. Ahlstrom Corporation Method and apparatus for treating zinc concentrates AUB2 (en) * -11-10 -07-08 Foster Wheeler Energia Oy Method and apparatus for treating zinc concentrates USA (en) * -05-21 -04-25 Gnb Technologies, Inc. Method for treating rotary slag USA1 (en) * -09-27 -03-29 Takafumi Sasaki Dry processing method and system for converter slag in copper smelting WOA1 (en) -09-21 -03-27 Valeas Recycling Ab Plasma induced fuming USB2 (en) -09-21 -06-26 Val'eas Recycling Solutions Ab Plasma induced fuming CNA (en) * -01-12 -04-08 昆明有色冶金设计研究院股份公司 Process for recovering valuable metals through smelting by blowing oxygen-enriched air into traditional fuming furnace

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Publication number Publication date AUB2 (en) -12-21 CAA (en) -07-01 SEB (en) -10-10 DEA1 (en) -03-10 SEL (en) -02-26 AUA (en) -02-23

The Thermodynamic Modelling of the Zn Slag Fuming with the Use ...

Process Description

Slag fuming is a well-established process, soon celebrating its 100th anniversary since the launch of the first commercial furnace.1 The process was initially developed to extract Zn from the slags of the lead-blast furnace and was used for Zn-rich slags; these days it is increasingly used for the extraction of a variety of metals from the complex feed allowing the content of valuable elements in the slag to be comparably low.2,3 The production of slags with a small concentration of impurities is also desirable for their potential use as construction materials.4 Adjustments of the operational parameters to maximize the extraction of target metals and produce cleaner slags require a flexible model applicable to the wide range of operational conditions.

RE TECH Product Page

The slag fuming process is based on the high volatility of metallic Zn at high temperatures and reducing conditions.5 It can be effectively separated from other components of the slag by reduction and evaporation according to the following simplified reactions:1,6,7,8,9,10

$$ {\text{ZnO}}_{{({\text{slag}})}} + {\text{C}}_{{({\text{solid}})}} \left| {{\text{CO}}_{{({\text{gas}})}} } \right|{\text{H}}_{{{2}({\text{gas}})}} \leftrightarrow {\text{ Zn}}_{{({\text{gas}})}} + {\text{CO}}_{{({\text{gas}})}} \left| {{\text{CO}}_{{{2}({\text{gas}})}} } \right|{\text{H}}_{{2}} {\text{O}}_{{({\text{gas}})}} $$ (1) $$ {\text{ZnO}}_{{({\text{slag}})}} + {\text{3FeO}}_{{({\text{slag}})}} \leftrightarrow {\text{ Zn}}_{{({\text{gas}})}} + {\text{3FeO}}_{{{1}.{33}({\text{slag}})}} $$ (2) $$ {\text{FeO}}_{{{1}.{33}({\text{slag}})}} + {\text{C}}_{{({\text{solid}})}} \left| {{\text{CO}}_{{({\text{gas}})}} } \right|{\text{H}}_{{{2}({\text{gas}})}} \leftrightarrow {\text{ FeO}}_{{({\text{slag}})}} + {\text{CO}}_{{({\text{gas}})}} \left| {{\text{CO}}_{{{2}({\text{gas}})}} } \right|{\text{H}}_{{2}} {\text{O}}_{{({\text{gas}})}} $$ (3)

After leaving the bath, Zn fumes are re-oxidised outside of the reactor by air. Zinc oxide precipitates from the gas stream as a solid phase (zincite), which is further refined using hydrometallurgy and electrowinning.

Typically, carbon and hydrogen are supplied to the reactor in the form of pulverised coal or oil, while other sources of carbon and/or hydrogen can be potentially used. They both act as reducing agents and fuel. Reaction (1) is highly endothermic. To achieve the energy balance, a portion of C and H is combusted by the oxygen from the blast, either directly or through reactions with the components of the liquid oxide (slag). Ferric iron (Fe3+) in slag is also reduced during the process, first into ferrous iron (Fe2+), and then potentially into metallic iron (Fe0). The precipitation of the metallic iron is undesirable because of the formations of accretions and inefficient use of reductant and should be avoided or minimised during the process.1 From the thermodynamic point of view, the conditions of formation of Fe0 correspond to the lowest achievable concentration of zinc in the slag among other given conditions. Other elements, such as Pb, Sb and As, can be reduced from their oxidic form and potentially separated from the slag into metallic and sulphide phases6 or into gas. The presence of As and Sb may cause the formation Fe-rich speiss instead of solid metallic iron.

