Journal ofMining and Metallurgy 36 (3-4)B (2000) 195 - 208

INVESTIGATION OF DUSTS FROM WASTE-HEAT

BOILER AND ELECTROSTATIC PRECIPITATORS

AFTER FLASH SMELTING FURNACE FOR COPPER

CONCENTRATES

Ts.Markova', BiBoyanov'", S.Pironkovl and N.Shopovl

I Non-Ferrous Metals Yield Ltd., 4009 Plovdiv, Bulgaria2 University of Plovdiv, 4000 Plovdiv, Bulgaria

(Recived 25 October 1999; accepted 10 May 2000)

Abstract

Dusts from different points of the waste-heat boiler (WHB) and the ElectrostaticPrecipitators (ESP) after the Frush Smelting Furnace (FSF) for copper concentrate havebeen thoroughly investigated. For this purpose, chemical, chemical phase, X-ray­structure and differential-thermal and thermogravimetric analyses have been used. Thephases of the copper and the iron have been determined quantitatively. The results fromthe analyses OJ the dusts from WHB and ESP show a high level of uncontroledsulphatization of the dusts, which runs mainly in the radiation section of WHB. This is amajor problem for obtaining deposits and residues leading to frequent stoppages of theFSF. The presence of Fe20.~ in the solid phase facilitates the process of sulphatization,catalyzing the oxidation of S02 to SOJ. The prevailing phases in all samples are FeJ04and CUS04. Of the sulphides in the samples CU2S is mainly present. The distribution of thetemperatures in the WHB has been determined and the possible influence of theconditions of work of the furnace on the processes of sulphate-formation In the dusts hasbeen found. Specific recommendations jor implementation in practice have been madewith the aim ofdecreasing the sulphate-formation and increase of the .duration of work ofFSF.

Keywords: blast furnace, boiler, electrical precipitater, pouders, phase analysis,sulphatization

*Corresponding author

J.Min.Met. 36 (3-4)B 2000 195

Ts. Markova et al.

1. Introduction

Increase of the productivity and competitive power, the processing oflarge quantities of imported copper concentrates, modification of the FSF,burners, furnace body, the introduction into exploitation of new furnaces,the replacement of four with one central burner, the preservation of theenvironment, etc. have been into notice of the major world producers ofcopper who use flash smelting process (FSP) [1,2]. Attention has also beenpaid to the influence of the dust in the furnace gases on the normal work ofthe WHB and the ESP of the FSF [3,4].

The flash smelting process by the method of the Finish firmOutokumpu OY has been put into exploitation in MDK-AD-Pirdop(Bulgaria) in 1987. In the past years a number of investigations have beencarried out, dedicated to the possibility for use of additional solid fuel [5],creation of a thermotechnical model for calculation of the flash smelting ofsulphide copper raw materials [6], determination of the forms of the lossesof copper with the slags [7], the possibilities for change of the phasecomposition of slags from the flush smelting furnace [8], the initial slagmelts in the flame of a copper flash smelting furnace [9], mathematicalmodel of cumulative energy consumption [10], etc.

One of the most important conditions for flawless exploitation of thecomplex for flush smelting, consisting of: flush smelting furnace (FSF) ­waste-heat boiler (WHB) - electrostatic precipitators (ESP), is themaintaining of the heat exchange and electrode surfaces clean, withoutdeposits and accumulations. This on its part depends mainly on thequantity and quality (the chemical and phase composition) of the dustsbrought out from FSF, as well as on the processes running in them atpassing of the gas non-homogenous system (gas + solid) through WHBand ESP [11-14]. These matters, however, have not been treated with thenecessary attention.

