Regular Article
Pre-modern Refining Process of “Okaji” without Deoxidation
2014 Volume 54 Issue 5 Pages 1059-1066
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2014 Volume 54 Issue 5 Pages 1059-1066
In Tatara process, steel “kera” and pig iron “zuku” were produced. Pig iron and low quality of steel “Bugera” in these products were decarburized with air by hand blowing to produce steel plates with low carbon content, called “Hochotetu” or “Waritetsu”. The decarburization in pre-modern refining process was called “Okaji” and composed of two processes of “Sageba” and “Honba”. In the Sageba process, pig iron was decarburized to steel with about 0.7 mass%C, called “Sagegane” and the yield was almost 100%. In the Honba process, the Sagegane was decarburized to steel with about 0.1 mass%C, called “Oroshigane”, without deoxidation and the yield was 60 to 70%. The Orosigane was promptly forged to make plates. Thus, the contents of oxygen in Hochotetsu was about 0.2 mass%. The Okaji process has been examined in the present work. In the Sageba process, pig iron was decarburized by FeO slag, called “Noro”, with CO gas bubbling at about 1400°C. In the Honba process, the temperature in furnace was kept about 1450°C. Sagegane was oxidized by oxygen gas in air to generate heat and the temperature of Oroshigane increased to near 1528°C. The temperature of furnace during Okaji process was carefully controlled by blowing rate and keeping moisture in hearth of wet charcoal bed. Water was sometimes poured during operation.
Tatara is the Japanese traditional ironmaking process to produce pig iron “zuku” and steel “kera” in a furnace from iron sand and charcoal during about 70 hrs. Tatara works were closed after world war II but reconstructed in Yokota-cho of Shimane prefecture in 1977. Pig iron and low quality of steel “Bugera” in these products were decarburized to produce low carbon steel in “Okaji” works. The Okaji process was composed of two processes of “Sageba” and “Honba”. In Sageba, pig iron was decarburized to steel with about 0.7 mass%C, called “Sagegane”. In Honba, the steel was decarburized to low carbon steel with about 0.1 mass%C, called “Oroshigane”. A block of Oroshigane was instantaneously forged to make plates, called “Hotyotetu” or “Waritetu”. The yields of Sageba and Honba processes were almost 100 mass% and 60–70 mass%, respectively. Old nails used in Japanese shrines and temples had been manufactured from Hochotetsu. The steel of old nails include carbon with about 0.1 mass% and over-saturated oxygen with about 0.2 mass%.1)
Unfortunately, The technique of Okaji process was ceased in the early 20 Century. Kuni-ichi Tawara2) reported on the Tatara and Okaji processes worked in the area of Izumo of Shimane prefecture and the west area of Hoki of Tottori prefecture in the end of the 19 Century. Fujio Sakaki3) represented the technique of “Okaji” and Seizo Tanabe4) introduced the skill of the master Heisuke Watanabe. There is the movie of Okaji process in the early 20 Century.5)
In the present paper, the mechanisms of decarburizing reaction of pig iron and steel by air in “Okaji” process is realized.
The resources of Okaji process were pig iron of “Zuku” and low quality steel of “Bugera” produced in Tarara process. Pig iron was classified in three types. The first was porous pig iron of “Nagare-zuku” that run out from Tatara furnace during operation, the second was dense pig iron of “Ura-zuku” that remained in the furnace and the third was “Kera-zuku” that adhered under “Kera” of steel bloom. Nagare-zuku and Ura-zuku were called as “Hachime-zuku” and “Korime-zuku”, respectively. The composition of pig iron was listed in Table 1. Silicon and phosphorus contents in pig iron were lower than modern blast furnace and pig iron was white cast iron. Dephosphorized pig iron was produced by the Kondo works in the west area of Hoki. The process was called “Tamebukiho” that molten pig iron was dephosphorized during dropping through a deep pool of slag “Noro” in the bottom of Tatara furnace. Because “Noro” is a slag with iron oxide rich near Fayarite composition, as shown in Table 2, and has high oxygen potential in equilibrium with iron. Thus, Dephosphorization of molten pig iron effectively proceeded at 1350 to 1400°C.