In practice, kinetic factors prevent achieving the thermodynamic limit in the zinc fuming process. On the macro level, the rate of gases that can safely be blown through the slag limits the speed of the process.11 Heterogeneous gas-slag reactions and gas combustion reactions are relatively slow.12 Coal particles entrained in the slag react with the slag components through the gas film; this reaction is limited by diffusion.10,13 The overall mass and heat balances of the reactor should comply with both thermodynamic and kinetic aspects of the process. Potential separation of liquid metal, matte or speiss phases would depend on the growth of droplets, coalescence and settling kinetics.

Process Models

The existing models of the Zn slag fuming can be divided into three groups: thermodynamic, kinetic and combined models. The models belonging to the first group assume the achievement of thermodynamic equilibrium at fixed time intervals in the reactor. Equilibrium calculations rely on the thermodynamic properties of the present chemical species and phases. They provide mass and heat balances. More advanced models give information on phase equilibria and behaviour of minor elements. The accuracy of these models depends on the quantity and quality of equilibrium experimental data of different types, used in the assessment of model parameters. Kinetic models consider a set of stoichiometric reactions. Their rates are established using empirical coefficients. To increase the accuracy, a larger set of industrial data would have to be used. The models of this type can give accurate predictions of the dynamic behaviour for a given reactor, but they would have limited predictive power outside of the range of the typical conditions. The models of the third type combine the advantages of thermodynamic and kinetic approaches and can be complicated. Thermodynamic foundation of these models is independent from the process application, and kinetic factors are specific for the process. Kinetic factors must be calibrated using process data of the furnace in question. These advanced models can be difficult to use daily by furnace operators. The following sections provide examples of the earlier studies, without an intention to make a complete review, at the same time outlining the main features of the previously developed models, their advantages and disadvantages, and the existing discussion on the thermodynamic or kinetic nature of the process.

Bell et al.6 prepared the thermodynamic model based on the activity coefficient of zinc oxide derived from the industrial data. The obtained model was used for the sensitivity analysis of the process. Several parameters, such as fuel composition, oxygen enrichment and blast preheating, were tested. The list of different reductants and fuels for the calculations included hydrogen gas, natural gas, fuel oil, coals of different volatility and carbon. The main output of the model was the time of the process. It was predicted to increase in order of the reductants/fuels given above. Two sets of industrial data resulted in close activity coefficients of ZnO, but the third set gave lower values in the range of 2–8 wt.% of Zn in slag.

Quarm14 proposed the kinetic equation to describe the concentration of zinc in slag as a function of time. The simple equation contained a single adjustable parameter, velocity coefficient, that had to be calibrated for each time period. The heat balance was calculated using heats of reactions.

Kellogg7 developed a two-stage computer mathematical model of the process. The first stage included the direct reaction of the reductant and blast with the slag phase with the formation of the reduced Zn-containing vapour. The second stage included the oxidation of the produced vapour with tertiary air, forming zinc oxide. The 90-min fuming cycle was divided in 900 steps with 0.1-min integration steps; the thermodynamic calculations were made for every step. The presented model was the first to encounter for the fuming of Pb but, due to the lack of the knowledge on the process, contained simplifications of the chemistry and heat balance.7 The industrial data obtained during normal and extreme operational conditions (temporary absence of reductant supply12) were successfully reproduced by the authors. Additionally, the effects of the coal rate and preheated blast were investigated similarly to Bell et al.6 The model did not consider the solid phases present in equilibrium and possible precipitation of metallic Fe. The activity coefficients of the slag species were assumed to be independent of the starting slag composition.

Sundstrom11 reported the mathematical model of the slag fuming process based on the numerous sets of plant data collected over the time. The model considered the ZnO content in the slag at different points in time, charge volume, slag composition and other not specified operating conditions. The presented model was based on the previously treated slag and was appliable only for the limited range of the slag compositions.