In recent years it often becomes necessary to stop the FSF because offailures in the waste-heat boiler and the electrostatic precipitators, whichwere caused by the obtaining of large quantities of deposits andaccumulations. Because of that, it is of special importance to define thecauses for that, and to apply measures which to lead to a decrease or totalliquidation of these negative processes. The knowledge about the

196 J.Min. Met. 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic..,

behaviour and the phase changes occurring in the dusts on the way afterthe smelting furnace, will allow suggesting appropriate decisions andmeasures, which would decrease the stoppages because of failuresituations. They are caused by frequent breaks on the heat exchangesurfaces of WHB, a low efficiency coefficient of ESP due to deteriorationof their electrical parameters as a result of sticking of the coroningelectrodes, etc.

In this connection, the purpose of the present investigation is, bymeans of a detailed chemical, chemical phase, X-ray-phase anddifferential-thermal and thermogravimetric analysis of dusts selected fromdifferent points of WHB and ESP after the furnace, to clarify the changesoccurring with them, and the causes which originate them; on the basis ofthe obtained results to suggest specific recommendations for introductioninto practice with the aim of decreasing the failures and increasing theduration of work of FSF.

2. Experimental

In the course of investigation of the dusts from WHB and ESP about130 samples have been taken from different poiints. They have beensubmitted to a detailed investigation by means of different methods.

The chemical phase analysis has been carried out on a certain numberof samples, which more specifically represent the respective section of theway of the dust-gas system after FSF [15].

DTA and TG studies (Q Derivatograph, MOM, Hungary) were carriedout under the folIoing conditions: sensitivity of DTA - 1 mV; DTG - 1mV; TG - 100 mg; heating rate, 100 min-I; sample mass 100 mg. Aplatinum crucible with a diameter 10 mm and a hight 12 mm was used. Allthe studies were performed in air medium [16].

X-ray diffraction analysis of the solid phases has been carried out witha TUR-M62 apparatus (Dresden, Germany) using Colv, radiation and aniron target [17].

The temperature measurements have been carried out using portableapparatus and sensors with thermocouples NiCr-Ni with a working lengthof 1000 mm. It has been worked simultaneously at 4 points, at 3 different

197

Ts. Markova et al.

distances on the inner side of the screen panels - 250, 500 and 800 mm.Measurements have been carried out at 43 points, located along the heightand width ofWHB.

In order to obtain a more complete information about the kind andlevel of running of the changes, occurred in the dusts from the selectedpoints after FSF, the following samples have been submitted to a detailedphase analysis: No.1 and No.2 - samples from the transition, i.e. from inletof WHB; No.3 and No.4 - from the radiation section (RS) of WHB; No.5and No.6 - from the convection section (CS) of WHB. Their chemicalcomposition is presented in Table 1.

Table 1. Chemical composition of the tested samples

No. Kind of sample Contents %

Cu Fe Sob Ss04 Sulphateratio

I From the transition 34.64 20.87 6.72 4.36 65

2 From the transition 33.79 21.70 5.51 4.02 73

3 From RS of WHB 32.85 22.32 7.45 6.22 83

4 From RS of WHB 29.40 23.77 8.06 5.98 74

5 From CS of WHB 31.90 20.38 7.85 7.64 97

6 From CS of WHB 28.72 21.13 8.44 8.08 96

A main conclusion, which can be made about the chemicalcomposition, having relation to the kind of the phases present in thesamples, is the high contents of sulphate phase. The preliminaryinvestigations showed that this is due to the presence of water-solublesulphates - mainly of the copper and to a smaller extent of the iron. Theexperiments carried out by us for many years, show that the sulphates inthem are with a non-stoichiometric quantity of integrated vapour from theair. This leads to a shifting of the lines of the X-ray patterns, which makesdifficult their correct identification through an X-ray-phase analysis. Thatis why, in advance, by a weight method, the total contents of the water­soluble sulphates have been determined. The obtained results are asfollows: sample No.3 - 30% water-soluble sulphates; sample No.4 - 31%;sample No.6 - 42%.

198 i.Min.Met. 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic...

