Optimisation Of Mungher Quarzite And Raigarg Quartzite Fractions In Making Of Silica Bricks For Use In Coke Oven

Optimisation Of Mungher Quarzite And Raigarg Quartzite Fractions In Making Of Silica Bricks For Use In Coke Oven

Joseph K. V 1, P. K Purushootambhai 1, P. Das 1, G. S Hebbar 1 and N. K. Ghose 2

1. Mechanical Engineering Dept, Christ University, Kumballgadu, Bangalore-560060,INDIA

2. Refractories Technology Group, Research and Development Center for Iron and Steel, SAIL, Ranchi- 834 002,INDIA

ABSTRACT

Silica Brick is generally used in Coke oven and in Blast furnace stove in an iron and steel plant.This work describes the detailed experimental work carried out in replacement of conventional Mungher Quartzite from Bihar,India by a non-conventional quartzite from Raigarh district ,MP,India, as was deemed necessary due to environment regulations .Detailed evaluation of physical and chemical properties indicated that a maxium of 25% replacement of Raigarh quartzite in a coarser fraction (25%) is possible and the refractories thus produced meet the ISO specifications

1. INTRODUCTION :-

   Silica Brick is generally used in Coke oven and in Blast furnace stove in an iron and steel plant . The main raw material is Quartzite which is found abundantly in Munger District , Bihar India. Quartzite should be of fine grain structure with a minimum of 96-97%, % silica (SiO2), Alumina (less than 1%), Iron Oxide (Fe2 O3) less than 1% and alkalies(Na2O and K2O) less than 0.5%

   Silica mineral has four different polymorphic transformations namely Quartz,Tridymite,Cristoballite and Silica Glass. Each of these forms has has its low and high temperature modifications and is stable at different temperature range. Thus -> conversion takes place at 573 0Cresulting in 0.82% volume expansion, quartz converts to Tridymite at 8670C and is stable upto 14700C.The conversion of tridymite to cistobaliteis associated with very little volume changeat higher temperature of firing.At 1723oC it melts to give silica glass. Specific gravities of Quartz,Tridymite, Cristobalite and Quartz glass are 2.65,2.27,2.33 AND 2.21 respectively.Almost irreversible phase(volume) change during tridymite to cristobalite makes application of Silica as refractories for furnace lining ideal.However this small polymorphic phase change in tridymite-> cristobalite is associated with a small volumeexpansion making masonary structure ideal for leak proof applications in coke oven refractories.

   Polymorphic Transformations in Quartz Mineral

   The physico chemical properties of indian Coke Oven Silica bricks is given below.

   India has quartzite deposits in every state but the raw material from Bihar Sarif andMungher district in Bihar Sarif District is ideally suited for silica brick production.

   After primary washing to remove the clayey impurities , the quartzite is passed through primary Jaw crusher for coarse grinding . This is further subjected to re-crushing in Gyratory Crushers, roll mill and impact mills for getting finer particles while the oversize is reground into finer fractions. The different grain sized particles are mixed on optimum proportions and mixed with an organic bond (organic Lye-1- 2%) and 2-3% milk of lime. The sequence of charging these ingredients is critically done so that the individual grains are thoroughly coated with milk of lime and the organic bond. The addition of milk of lime helps in easy polymorphic transformation of quartz at higher polymorphic transformation temperatures. A suitable blend size with different proportions of coarse (1-3mm), medium (appx 1mm)

   and fines(less than 1mm) size fractions are ideal for the blend mixture to achieve green strength and lowering of porosity in the green compacts

   In Steel Authority of India silica bricks are manufactured inBhilai Refractories Plant. Quartzite from Munger district is used is generally used for preparation of Silica Bricks.Quartzite is use in the manufacture of silica bricks as it has high refractoriness. Refractoriness is lowered by the presence of fluxes such as lime, iron oxide, magnesia and alkalies. The purity of raw material i.e., high silica content is essential with least possible Al2O3. The presence of 0.01% Al2O3 lowers the refractoriness of silica. Silica rock of metamorphic origin is better than that of igneous origin because the silica grains are cemented with cristobalite and tridymite and are stable phases of silica. Physically, quartzite should be of fine grained, compact and cryptocrystalline type.

   The raw materials sub-committee of the directorate general of technical development on refractories in its report dated September 1985 has stipulated the following specifications for quartzite.

   Physical characteristics:

   Grade-I	Grade-II
   Medium to fine grained, compact, granular texture, homogenous, free from iron bands, patches, pyrite spots, pyrophyllite, coating, devoid of mica coating.	Occasional iron patches may be allowed free from iron bands.