Blaskett15 developed a simplified thermodynamic model to study the effect of preheated blast on the operational parameters of the process. The chemistry of slag was described by the concentration of ZnO only, limiting the predicting power of the model.

Grant and Barnett16 prepared the computer model based on the one presented earlier by Kellogg.7 The previous model was extended by the incorporation of the recuperative heating of secondary air and waste heat boiler simulation “thereby closing the loop and enabling the model to calculate continuously varying secondary air preheat temperatures”.16 The predictions were shown by the authors to be in good agreement with the industrial data. In the following study, Grant17 showed that activity coefficients of slag species can be derived from the plant data using the discussed model to account for the difference in the slag complexity in a limited range of compositions. The developed model was compared with the industrial data in later studies by Richards et al.9,10,18,19 and was shown to underestimate the fuming rates (note that the thermodynamic predictions can only overestimate the fuming rates and not underestimate them20).

Richards et al.9,10,18,19 proposed the mathematical model of the process based on the kinetic principles. Authors developed a system of 11 equations considering mass and heat balances of the reactor and rate terms of selected reactions. The change in ZnO, FeO and Fe2O3 activity coefficients in the slag were accounted for by introducing the equations based on the available literature data and depended on the temperature and the lime-to-silica ratio of the slag. The potential fuming of other species, such as Pb, As or Sb, was not incorporated into the model. The model was shown to replicate most of the available industrial data reported. Cockroft et al.21 extended the discussed model by adding the oxidation/reduction and evaporation of Pb into the list of considered chemical reactions. In the later study Richards22 demonstrated the applicability of the model to the optimisation of the continuous slag fuming process.

Jak and Hayes23 described the phase equilibria of the slag fuming using Factsage thermodynamic software.24 The chemical behaviour of the slag, simplified by the authors to the PbO-ZnO-FeO-Fe2O3-CaO-SiO2 system, was predicted for the wide range of ZnO concentrations and CaO/SiO2 and Fe/SiO2 ratios. The model was tested using the plant data presented earlier by Grant.17 The predictions were shown to be in good agreement with the plant results, and the difference in predicted and plant data was related by the authors to the kinetic limitations of the process taking place when the concentration of Zn in the slag decreases. The described model considered the interaction between liquid oxide and gas phases only and did not account for the heat balance of the process.

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Verscheure et al.2,25,26,27 developed a mathematical model using Factsage thermodynamic software24 to describe chemical, thermal and heat transfer outcomes of the submerged plasma and oxidative Isasmelt processes.25,26 The authors studied the effect of various process parameters on slag chemistry. Even though the formation of matte phase was considered, the thermodynamic database used could not describe the possible matte immiscibility, which can affect the predictions.

Lotfian28,29 studied the partial replacement of coal in the Zn fuming process by plastic materials of various types. After the industrial trial the author created the thermodynamic model of the process using public databases of Factsage thermodynamic software.30 The calculations were made to reproduce the plant trial data. The duration of the process was divided into 10-min intervals with the thermodynamic calculation corresponding to each of the intervals. The temperature of the process was kept constant at °C. Only spinel, slag, monoxide and olivine were selected from all solid solutions available. All possible pure solids and liquids formed in the PbO-ZnO-FeO-Fe2O3-CaO-SiO2-Al2O3-MgO system were selected from FactPS and FT-Oxide databases. Cu and S were not included into the calculations, and the formation of matte and the heat of the bath were not considered.

Nagraj31,32 used private33 and public thermodynamic data of Factsage thermodynamic software30 to develop a combined thermodynamic-kinetic model of the submerged plasma fuming process. The kinetic correction factors were introduced by the author to “close the heat and mass balance” of the process. The slag chemistry was simplified to the Cu2O-PbO-ZnO-FeO-Fe2O3- CaO-SiO2-Al2O3 system. The predictions were stated by the author to be in a good agreement with the industrial data. The phases considered in the model were not specified by the author.

Cheng et al.34 developed a kinetic model of Zn fuming based on the conducted laboratory study. The activity coefficients of the slag species were derived from the available thermodynamic data and used in a set of kinetic equations describing the volumetric mass transfer coefficients. In a later publication,35 a combined thermodynamic and kinetic model using an effective equilibrium reaction zone approach has been presented. The resulting model was tested against the available experimental data. Rather complex functions for the reactive gas and slag fractions were necessary to describe the experimental data. The model focused on the Zn concentration profile without detailed review of phase equilibria, energy balance and distribution of minor elements. The temperature was kept constant.