An increase of the contents of the water-soluble sulphates on the wayof movement of the gases could be due to [18,19]:

a) an additional oxidation mainly of sulphides of the copper and the iron;b) a sulphatization of oxides (to a smaller extent of ferrites) from S02 and

O2 (S03), found in the furnace gases. The presence of Fe203 in thesolid phase could catalyze the oxidation of S02 to S03 and thus tofacilitate the process of sulphatization.The data from the test of the sulphates show that their stability

decreases in the following order:

The oxides of Ca and Mg can also sulphatize, but they remaincomparatively stable up to 1000°C.

The carried out X-ray-phase analysis (XRPhA) of the samples fromRS and CS of the waste-heat boiler (Fig.l) give grounds to make thefollowing main conclusions:

The tested samples do not differ considerably by their phasecomposition. Differences can be observed in the quantitative relationbetween the different phases.

A prevailing phase in all samples is Fe304. The presence of Fe203 inthe samples is much smaller in comparison to Fe304. The observed shiftingof the lines of Fe304 on the X-ray patterns shows that possibly CuFe204 orCu is present, dissolved in Fe304, and a formation of CUxFe3-x04.

A second main phase in the samples is CUS04. This conclusion cannotbe made simply on the base of the X-ray patterns because of the abovepointed out reasons. The help of other methods is also needed, whichconfirmed the presence of CUS04. To a considerably smaller extent FeS04is also present in the samples.

Another phase containing Cu and Fe appears the ferrite of theunivalent copper CuFe02 (CUZO.Fe203).

By the data from the X-ray patterns, it could be claimed that possibleSi02 is connected mainly as fayalite - 2FeO.Si02.

Because of the low contents of Zn, Pb, Ca, Mg and others, theirpresence is not expressed clearly enough on the obtained X-ray patterns.

199

Ts. Markova et al.

Fig.l . X-ray patterns ofsamples from RS (L] and es (2) ofWHB

In order to obtain a more detailed picture regarding the quality phasecomposition, a water extraction of the samples was made, and an XRPhAwas made on the residuum (Fig.2). This gave the opportunity to remove thelines of the water-soluble sulphates and to reinforce the lines of the otherphases. In this case, the diffraction maximums of CU20, Fe203 and ZnOare clearly seen.

Fig. 2. X-ray patterns of samples from RS (I ) of WHB, extracted with water (2), extractedwith water and heated at ] ooooe for] hour (3)

200 i. Min.Met. 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic ... ~ _

From the derivatogram of sample No.3, after extraction of the water­soluble sulphates and heating at 1aaaoc during 1 hour, it is possible tomake additional conclusions about the present phases.

The lines of CU20 are not observed, on their place those of CuOappear. The intensity of the lines of CuFe02 relatively decreases, while theintensity of the lines of CuFe204 (Fe304) increases.

The implementation of heating in an oxidizing atmosphere and thereaching of high temperature (1aaaOC) give an explanation for the obtainedresults. As a result of the thermal treatment CU20 oxidizes to CuO, whilethe ferrite of the univalent copper oxidizes to ferrite of the bivalent copper.

DTA and TGA have been carried out in an air atmosphere, at heatingup to 1aaaoc, at a heating speed - 1aD/min. The obtained derivatograms ofsamples No.2, 4 and 5 are presented respectively in Fig. 3,4 and 5.

From the obtained results the following more importantgeneralizations can be made:

OTA

~~ V-------

200

10

400 600

V760

,800

DTG

I

1000 t.·c

270~. 01------­E..., , 780------'

TG

Fig.3. Derivatogram ofsample from transition

201

Ts. Markova et al.

OTA

730

DTG

790

200 400 600 800 1000 I,·C

or 10340

E

~ 20TG

Fig.4. Derivatogram ofsample from RS of WHB

CTA

480

.1!!!l. --~ ,--- CTG

dl 'v'--.780195

200 400 600 800 1000 T,·C

20

100 160

TG

202

Fig.5. Derivatogram ofsample from CSofWHB

J.Min.Met, 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic...