| Area | Works | C | Si | Mn | P | S | Ti |
|---|---|---|---|---|---|---|---|
| Izumo | Tanabe | 4.46 | 0.15 | 0.19 | 0.043 | 0.003 | trace |
| Iwami | Yonehara | 3.63 | trace | trace | 0.1 | 0.003 | trace |
| Hoki | Tonami | 3.61 | 0.03 | 0.01 | 0.033 | 0.010 | trace |
| Aki | HiroshimaTesuzan | 3.80 | trace | trace | 0.15 | 0.020 | 0.12 |
| Hoki* | Kondo | 3.22 | 0.18 | 0.049 | 0.009 | 0.018 | trace |
| Works | T.Fe | FeO | Fe2O3 | SiO2 | MnO | Al2O3 | CaO | MgO | P2O5 | TiO2 | V2O5 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Tsugoyama | 53.42 | 63.17 | 6.18 | 17.82 | 0.63 | 6.12 | 1.78 | 0.88 | 0.19 | 0.64 | 0.09 |
| Sakanokaji | 48.02 | 59.22 | 2.87 | 21.16 | 0.33 | 7.02 | 2.56 | 1.04 | 0.20 | 2.56 | 0.53 |
In Tatara process, Zuku and Kera were produced each about 1.5 tons from 12 tons of iron sand and 12 tons of charcoal during about 70 hrs. The size of Kera bloom was approximately 2 m of length, 1 m of width and 0.3 m of thickness. The big steel bloom was broken in fist size by dropping down the big chisels with 1 and 0.5 ton from tower. The broken steel lumps were classified into several grades. Zuku and the low grade steel with low carbon content including Noro, called “Bugera”, were used as the resources of Okaji process.
2.2. FuelThe charcoal for Okaji, called “Kozumi”, was produced by a expert, “Yamako”. A large quantity of branches of big trees that made charcoal for Tatara were employed. Old Mr. Mansuke Takada who was “Yamako” of “Ashiya” Okaji works told that in order to make “Kozumi” charcoal, the best place was slope. The ground was slightly dug in round with about 8 m diameter and surrounded by bank. Branches were burned and extinguished with soil. Farmers produced “Kozumi” in a square hole with 2 m side. Branches were cut in about 1 m length and burned. Wood was piled up on branches covering with green grass to smoke during firing. Finally charcoal was extinguished with soil. Because “Kozumi” charcoal was absorbent, it had to carry to Okaji works in the daytime to avoid night dew.
The consumption of charcoal in Okaji works was 11 to 13 bamboo baskets a day. The volume of basket was approximately 90 cm in diameter and 90 cm depth. A large volume of charcoal was carried by human back without horse. Thus, the works was usually moved to follow charcoal site. On the other hand, heavy and dense iron sand and steel products were carried by horse.
2.3. Okaji WorksFigure 1 shows the layout of “Tugoyama” works in the west area of Hoki. A hearth with a piston-type blower was installed in each “Sageba” and “Honba”. The layout was rather different from other works.
Layout of Okaji works of Tsugoyama.4)
As shown in Fig. 2, the hearth of Sageba and Honba had a pit of 1.5 m in length, 1.2 m in width and 1.3 m in depth with drain covered by wood plates at the bottom. The pit were covered with clay to become 1.2 m in length, 0.3 m width and 0.85 in depth and stamped with charcoal powder.
The blower and hearth was divided by a fire wall with 24 cm in thickness. Under the wall, a bamboo pipe of “Kirokan” was passed from blower to hearth and connected to a clay tuyere. The tuyere was made of clay with 36 cm in length and 3 cm in diameter at the exit for Sageba and 48 cm in length and 4 cm in diameter for Honba and inclined by 18 degree for Sageba and 4 degree for Honba. In front of the tuyere, the oblong hearth was trenched a little sloping with about 10 cm in depth and about 1 m in length.
The wood blower had a piston sealed with raccoon dog’s fur. Its size was 1.4 m in length, 0.36 m in width and 0.8 m in height and the volume was 0.36 m3.
Honba was installed an anvil, called “Kanatoko”, and a pig iron plate, called “Kanashiki”, of wrought iron. The size of anvil was 21 cm in length, 9 cm in width and 54 cm in height. The anvil was buried in ground by 10 cm depth on round stones and faced its long side to a master, called “Daiku”. It reinforced with stones in both sides and a steel wedge. Daiku sit on the hill in 30 cm height from ground. The anvil was inclined outside by 3 degree to fit forged plate on the flat surface of anvil. The size of “Kanashiki” was 54 cm in length, 48 cm in width and 10 cm thickness and set outside of anvil.
