A study of synthetic forsterite refractory materials usingwaste serpentine cutting

T.W. Cheng *, Y.C. Ding, J.P. Chiu

Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, Taipei, Taiwan, ROC

Received 5 November 2001; accepted 31 January 2002

Abstract

There are more than 40 serpentine mines in Taiwan, and most of them are located in Hualien (Eastern Taiwan). Almost all the

serpentine dimension stones produced from these mines are exported. Crushed serpentine is used as a ux material for iron-making.

There are more than 0.54 million tons of serpentine waste produced per year during mining operation. This serpentine waste, at the

moment, has no commercial value. The purpose of this research is to develop a process to manufacture synthetic forsterite refractory

using serpentine waste with the addition of magnesium-based compounds by sintering technology. The test results show that,

compared with MgO, the addition of MgOH2 or MgCO3 can react with serpentine completely to produce better forsterite at lowertemperature. The process is feasible and provides a potential usage of serpentine waste in the future. 2002 Elsevier Science Ltd.All rights reserved.

Keywords: Industrial minerals; Environmental; Recycling; Wasteprocessing

1. Introduction

The serpentine in Taiwan was formed from gabbroand olivine during metamorphism process. The majormineral is serpentine, and it also contains pyrite, mag-netite, ilmenite, olivine and other traces of minerals. Ithas dark green appearance with hardness of 35. Theutilization of serpentine is divided into two categories:dimension stone and crushed rock. There were24; 266 m3 of dimension stone produced (approximatevalue US$4,050,000) in year 2000, and most of themwere exported for building decoration. For crushedserpentine rock, the production was 328,840 tons (ap-proximate value of US$4,900,000) in the same year, andthe crushed serpentine was mainly supplied to ChinaSteel as a ux material for iron-making (Chen, 2001).On top of this, there are 0.54 million tons of serpentinewaste produced each year (Ministry of Economic Af-fairs, 1996). This is due to the well-developed joints inthe serpentine orebody as well as the stringent require-ments of the particle size (680 mm) and chemicalcomposition of serpentine by iron-making company.

The waste also creates considerable environmentalproblems.

There have been several investigations on the use ofserpentine waste in various elds (Ministry of EconomicAairs, 1996; Kanari et al., 1998; Carniglia, 1992; Tsaiet al., 1988; Song, 1978; Levin et al., 1964). For example,fertilizer production (Ministry of Economic Aairs,1996), soil amelioration (Ministry of Economic Aairs,1996), extraction of amorphous silicate (Ministry ofEconomic Aairs, 1996), and extraction of pure mag-nesium compounds (Kanari et al., 1998; Tsai et al.,1988) have been investigated. Due to its theoreticalcomposition of SiO2 (34.3%), MgO (44.1%), Fe2O3(6%), Al2O3 (0.2%) and CaO (0.45%), serpentine is alsofrequently used to manufacture forsterite-based refrac-tory that contains about 85% of forsterite and 15%magnesioferrite. This type of refractory is made fromserpentine, talc or olivineserpentine using high tem-perature (Song, 1978). When serpentine is used to makeforsterite, it needs to be calcinated to release its struc-tural water that results in 12% volume reduction. Fromthe MgOSiO2 binary phase diagram and MgOAl2O3SiO2 ternary phase diagram (Levin et al., 1964;Carniglia, 1992), it was found that refractoriness forserpentine is low. In order to form forsterite with ser-pentine as raw material, the ratio of MgO, Al2O3 and

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*Corresponding author. Tel.: +886-2-2771-2171; fax: +886-2-2731-

7185.

E-mail address: twcheng@ntut.edu.tw (T.W. Cheng).

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SiO2 need to be controlled under certain optimumfractions. In particular, sucient magnesite must beadded to convert the low-melting metasilicate to thehigh-melting orthosilicate. The literature is relativelyscarce in this eld. Carniglia (1992) has noted that for-sterite basic refractories must be kept to the MgO-richside of the forsterite composition P58 wt% to ensurethe MgOMg2SiO4 eutectic at 1850 C which governsinitial melting. The purpose of this research is to seek theoptimum ratio of magnesium-based compound, such asMgO, MgCO3 or MgOH2, for making synthetic for-sterite, which involves a potential usage of the serpentinewaste in Taiwan.