The tested samples have a similar character of behaviour at thermaltreatment in air atmosphere. At a temperature above 350-400°C a gradualincrease of the mass of the samples begins, which is connected to theintegration of oxygen from the atmosphere air. This could be due to anoxidation of sulphides to sulphates and of lower oxides to higher ones:

4CU2S + 902~ 4CUS04 + 4CuO2CU20 + O2~ 4CuOFeS + 202 ~ FeS04

This process is more clearly expresse at samples No.3 and No.4. Withthem, the increase of the mass is of the order of 5-6%, while with the othersamples this value is 3-4%.

At a temperature over 650°C a dissociation process begins, which runsintensively up to about 820°C, and after that slowly continues up to1000°C. The intensive process is due to the dissociation of CUS04 with asmall additional contribution of FeS04.

The total loss of mass for all four samples from RS and CS of WHB isof the order of 20-21% and runs in one and the same way. This givesgrounds to make the conclusion that the tested samples slightly differ in thequantitative relation between the separate phases.

DTA and TOA of the cake from sample No.3 were carried out afterextraction with water, and after that heated up to 1000°C. The obtained TOcurves showed that about 6% of the loss of mass at 1000°C is due to thedissociation of the products of oxidation, i.e. mainly to the formedsulphates. The heated sample has a TO curve, which categorically showsthat the oxidation and dissociation processes in this case have finishedcompletely. The fact itself that the phase CuO is observed on the X-raypattern of this sample shows that the thermal treatment to lOOO°C does notlead to a full dissociation of the high copper oxide. This is «n importantresult because it gives grounds to consider that the decrease of the mass ofthe samples in the temperature interval 650-820°C is due mainly to thedissociation of metal sulphates.

The obtained results regarding the distribution of the temperaturealong the length of WHB (Fig.6) show an abrupt decrease of thetemperature of the gases after the inlet in WHB, and a great temperaturefall along its height. This testifies that there exist appropriate temperatureconditions for running of a secondary sulphatization of the metal oxides.

203

Ts. Markova ct al.--- _.._._.-..._....._._--------------------------

410'

VI OUTPUT

VI OUTPUTI c>

370'

\JEST

EAST

459'265'

258'

I I Do

645'

325'

570'

710' l50' 778'

380'

::::::=======::::~=60'~____

I I I I11 III IV V

1200'

1000'

800'

600'

400'

200'

I IINPUT I 11

1200' ~

Fi[;.6. Distribution of the temperature along the length of WHB

The taken derivatograms of the samples from the transition, i.e. frominlet in \VBB (No.1 and No.2) resemble the derivatograms of the otherfour samples. but there are differences observed:

Because of the higher contents of S, in sample No.1 in comparison tosample No.2. with it a gradual oxidation process begins at temperatureabout 320°C This process is connected with a release of heat (maximum ofthe exothermal effect at 430°C) and an increase of the mass of the sample,which is clearly expressed. A similar phenomenon, but to a lower degree,is observed also with sample No.2 (Fig.3).

The dissociation of the sulphates (mainly CUS04) runs by analogy tothe rest of the samples, but their quantity is less. In sample No.1 thecontent of t:opper, as a sulphate is about 8%, while in sample No.2 - about6%.

The total loss of mass at 1000°C for sample No.1 is 16%, while forsample No.2 .- 14%. These values are lower in comparison to the samplesfrom RS and CS of \VHB. This fact proves that with the movement of thedusts from the transition towards the outlet of the waste-heat boiler, aprocess of obt.aining additional quantity of sulphates is running, due tooxidation of sulphides and sulphatization of oxides as a result of aninteraction with the furnace gases.

204 J.Min.Met. 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic...

The carrying out of a chemical phase analysis of samples with a multi­component composition, as the tested samples are, is an unusually difficulttask. In the literature, the behaviour of all cooper and iron containingphases is not clearly enough pointed out. This necessitated the taking of anumber of additional tests, giving the opportunity to clarify morecompletely the behaviour of the separate phases. A summary of theobtained results is presented in Table 2 and Table 3. The data in the tablescould serve as a basis to make some conclusions:

The copper connected as a sulphate is of the order of 38-45% of thetotal copper. In the samples from CS it is slightly more, which points out toan additional sulphatization of the dust-form material.