The workers of Sugitani works were a master “Daiku”, a “Sage” craftsman, 4 hammering men, called “Teko”, and 2 blower men. The first to forth hammering men surrounded an anvil in opposite side of the master and forged clockwise according to the sign of master. The weight of hammer was 5.6 to 7.5 kg. “Daiku” and “Sage” craftsman wore black nets to protect their eyes from high temperature.
2.4. Operation 2.4.1. Sageba Process319 kg of pig iron blocks was used a day in Tsugoyama works and 200 to 250 kg a day in Sugitani works. In Uchitani works at Izumo, 255 kg of pig iron and Bugera was used. In Ichikisakamura works at Iwami of the west area of Shimane prefecture, 225 kg of pig iron was used.3)
The big blocks of pig iron were set in tunnel in the front of tuyere and the other blocks were piled up on the tunnel, as shown in Fig. 3. The piled blocks were covered with charcoal and fired with blowing air. In initial stage, blowing was moderate. After about 1 hr, pig iron blocks near tuyere were melted and decarburized to about 0.7 mass%C. When molten steel was flowed down in just front of tuyere, Sage craftsman checked the viscosity of molten steel with an iron stick, called “Sokotsuki”. When molten steel was viscous, blowing rate was increased. However, high temperature caused to dry in the hearth and to make steel hard and brittle. Thus, moisture in hearth should be always controlled with sprinkling water taking into account of the state of charcoal burning and blowing rate. The decarburization of pig iron under controlling temperature was most difficult. After 20 to 30 min, decarburized steel blocks, “Sagegane”, were pulled out from the front of tuyere every few min, as shown in Fig. 4. The Sageba process proceeded during about 2 hrs. The yield of iron was almost 100% and the consumption of charcoal was 450 kg per 300 kg of “Sagegane”.3)
“Sagegane” blocks taken out from hearth with a iron stick of Sokotsuki.5)
In Honba process in Sugitani Okaji works, the mixture of about 200 kg of Sagegane and about 100 kg of Bugera was divided in 9 groups. In Tsugoyama Okaji works, it was divided Sagegane in 10 groups. The ratio of mixture was different in each Okaji works.
About 30 kg of Sagegane blocks were piled up as the same manner as Sage process and covered by charcoal. Charcoal was burned and blocks were heated. Blowing was rather weak in the initial stage but strong when combustion became active. After about 20 min, 70% of Sagegane started to melt and drop down to hearth. Molten steel was partially solidified on hearth and Sage craftsman agglomerated in a block with a iron stick of Sokotsuki. The block was always turned to heat with fire and decarburized. Then, the block was kept a little away from tuyere and the other 30% of Sagegane blocks were melted, decarburized and agglomerated with the previous block in a block. Finally, Sage craftsman took out the hot and decarburized steel block with a pincher and covered it with straw ash. He put it on a Kanashiki plate and handed it over to Daiku. The Sage process took about 30 min.
Daiku held the hot block on a Kanashiki plate and four Teko hammers forged it to form in rectangular and cut gap to separate it into 2 pieces. The first forging was called “Hito-yaki” or “Dokiri”. The block was heated again by Sage craftsman and handed it over to Daiku. Daiku put it on an anvil of Kanatoko to form in rectangular plate and cut it into 4 pieces to the longitudinal direction. The second forging was called “Futa-yaki” or “Nibangiri”. In the third to sixth forging, each plate was heated and extended to one way. One end of plate was cut to form in rectangular and heated again. The cut was called “Hanagiri”. After then, each plate was cut notch in the center, as shown in Fig. 5. The size of plate was 60 cm in length, 20 cm in width and 1 cm in thickness. In the Honba process, the weight of plate was about 5 kg per plate. The yield was 60 to 70%. One Honba process took about 1 hr and Total Honba processes took about 10 hrs a day.
(a) Oroshigane forged first time “Hito-yaki” and (b) Hocho-tetsu cut notch in the center.5)
Mr. Heisuke Watanabe of “Daiku” said that it was clever for Daiku to make a plate with less hammering because steel became harder by more hammering. Steel plate during the third forging was very soft like a fresh rice cake. A hammering man, Teko, never hammered twice at the same point and never hit with the edge of hammer because of the cause of its damage. It was very important for Teko to hammer with the flat surface of hammer at the center of Kanatoko. Daiku always put the hammering point of plate on the center of an anvil and took into account of all of the forging processes.
