Contribution to characterization of the diatomite for industrialapplication

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ScienceDirect

Energy Procedia 00 (2015) 000–000

www.elsevier.com/locate/procedia

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD).

International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES15

Contribution to characterization of the diatomite for industrialapplication

Hazem Meradi

^a*

, L’Hadi Atoui

^b

, Lynda Bahloul

^a

, Kotbia Labiod

^a

,Fadhel Ismail

^c

aWelding and NDT Research Center (CSC). BP 64 Cheraga - Algeria

bDepartment of Metallurgy and Materials Engineering, University of Annaba, Algeria

cDepartment of Process Engineering,Laboratory LOMOP, University BadjiMokhtar of Annaba, Algeria

Abstract

Diatomite also known Kieselguhr, is a non metallic mineral composed of the skeletal remains of microscopic single-celled aquatic algae called diatoms.The aim of this study was to test and to evaluate the diatomite of Sig region (West Algeria) tosubstitute the main mold powder used in continuous casting of steel for thermal insulation and lubrication. Generally, fluorine is added to mould fluxes to improve the viscosity. But this leads to the environmental pollution and the equipment corrosion. The laboratory and industrial investigations of diatomite have indicated the good results obtained in continuous casting of steel for thermal insulation and environmental protection against pollution (without fluorine). Also the characterization showed the hot behavior of this product with the various transformations and could give the possibility to other use.

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD).

Keywords:Diatomite, thermal insulation, lubrication, mold fluxes, fluorine

* Corresponding author. Tel.: +(213) 661697553; fax: +(213)38875982 . E-mail addresse: meradi213@yahoo.com

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1. Introduction

In the continuous casting of steel, the choice of mold slag is decisive for the lubrication and the heat transfer control in the mold. The composition, the viscosity, the solidification temperature and the crystallinity, show how the mold powder which is added to the top surface of molten steel, will melt into a liquid layer, (called mold flux), infiltrate into the gap between the shell and the mold during continuous casting and may control the behavior of lubrication and mold heat transfer [1-2].

The functions of the mould flux constitute a task of great complexity which can be summarized as follows [2,3,5,6]:

- a lubrication of the strand through the mould,

- an uniform heat transfer across the infiltrated slag layer formed between steel shell and mould, - a protection of the molten against oxidation,

- an absorption of non-metallic inclusions - and a thermal insulation of molten steel.

The mould fluxes are synthetic slags constituted by a complex mix of oxides, minerals and carbonaceous materials. The typical composition is shown in Table 1:

Table 1: Typical composition of mould fluxes [7].

Compound SiO2 Al2O3 B2O3 Fe2O3 CaO MgO BaO

Wt (%) 17 - 56 0 - 13 0 – 19 0 – 6 22 - 45 0 - 10 0 - 10

Compound SrO Na2O Li2O K2O F MnO C

Wt (%) 0 – 5 0 – 25 0 – 5 0 – 2 2 -15 0 – 5 2- 20

Fluorine is an important constituent of the mould powders that aids in reducing the melting point of slag as well as increasing its fluidity by lowering its viscosity. However, fluorine contained in mould powders, liberates gaseous fluorides to the atmosphere resulting from its evaporation and chemical reactions in the mould powders [8]. This alters the composition and subsequently the thermo-physical properties of the flux. Moreover, the volatile fluorides liberated could cause an environmental degradation,a corrosion of equipment, an acidification of the cooling water and are a potential hazard for safety and health [9-11].

Within this context, the elimination of fluorine from the mould powder composition becomes essential. For this purpose, diatomite is used in order to substitute the fluorine.

Diatomite is a non metallic mineral composed of the skeletal remains of microscopic single-celled aquatic algae called diatoms, itis a natural material formed from the remains of diatoms[11].

Diatomite products are used in many ways such asreinforcing, stiffening and hardening of organic solids, reducing adhesion between solidsurfaces, increasing adhesion, increasing viscosity. Diatomite is abundant in many areas of the world and has unique physicalcharacteristics such as high permeabilityand porosity, small particle size, low thermal conductivity and density and high specific surface [12].