2. Experimental procedures

Serpentine samples were collected from the undersize(less than 6 mm) raw materials, where the oversize oneswere supplied to China Steel. Table 1 shows the chem-ical composition of the serpentine sample. After groundto 74 lm, the samples were treated with a wet magnetseparator in order to remove the magnetite. The sampleswere ltered and dried for subsequence experimentalusage. Thermogravitational and dierential thermalanalyses (TGA/DTA) were then used to evaluate thesamples thermal behavior. Blended samples with MgO,MgCO3, or MgOH2 powder in the ratio of 10%, 15%,and 25%, respectively, were then ground for 30 min in aball mill. The ground samples were pressed into pellets(1 cm diameter) and heated by a programmable controlelectric furnace to 400 C for 1 h in order to release thestructural water. The temperature was slowly increasedat a heating rate of 10 C/min until certain temperatures,(i.e. 780, 1250 and 1500 C) were reached. The desiredtemperature was held for 30 min and then cooled toroom temperature. Following the high temperaturetreatment process, samples were ground to 74 lm forXRD (Rigaku D/MAX-VB diractometer with CuKaradiation) determination and the phase transformationcharacteristics were evaluated. The relative proportionsof the phases as a function of temperature, additionalpercentage of magnesium-based compound, or sinteringtime were estimated from the ratio In=Itotal where In is the

intensity of a chosen X-ray peak for each phase and Itotalis the sum of the intensities of all peaks. The peaks usedfor the In=Itotal calculation were 2.45 AA [forsterite (1 0 0)],14.30 AA [clinochlore (0 0 1)], 2.87 AA [enstatite (6 1 0)],8.45 AA [Cordierite (1 1 0)] and 2.10 AA [magnesia (2 0 0)].

3. Results and discussion

3.1. DTA/TGA analyses of thermal behavior

Fig. 1 shows the DTA/TGA result of the serpentinewaste. An endothermic peak was found at temperaturesbetween 400 and 430 C with a slight weight reduction.This is probably due to the heat eect caused by bruciteand other impurities (Huang, 1987). Fig. 1 also shows anobvious endothermic reaction when temperature in-creased from 740 to 780 C, due to the release ofstructural water. At temperature between 800 and 850C, a small exothermic peak was found which representsthe destruction of serpentine crystalline structure andthe formation of forsterite and enstatite (see XRD re-sults). This indicates that the structure of serpentine isnot perfect; otherwise, only forsterite is observed (Hu-ang, 1987; Deer et al., 1992). For temperatures higherthan 850 C, TGA analysis shows no weight variation.After comparing the test results with those obtained byNagamori et al. (1980), the serpentine in Taiwan has ahigher endothermic region (740780 C) than that ofserpentine found elsewhere (650750 C) (Nagamoriet al., 1980). This is probably due to the minor dierencein their mineral composition. However, the weight re-duction measured 12% was in good agreement withthat obtained by Nagamori et al. (1980).

3.2. XRD Determination of phase transfer

Fig. 2 shows the plot of In=Itotal versus sinteringtemperature. The sintering time was half an hour and10% MgO was added to the sample. It is obvious that

Table 1

Chemical composition of serpentine sample

Chemical composition Wt%

SiO2 39.23

Al2O3 2.52

MgO 37.06

CaO 0.55

Fe2O3 7.48

MnO 0.09

TiO2 Tr.

Ig. Loss 12.14Fig. 1. DTA/TGA result for serpentine in Taiwan.

272 T.W. Cheng et al. / Minerals Engineering 15 (2002) 271275

the serpentine decomposed to free magnesia, andtransformed to forsterite in the range of 7801500 C.The intensity peak of forsterite increases with raisingsintering temperature. Enstatite exhibits similar char-acteristic but shows a decrease in intensity at 1500 C.For clinochlore and magnesia, the peak intensities de-crease sharply as the sintering temperature increases.