   The thermal characteristic of quartzite should be such that (i) there is uniformity during thermal conversion; (ii) specify gravity of fired quartzite lumps in conventional kilns at 1430 C with proper firing schedule should be less than 2.46 %, and (iii) fired quartzite lumps should be clean, white and spot free.

   Chemical Characterstics

   Chemical Composition	Grade-I	Grade-II
   SiO2	Above 98%	Above 96%
   Al2O3	Less than 0.75%	Less than 1%
   Fe2O3	Less than 0.75%	Less than 1.5% (in distributed form)

   However due to environmental problems the mining in Mungher area is restricted. Although sufficient quantity of quartzite is available from Raigarh area its use is restricted as it exhibits significant expansion while firing. The present work was therefore undertaken to explore the

   amount of quartzite from Raigarh to be mixed with Mungher Quartzite while retaining the properties suitable for application as an ideal Silica Refractory material

2. Experimental:

   Experiment 1 :Determination of optimum grain size distribution of Mungher Quartzite :

   Coarse (1.4-3.0 mm) and medium (0.3-1.4mm) sized grains were mixed with lime water , calcium salt, sodium lignosulphate, fine silica and fine (0.3-1.4mm) quartzite and the mixture was finally mixed with molasses which acts as a bond. A pressure of 15 tonnes was applied on cylindrical samples and the pressure was held for 30 seconds after which the samples were cured at room temperature. The details of the batch compositions are given in Table -1

   Table-1: Batch compositions of Mungher quartzite with different grain size additions

   Ingredients	Batch-1 Munger (gm)	Batch-A (gm)	Batch-H (gm)	Batch-S (gm)
   Munger
   Coarse	100	120	140	140
   Medium	200	200	100	172
   Fine	100	80	160	88
   Total quartzite	400	400	400	400
   Lime(dry)	10	10	10	10
   Mill scale	1.6	1.6	1.6	1.6
   Caicium salt	3.2	3.2	3.2	3.2
   Fine silica	4	4	4	4
   Lignosulphate	4	4	4	4
   Molasses	18	18	18	18
   Additives	40.8	40.8	40.8	40.8
   Total batch	440.8	440.8	440.8	440.8
   Water for making milk of lime	16.5	16.5	16.5	16.5

   The samples were fired in a rotary hearth furnace at 14000 C and were soaked for 7.15 hours. The AP (open porosity) and BD (Bulk Density) of the samples are given in Table 2 below

   Table -2 Density, porosity and Volume expansion of samples prepared from different size fractions of Mungher Quartzite

   Batch no.	Dry W.t. (g)	Sus. W.t. (g)	Sat. W.t.(g)	A.P. (%)	Fired B.D(df). (gm/cc)	Green BD(dg) (gm/cc	C.C.S. (kg/cm2)	Volume expansion (1/df – 1/dg)%
   1/1	204.34	120.20	225.63	20.19	1.94	2.21	–	6.0
   1/2	204.21	120.32	225.58	20.30	1.94	2.20	–	6.0
   A/1	206.07	121.42	226.03	19.08	1.97	2.23	–	5.8
   A/2	206.02	121.34	227.68	20.37	1.94	2.18	–	5.6
   H/1	206.34	121.06	228.75	20.81	1.92	2.14	–	5.4
   H/2	206.50	121.30	228.89	20.81	1.92	2.14	–	5.4
   S/1	205.30	121.17	225.99	19.74	1.96	2.22	–	5.9
   S/2	205.66	121.16	226.30	19.63	1.96	2.22	–	5.9

   The general observation from the above mentioned experiment is that even with change of coarse, medium and finer fractions of Mungher quartzite there is no appreciable change in the volume expansion of the samples

   Experiment 2 :Optimisation of Raigarh quartzite and Mungher Quartzite blends

   The batch mixtures of various combinations of Raigarh and Mungher Quartzite is given in the table below (Table 3)Coarse (1.4-3.0 mm) and medium (0.3-1.4mm) sized grains were mixed with lime water , calcium salt, sodium lignosulphate, fine silica and fine (0.3-1.4mm) quartzite and the mixture was finally mixed with molasses which acts as a bond. A pressure of 15 tonnes was applied on cylindrical samples and the pressure was held for 30 seconds after which the samples

   were cured at room temperature for a period of 40 hours. The details of the batch compositions are given in Table -3. The samples were fired in a rotary hearth furnace at 14000 C and the BD and Open porosity data are given in table -4.