The commercial diatomite contains the following chemical composition [13]: 85 - 94 % SiO2, 1 - 7% Al2O3, 0.4 - 2.5% Fe2O3, 0.1 - 0.5% TiO2, 0.03 - 0.2% P2O5, 0.3 - 3% CaO, 0.3 - 1% MgO, 0.2 - 0.5% Na2O, 0.3 - 0.9% K2O and 0.1 - 0.2% organic matter and soluble salts.

Due to its specific properties (porous structure, high silica content, low density, low conductivity coefficient, etc.

[14], the diatomite has extensively been applied in many ways, such as filter aid [15-17], insulating materials [18,19], catalyst support or carrier [20,21] and cement production [22].

The diatomite reserve in Algeria is estimated at several million tons and it is located in Sig region (from 50 km of Mascara, west of Algeria).The aim of this study was to test and to evaluate this diatomite to substitute the main mold powder used in continuous casting of steel for thermal insulation and lubrication.

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2. Experimental work 2.1 Materials

In the present work the diatomite powder from Sig Deposit (West Algeria) have been studied. The chemical composition of this powder is presented in Table2. Industrials trials were realized in steelworks with ordinary steel with chemical analysis shown in Table3.The Thermogravimetric Analysis and Differential Scanning Calorimetry (TGA-DSC) were carried out using a fully computerized Netzsch STA 409 PC simultaneousTGA-DSC instrument.

About 10 mg of diatomite powder was placed into Al2O3 crucible for simultaneous TGA-DSC analysis and was heated at a rate of 10 ^°C/min, from room temperature to 1100 ^°C in a static air environment.

The structure of diatomite sample without any treatment has been viewed microscopically using scanning electron microscope (SEM) type Philips XL30. This system provides both secondary electron and backscattered electron imaging along an integrated EDAX system, the resolution was 20.0 kV.

Table 2.Chemical composition of the diatomite powder used during the industrial test.

Component MgO Fe2O3 SiO2 TiO2 CaO K2O Al2O3 Na2O Cr Fire losses

Wt (%) 2.15 1.19 73.4 0.027 13.58 0.78 3.15 0.002 0.51 5

Table 3.Chemical composition of the steel used during the industrial test.

.Component C Mn Si P S Mo Ni W Cr

Wt (%) 0.07 0.35 0.15 0.005 0.005 0.002 0.003 0.002 0.001

2.2 Plant trials

Industrial tests with the diatomite were performed in two heats using the same steel grade. Each heat was about one hour duration and was applied in one strand.These two heats constitute one sequence of casting. Technological parameters were monitored during continuous casting with the objective to evaluate the performance of diatomite for insulation thermal and lubrication of mold.

The powder of diatomite is constantly added to the surface of the bath (Fig.1)and temperature was measured by pyrometric cane every 10 minutes.

Fig. 1.Schematic representation of the mould flux in the continuous casting mould (Adapted from Reference [24]).

Taking Temperature

Mould

Crystalline Layer Amorphous Layer

Solidifying Shell

Mould Flux: Diatomite Sintered Layer C-enriched zone Liquid Layer

Molten Steel

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3. Results and discussion

The trials realized with the diatomite powder in continuous casting of steel showed a good behavior of this powder in the thermal insulation.It is visible clearly by a standard deviation which is 06 °C for the first ladle and 04 °C for the second, this proves that the powder has well played its role of thermal insulation. These results are acceptable and very encouraging by steelmakers.Scanning Electron Microscope (SEM) for sample diatomite showed porous structures with several diameters (Fig. 2). This SEM picture showed that the pores openings of the diatomite porous structure. We can note that the pores are predominantly in circular form and we can also see the presence of impurities.The SEM micrography of the natural diatomite was typical. It was found to be essentially amorphous but also it contained ankerit, calcite and quartz [25].The results of simultaneous analyses TGA-DSC for diatomite sample without any treatment are shown in Fig.3. The results indicate the loss of mass when the temperature is increased and revealed that the diatomite has four mass losses. The first loss (about 5%) between room temperature and 200 ^°C, the second mass loss (about 4.10 %) in the temperature range from 200^°C to about 600 ^°C, the third loss (about 2.76 %) in the range from 600 ^°C to 800 ^°C and the last loss (about 0.63%) in the range from 800 ^°Cto1100