The content of copper as a phase CU20 (free CU20, as well asconnected as CU20.Fe203) is about 30% of the total copper. By means ofthe chemical phase analysis it is difficult to determine the contents ofcopper as CU20.Fe203, but for this reason the help of the Mossbauerspectroscopy and the XRPhA was used.

Table 2. Results from the chemical phase analysis by copper

No. Sample Component Contents, %1 3 Cu sulphate 12,12 Cu oxide (CU20, CU20.Fe203) 9,23 Cu sulphide (CU2S, CuS, CuFeS2) 5,74 Cu ferrite (CuFe204,) 4,35 4 Cu sulphate 11,46 Cu oxide (CU20, CU20.Fe203) 8,37 Cu sulphide (CU2S, CuS, CuFeS2) 6,88 Cu ferrite (CuFe204,) 1,6..-9 5 Cu sulphate 12,810 Cu oxide (CU20, CU20.Fe203) 9,511 Cu sulphide (CU2S, CuS, CuFeS2) 2,312 Cu ferrite (CuFe204,) 5,313 6 Cu sulphate 12,914 Cu oxide (CU20, CU20.Fe203) 8,615 Cu sulphide (CU2S, ces, CuFeS2) 2,416 Cu ferrite (CuFe204,) 4,1

205

Ts. Markova et al.

The sulphide copper decreases from RS towards the CS of the waste­heat boiler. Its part of the total copper is about 20% for the samples fromRS and about 8-10% for the samples from CS.

The ferrite copper (CuFe204) varies within wider limits in the testedsamples.

The main phase, containing iron, is Fe304. The Mossbauer spectre,made to sample No.3, showed that the magnetite is not stoichiometric andthere is copper dissolved in it, for which the widths of the obtained linestell. It is more correct to write down that the main iron-containing phase isMexFe3-x04. where the Me is mainly Cu, but it could also be Zn, Ni, Co,Ca, Mg, Pb, etc. Because of their low contents it is very difficult to identifythem.

A problem represent the ferrites, which are in a ferromagnetic form atroom temperature, because they create difficulties for defining andidentifying them because of the close X-ray patterns with the magnetiteand an overlay of the lines at the Mossbauer spectres.

Table 3. Results from the chemical phase analysis by iron

No. Sample Component Contents, %

I 3 Fe sulphate 1.262 Fe sulphide (FeS, FeS2, CuFeS2) 0,433 Fe silicate, ferrite (Fe2Si04, CaFeSi04, 2PbO.Fe203) 1,874 Fe oxide 2,265 Fe oxide, ferrite (Fel04, CuFe,OJ., CuFeO,) 15,56 4 Fe sulphate 0,987 Fe sulphide (FeS, FeS2, CuFeS2) 0,298 Fe silicate, ferrite (Fe2Si04, CaFeSi04, 2PbO.Fe203) 2,139 Fe oxide 1,1910 Fe oxide, ferrite (Fe10J., CuFe,OJ., CuFe02) 20,2II 5 Fe sulphate 1.5112 Fe sulphide (FeS, FeS2, CuFeS2) 0,3413 Fe silicate, ferrite (Fe2Si04, CaFeSi04, 2PbO.Fe203) 2,4114 Fe oxide 1,3715 Fe oxide, ferrite (Fe10J., CuFe,OJ., CuFeO,) 15,7516 6 Fe sulphate 1,5517 Fe sulphide (FeS, FeS2' CuFeS2) 0,3618 Fe silicate, ferrite (Fe2Si04, CaFeSi04, 2PbO.Fe203) 2,9119 Fe oxide 1,1620 Fe oxide, ferrite (Fe10J., CuFe,OJ., CuFe02) 16,75

206 r.Min.Met. 36 (3-4)B 2000

Investigation of dusts from waste-heat boiler and electrostatic...