In the previous publications,6,7,8,12,16,20,23,36,37 the authors discussed whether Zn fuming process reaches thermodynamic equilibrium at a given time step. General agreement is that purely kinetic or thermodynamic models cannot be used for the accurate description of the process if the wide range of input conditions is expected. Therefore, the combination of both should be applied, as done in the later publications.31,32,35 Rather complex models have been developed in these studies. In the present study, a relatively simple equilibrium thermodynamically based model is used to describe the plant data, and more attention is given to the predictions of phase equilibria and partitioning of minor elements. The thermodynamic data used in the presented model are those based on the recent outcomes of the integrated experimental and thermodynamic modelling study of the gas/slag/metal/matte/speiss/solid phase equilibria in the Cu-Pb-Zn-Fe-Ca-Si-O-S-Al-Mg-As-Sb system.38,39,40,41,42,43,44,45,46 The use of the self-consistent thermodynamic data allows a wide range of the slag compositions to be used in the presented model without limitations on the concentration of the major slag elements (Cu-Pb-Zn-Fe-Ca-Si-O-S-Al-Mg).

Phase Equilibria

Earlier review of the phase equilibria in the Zn slag fuming systems was done by Jak and Hayes.23 The process is usually operated with fully liquid slag, but a small presence of solid phases is possible. According to Jak and Hayes,23 the potential phases to be formed in the system are spinel, wüstite, melilite and pseudowollastonite. In the present study, zincite and matte/metal/speiss phases were also considered; Table I includes the formulae of the main phases and species used in the present study and mentioned in the following text.

The thermodynamic model of the system was developed in the integrated experimental and thermodynamic modelling study of binary, ternary and higher order gas/slag/metal/matte/speiss/solid systems.38,39,40,41,42,43,44,45,46 The availability of accurate experimental data on the interactions between gas/slag/metal/matte/speiss/solid phases in the internal database is an advantage of the present model.

To ensure that the proportion of solid phases during slag fuming do not exceed the desirable level, fluxing diagrams similar to those given in Fig. 1 can be used. The common Zn fuming slag compositions can be described by the ZnO-FeO-Fe2O3-CaO-SiO2-Al2O3-MgO system. Even though the concentrations of the Al2O3 and MgO in the slag are usually small compared to other elements, their presence stabilises the spinel over wüstite and therefore should be accounted for. Different ternary sections of the system such as ZnO-(FeO + Fe2O3)-(CaO + SiO2) (Fig. 1A) or (FeO + Fe2O3 + ZnO)-CaO-SiO2 (Fig. 1B) with constant concentrations of Al2O3 and MgO in the slag and fixed (FeO + Fe2O3)/ZnO and CaO/SiO2 ratios, respectively, can then be used in industrial practise to adjust the fluxing strategy for any given batch. In addition to the slag (liquid oxide and solid phases), metal, matte and speiss can be formed in the bath collecting a part of the Cu, S, Sb and As. This presents an opportunity to settle and separate a portion of these elements from the slag.

Decarbonisation of the Process

Ammonia is considered one of the replacements for fossil fuels in a wide variety of industries.47 Replacing coal in metallurgy is challenging in processes such as iron and lead blast furnace reduction but more possible in others such as slag fuming as described in the present study. One of the goals of the present study is to thermodynamically analyse the difference in the slag fuming process parameters caused by the complete replacement of coal with ammonia. The findings of this study can then provide information to the metallurgical engineers seeking decarbonisation opportunities.

However, the presented study does not consider the difference in the kinetics of the process related to the difference in the reducing agents used. The kinetic aspects of the interaction of ammonia with slag should be additionally studied before any conclusions on the feasibility of the ammonia as a reductant in the described process can be made. The use of ammonia also brings other challenges such as storage and introduction into the reactor, which should be considered.