°C.The results from DSC measurements on the diatomite indicate that when the temperature is increased, several reactions take place. These are clearly detected in the graph in Fig.3.

We can see from DSC spectrum three endothermic pics at 84.7, 576.1 and 783.5 ^°C, and one exothermic pic at 894.9

°C. The endothermic peak centered at 84.7^°C and a shoulder around 165 ^°C was assigned to the loss of water absorbed on the diatomite. The small peak at 576.1 ^°C might be due to the quartz transformation. The large endothermic peak at 783.5 ^°C has been assigned to a formation of siloxane bridges resulting from dehydroxylation of isolated silanol groups on the internal surface of the diatomite that corresponds to the maximum loss of mass (20,96 %). At 894.9 ^°C, we can see an exothermic pic caused by the cristalization. The exothermic direction is shown along the vertical axis.Generally, quartz is known to give an endothermic reaction between 565 and 575 ^°C, in figure 3, the endothermic pic at 576.1 °C is very low because the amorphuous structure. The curve of thermal gravimetric analysis of diatomite sample is also given in Fig. 3. This curve can be also utilized in the detremination of the optimum temperature in flux calcination process for eliminate impurities.

Table4: Evolution of steel temperature every 10 minutes.

Temperaturesteel

(^°C) First ladle

Time (h : mn)

1520 “ 00:06

1520 “ 00:09

1518 “ 00:19

1520 “ 00:30

1510 “ 00:48

1500 “ 01:07

Second ladle

1530 “ 01:22

1535 “ 01:31

1525 “ 01:47

1528 “ 01:51

1528 “ 02:00

1525 “ 02:10

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Table 5.Results of losses temperature of steel in mold.

FIRST LADLE SECOND LADLE

Minimum (^°C) 1515 1525 Maximum (^°C) 1520 1535 Average(^°C) 1518

Standard deviation (^°C)06

1528 04

Fig.2. SEM micrographyof diatomite powder with 3000X magnification

Fig.3. Simultaneousthermal analysis TGA-DSC for diatomite sample from Sig deposit.

894.9 °C

576.1 °C 84.7 °C

783.5 °C

-11.86 % -9.10 %

-0.63 % -21.60 %

TGA (%)

80 90

85 95 80

-0.6 -0.4 -0.2 0.0 0.2 0.4

DSC/(mW/mg)

200 400 600 800 1000

TGA DSC

Temperature /°C

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4. Conclusion

Because fluoride in mold caused hazards during application of gaseous emission and leaching from mold flux in cooling water corroding thus the plant equipment,the aim of this study was the substitution of mold fluxes containing fluorine by diatomite.The results obtained showed that diatomite have a porous structure with ordered size distribution of the pores, this give low density and the chemical composition is predominantly consists of SiO2

with additions of Fe, Al, Ca, Mg, Na, K.SEM figure of the sample diatomite showed the presence of intact diatomite skeletons but with impurities. Thermal analysis of diatomite sample was depicted and the loss mass was maximal at 800 ^°C.The peak at 576^°C analyzed by DSCshowed the transformation of quartz which is more important for sand.

Diatomite from Sig deposit (West Algeria) can be successfully beneficiated with very good efficiency for thermal insulation in continuous casting of steel without treatment. The obtained results showed a low temperature loss of steel during continuous castingand which allowsto a good thermal insulation. Further trials are necessary to confirm the behavior of diatomite with high carbon in steel grade. In a first stage, we can confirm the good behavior diatomite for heat insulation during continuous casting of steels with low carbon.

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