After heating to 1500 C and with increasing MgOcontent, the peak intensities of forsterite and magnesiadecreased gradually. However, there is an increase inenstatite due to the re-crystallization of free magnesiaand silica to form forsterite and enstatite (Fig. 3). At thistemperature, strong contraction of the material wasobserved depending upon the degree of serpentinizationof the raw materials. Enstatite crystal grew and the rawmaterials were completely sintered. As seen in Fig. 4, thepeak intensity of forsterite increases inversely with thepeak intensity of enstatite. Similar results were also re-ported elsewhere (Pan, 1982).

Fig. 5 shows the XRD results of serpentine with theaddition of 25% MgCO3. Forsterite appeared when the

samples were heated to 780 C along with clinochloreand cordierite. As the temperature increased, the inten-sity of forsterite peak increased and its phase becamemore prominent while those of other mineral phasesstarted to decrease. The intensity of enstatite peak de-creased gradually with rising temperature. However,with sintering time increasing from 30 to 60 min, theintensities of forsterite peaks increased slightly (Fig. 6).With increasing amount of MgCO3 added, the intensi-ties of forsterite decreased due to formation of enstatiteat the sintering temperature of 1500 C.

Figs. 7 and 8 show the results obtained under thesame conditions as shown in Figs. 5 and 6 but with theaddition of MgOH2 instead of MgCO3. Both condi-tions produced similar results, but with lower forsteriteintensity when MgOH2 was used. As can be seen, theintensity of enstatite peak is related to the amount ofMgOH2 added. When retention time reached onehour, enstatite peak intensity decreased for bothMgOH2 and MgCO3 additions (10%).

Fig. 2. Plot of In=Itotal versus sintering temperature for 0.5 h with ad-ditional 10% MgO.

Fig. 3. Plot of In=Itotal versus MgO% additional for sintering 0.5 h at1500 C.

Fig. 4. Plot of In=Itotal for forsterite and enstatite versus sintering timewith additional MgO at 1500 C.

Fig. 5. Plot of In=Itotal versus sintering temperature for 0.5 h withadditional 25% MgCO3.

T.W. Cheng et al. / Minerals Engineering 15 (2002) 271275 273

From the results discussed above, it can be concludedthat the addition of MgOH2 and MgCO3 into ser-pentine crystal structure produces better forsterite. It isprobably due to the higher decomposition temperature

for MgO. A higher temperature is required in order todecompose MgO that can then react with serpentinecompletely to form forsterite. Because of the lower de-composition temperature of MgOH2 and MgCO3,magnesium ions from MgOH2 and MgCO3 can reactwith serpentine more easily and transform to forsterite.Therefore, the addition of MgOH2 and MgCO3 arebetter choices for making forsterite. Besides better for-sterite phase, cheaper cost of MgOH2 and MgCO3compared to that of MgO is another major consider-ation. Generally the particle sizes of forsterite for com-mercial usage range from 6 mm to 44 lm. The size of thesynthetic forsterite produced from waste serpentine canbe controlled by mineral processing operation (i.e.pelletizing, comminution, screening, and classication).A schematic owsheet for manufacturing the syntheticforsterite powder plant is illustrated in Fig. 9.

4. Conclusions

This work demonstrated that it is feasible to produceforsterite using serpentine waste with the addition ofmagnesium carbonate and hydroxide at certain ratios.Forsterite and enstatite are formed from 780 to 1500 Cdue to the decomposition of serpentine crystallinestructure. From XRD results, the peak intensity offorsterite increased with rising temperature, implyingbetter forsterite crystal structure. The completeness offorsterite crystal structure is directly related to the re-tention time. Among the three additives tested,MgOH2 and MgCO3 are considered to be betterchoices than MgO due to their lower cost and lowerdecomposition temperature, thus resulting in lesseramount of additive and lower reacting temperature be-tween magnesium oxide and serpentine.

Fig. 6. Plot of In=Itotal for forsterite, enstatite and magnesia versussintering time with additional MgCO3 at 1500 C.

Fig. 7. Plot of In=Itotal versus sintering temperature for 0.5 h with ad-ditional 25% MgOH2.

Fig. 8. Plot of In=Itotal for forsterite, enstatite and magnesia versussintering time with additional MgOH2 at 1500 C.

Fig. 9. Schematic owsheet of forsterite plant.

274 T.W. Cheng et al. / Minerals Engineering 15 (2002) 271275

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