   Table 3: Optimization of Mungher and Raigarh Quartzite blends

   Ingredients	Batch-1 Munge r (gm)	Batch-2 Raigar h (gm)	Batch -3 R25m (gm)	Batch -4 R25f (gm)	Batch -5 R25c (gm)	Batch-6 R25m25 f (gm)	Batch -7 R50m (gm)	Batch-8 R25c25 m (gm)
   Munger
   Coarse	100		100	100	0	100	100	0
   Medium	200		100	200	200	100	0	100
   Fine	100		100	0	100	0	100	100
   Raigarh
   Coarse(c)		100	0	0	100	0	0	100
   Medium (m)		200	100	0	0	100	200	100
   Fine (f)		100	0	100	0	100	0	0
   Total quartzite	400	400	400	400	400	400	400	400
   Lime(dry)	10	10	10	10	10	10	10	10
   Mill scale	1.6	1.6	1.6	1.6	1.6	1.6	1.6	1.6
   Calcium salt	3.2	3.2	3.2	3.2	3.2	3.2	3.2	3.2
   Fine silica	4	4	4	4	4	4	4	4
   Lignosulphat e	4	4	4	4	4	4	4	4
   Molasses	18	18	18	18	18	18	18	18
   Additives	40.8	40.8	40.8	40.8	40.8	40.8	40.8	40.8
   Total batch	440.8	440.8	440.8	440.8	440.8	440.8	440.8	440.8
   Water (ml)	16.5	16.5	16.5	16.5	16.5	16.5	16.5	16.5

   Table-4 Density ,Porosity and CCS (Cold Crushing Strength) ofMmunger and Raigarh Quartzite mixtures In different grain size fractions.

   Batch no.	Dry W.t. (g)	Sus. W.t. (g)	Sat. W.t.(g)	A.P. (%)	Fired B.D(df) (gm/cc)	Green B.D(dg) (gm/cc)	Volume expansion (1/df – 1/dg)%	C.C.S. (kg/cm2)
   2/1	204.04	119.15	228.37	22.28	1.87	2.15	6.8	408.30
   2/2	201.88	119.16	227.33	23.53	1.87	2.10	6.7	307.39
   3/1	204.01	120.18	225.67	20.53	1.93	2.17	5.8	302.67
   3/2	198.21	118.31	218.76	20.46	1.97	2.17	5.0	395.16
   4/1	206.14	121.56	228.62	21.00	1.93	2.17	5.8	398.80
   4/2	203.05	121.09	224.45	20.70	1.96	2.17	5.0	264.17
   5/1	205.88	121.06	227.18	20.07	1.94	2.20	6.1	424.00
   5/2	204.27	121.55	224.37	19.55	1.99	2.20	4.8	336.62
   6/1	201.58	118.36	224.46	21.56	1.90	2.13	5.7	321.59
   6/2	203.35	120.69	225.22	20.92	1.95	2.15	4.3	330.45
   7/1	204.44	119.74	227.28	21.24	1.90	2.16	5.7	384.55
   7/2	202.84	120.28	224.30	20.63	1.95	2.17	5.2	384.24
   8/1	204.74	119.66	226.25	20.18	1.92	2.18	6.1	489.96
   8/2	203.39	120.46	223.85	19.79	1.92	2.19	6.5	417.86

   It may be noted that Batch 4 which consists of 25% Raigarh fines and Batch 5 which contains 25% of Raigarh coarse fractions although did not indicate substantial change in volume expansion and open porosity values , they exhibited significant changes in their CCS values such that the inclusion of finer fractions (Batch-4/1 and 4/2) brings down the CCS values significantly lower (398.8 and 264.17) respectively as compared to the coarser fractions (Batch 5/1 and 5/2) which shows higher values of CCS ( 424.0 and 336.6 respectively ).

   Conclusion :

   It has been made possible to produce silica bricks using a replacement of Mungher Quartzite by Raigarh Quartzite fractions up to a maximum level of 25%. However the addition of finer variety brings down the ColdCrushin Strength (CCS) values . The AP values however did not show significant variation. The samples made with replacement of Mungher quartzite with Raigarh fractions followed ISI standards

   ACKNOWLEDGEMENT:

   The authors are grateful to the management of Christ University and Research and Development Center for Iron and Steel , Ranchi for their help in association with the project carried out at RDCIS Refractory Technology Group.

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