Table III provides the comparison between the model predictions and industrial data17 for Cases 1 and 2. The concentrations of Zn and Pb in slag as functions of time are shown in Fig. 4. The calculations for Case 1 with 81 or 8 equilibrium time steps are compared to industrial data17 as well as an earlier modelling study by Jak and Hayes.23 The concentrations of Zn and Pb in slag for Case 2 with 81 equilibrium time steps turned out to be practically the same as for Case 1 with 81 equilibrium time steps, so they are not shown in Fig. 4. A larger number of equilibrium time steps is demonstrated to have a small but noticeable effect on the results: a higher number of calculations resulted in higher removal of Zn and Pb at fixed total amount of air and coal. Overall, good agreement is observed for wt.% Zn in slag: industrial data17 points are in between those for Case 1 with 81 or 8 equilibrium time steps for all points, except for the last one. The difference between the model by Jak and Hayes23 and the present study can be explained by the difference in thermodynamic models. The replacement of coal with ammonia in the role of reducing agent resulted in the same change in the concentration of Zn in Pb in the slag with time (Fig. 4). For Pb, the model predicted higher removal, which is an interesting topic for investigations. This can be a kinetic factor: the reactions with Pb at very low concentrations do not reach equilibrium, even if reactions with Zn do; possible kinetic study of the evaporation of Pb from the slag can be beneficial for the future modelling studies of the process and will provide opportunities for reaching lower concentrations of Pb in the final slag. There also can be a sampling issue: concentration of elements < 0.05 wt.% are not measured with high accuracy with the x-ray fluorescence spectroscopy used by the author of the industrial data.17 At the same time these results demonstrate that discussion of thermodynamic and kinetic factors should be supported by the data for several elements, not just Zn.

The calculated furnace energy balance for Cases 1 and 2 is shown in Fig. 5. In the beginning of the process, between 0 min and 10 min, the energy balance is endothermic, which is likely caused by the lack of temperature measurements in this interval. The bath is heated over a period of time before the fuming, and some energy supplied was not reported. For the remaining duration of the process, the system was exothermic with the mean bath energy balance estimated to be − 0.6 MW. This value can be treated as a calibration parameter to investigate changes in the process for the same bath. The sensitivity of the energy balance for the described process parameters was found to be approximately 0.2 MW/K. The balance includes some of the uncertainties, such as in enthalpy values of amorphous coal, the accuracy of the temperature measurements and others.

The modelled change in the phase assemblage of the bath content with time is shown in Fig. 6. The solid phases, represented by spinel and melilite, dissolve in the oxide liquid with time because of the change of the bulk composition and increase of the system temperature. This can be correlated with the change in slag composition presented in Fig. 1 showing the decrease in the liquidus temperature with time. The use of ammonia resulted in the complete dissolution of the spinel before the end of the process and reduced stability of speiss and matte. These effects are explained by the absence of the constant introduction of Fe, Ca, Si, Al, Si oxides and S into the system (present in coal, but not ammonia). The dissolution of the spinel affected the bath heat balance closer to the end of the process making it more exothermic (Fig. 5). The mass of the oxide liquid also decreases with time, mostly because of the evaporation of the volatile elements and also the formation of matte phase stabilised by the presence of Cu in the system (added for the model demonstration purposes). The mass of matte increases closer to the end of the process, which is related to the partitioning of larger amount of Fe in this phase. The Cu-As-Sb-S speiss was also formed in the bath but existed only in the limited range of the conditions and disappeared by the end of the process.

Figure 7 presents the calculated partitioning of Pb, Sb, As, Zn, Cu and S among gas, slag (oxide liquid + oxide solids) and matte + speiss with time. Strong evaporation of Pb in the form of PbS is observed, which requires further testing of the thermodynamic model. For Sb and As, a significant amount is removed to the gas and from the remaining amount approximately half accumulates in matte and speiss phase. These phases can potentially be settled from the slag. Zn does not partition into the matte and speiss in significant quantities and is therefore concentrated in the gas. Cu, opposite to Zn, does not partition into the gas in significant quantities and is distributed between the slag and matte and speiss. S is partially removed from the system with gas but most of it is concentrated in the slag. The calculation results for minor elements were not verified by the industrial data, but they highlight the importance of including such information when studying the kinetics and thermodynamics of zinc fuming. The absence of an additional feed source of sulphur has also affected the partitioning of Sb, As and S (Fig. 8) allowing a bigger part of these elements to be removed with the gas phase while reducing the partitioning to the matte and speiss.

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