In the magnetite and the ferrite phase about 72-80% of the total iron ofthe samples are included.

The content of Fe203 has been evaluated with the help of additionaltests and is about 6-10% of the total contents of iron. The presence of theother iron-containing phases is with low-percentage contents.

3. Conclusions

1. The results from the complex investigations with the help of XRPhA,DTA, TGA, Mossbauer spectroscopy and chemical analyses of thedusts from WHB and ESP show the running, to a large extent, ofuncontroled sulphatization of the dusts, which occurs mostly in theradiation section of WHB. The presence of Fe203 in the solid phaseprobably facilitates the process of sulphatization, catalyzing theoxidation of S02 to S03.

2. The prevailing phases in all samples are Fe304 and CUS04. Of thesulphides in the samples CU2S is present.

3. With the aim of decreasing the sulphate-formation after RS it isrecommended to organize additional feeding of heated to 250°C air inthe upper part of the RS of WHB. This will reduce to minimum thedanger of formation of deposits and covers in the convection section ofWHB and the precipitation fields of ESP and will facilitate theexploitation of this equipment.

4. The results obtained and the conclusions made have been taken intoconsideration at the engineering and reconstruction of the waste-heatboiler, carried out in 1999.

References

1. A.K.Biswas and W.G.Davenport, Extractive Metallurgy of Copper, Pergamon,Oxford, New York, Tokyo, 1994, p.489.

2. Proceedings of the 8th International Flash Smelting Congress, 14-18 October, Tusconand Salt Lake City, 1996.

3. Y.C.Kang and S.S.Park, JOM, 10 (1997) 44-48.4. AP.Skuratov, AV.Finkelshtein, AM.Ptytsin and O.M.Grigoreva, Theory of

Processes of Non-Ferrous Metals Production, GINTSVETMET, Moscow (1996) 96­100.

5. V.Stefanova, T.Genevska, B.Stefanov and P.Bakarjiev, Erzmetal, 44 (1991) 538-541.

207

Ts. Markova et at.

6. B.Stefanov and E.Marinov. Metallurgy, 5 (1995) 16-20 (in Bulgarian)7. K.Genevski, Metallurgy, 5 (1995) 11-15 (in Bulgarian)8. K.Genevski, I.Gruev and V.Stefanova, Proceedings of the Balkan Conference on

Metallurgy, Varna, Bulgaria, May 28-30, 1996,29-34,35-40.9. K.Genevski, V.Stefanova and B.Stefanov, Proceedings of the 3th International

Conference "Non-Ferrous Metals '97", Krakow, Poland, September 11-12, 1997,88­95,96-104.

10. Sn.Magaeva and G.Patronov, Proceedings of the International Seminar"Contemporary Problems of Thermal Engineering", Glivice-Ustron, Poland,September 2-4, 1998, 191-198.

11. V.A.Kukoev, Z.N.Voronina and L.M.Shternberg, Theory of Processes of Non­Ferrous Metals Production, GINTSVETMET, Moscow (1996) 52-54.

12. Y.B.Hahn and H.Y.Sohn, Metallurgical Transactions B, 21 B (1990) 945-966.13. N.Kemori, W.T.Denholm and H.Kurokava, Metallurgical Transactions B, 20 B

(1989) 327-336.14. R.D.Hagni, Journal of Metals, 39 (1987) 8-10.15. N.A.Filippova, Phase analisys of ores and products of their processing, Chemistry,

Moscow, 1975, p.280 (in Russian)16. B.S.Boyanov, Thermochimica Acta, 302 (1997) 109-115.17. B.S.Boyanov, Thermochimica Acta, 322 (1998) 69-75.180 Zivkovic and V.Savovic, Principi Metalurske Termodinamike, Bakar, Bor, 1997,

p.890 (in Serbian).19. i.Zivkovic, N.Mitevska and V.Savovic, Thermochimica Acta, 282/283 (1996)

121-130.

208 i.Min.Met. 36 (3-4)8 2000