The hearth with 30 cm in width, 42 cm in length and 55 cm in depth is constructed by alumina bricks in steel box of 61 × 80 × 60 (cm). A tuyer of iron pipe with 21 mm ID was installed on the hearth wall with a downward angle of 10 degree and inserted into hearth by 7 cm. The tip of tuyere was coated by fire-resistant clay. In the opposite side of tuyere, the hearth was made with 30 cm width, 22 cm length and 20 cm depth. Charcoal powder was stamped on the bottom of the hearth and about 4 litter of water was poured. Cold air was continuously blown with a electric fan. Before starting Okaji process, the hearth was enough heated by charcoal burning.
In Sageba process, three blocks of white cast iron with 3.9 mass%C were piled up in front of the tuyer with distance of 3 to 4 cm, as shown in Fig. 6(a). The pig iron block was produced from iron sand and was casted in trapezoid with the bottom size of 11.0 × 6.5 cm, the upper size of 7.5 × 3.5 cm and the thickness of about 3.0 cm. Its weight was 1.25 kg. The blocks were covered with pine charcoal. Air was blown to burn charcoal and the blocks were heated. After white sparks of “Wakibana” appeared in flame, blowing rate was increased. Water was sometimes poured on charcoal with a sprinkling can. When the blocks were melted down, blowing was stopped and a solidified product of “Sagegane” was taken out and cooled in water.
Set (a) pig iron for Sageba process and (b) Sagegane block for Honba process and oxygen sensors.
In “Honba” process, a block of Sagegane was set upside down in front of tuyer, as shown in Fig. 6(b), and covered with charcoal. Then, the block was heated by blowing air. After “Wakibana” appeared, blowing rate gradually increased and was controlled as the “Wakibana” appeared in flame uniformly and strongly. Water was also sometimes poured on charcoal with a sprinkling can. When the block was melted down, a hot steel block of “Oroshigane” was taken out and covered with straw ash. Oroshigane blocks of No. 1 and 2 were promptly forged with a forging machine to form it in rectangular.
In order to measure temperature and oxygen partial pressure, three sensors with R-type thermocouple and needle-type oxygen sensors were set in front of the tuyer, behind and above the pig iron blocks, respectively, as shown in Figs. 6(a) and 6(b). The oxygen sensor is a Galvanic cell composed of ZrO2-9mol%MgO as a solid electrolyte and Cr/Cr2O3 mixed powder as reference electrode. The solid electrolyte was one-end closed tube with 4 mm OD, 3.5 mm ID and 35 mm in length. The tip of the sensor was protected by MgO tube to prevent it from eroding by molten FeO-rich slag of Noro.
The behavior of pig iron blocks and Sagegane block in front of tuyer was observed using a video camera.
Sagegane block of No. 5 and Oroshigane block of No. 3 were cut in plate of 5 mm thickness at the center in the direction to tuyere. The plates were cut in mesh to measure the concentrations of carbon and sulfur with the combustion method by LECO and those of the other elements with X-ray fluorescence method.
Sageba processes were performed 5 times with their yields of 92 to 100% and Hon-ba processes were done 3 times with their yields of 47% to 87%, as shown in Table 3.
| Process | Day & Time | Initial weight | Product weight | Yield | Wakibana appears | Start melt* | Water in hearth | Note |
|---|---|---|---|---|---|---|---|---|
| Sageba No. 1 | 2005/Nov/27 12:01–12:40 | Pig iron 4.15 kg | Sagegane 3.8 kg | 92% | 12:20 | 1.5 l | A block was falled down | |
| Honba No. 1 | 13:13–13:50 | Sagegane | Oroshigane 0.812 kg | 13:25 | 13:35 | 1.5 l | Forged | |
| Sageba No. 2 | 2006/Nov/12 10:40–11:21 | Pig iron 4.56 kg | Sagegane 4.50 kg | 99% | 10:49 | 11:12 | 1.9 l | |
| Sageba No. 3 | 13:05–13:56 | Pig iron 4.54 kg | Sagegane 4.43 kg | 98% | 13:26 | 13:27 | 2.6 l | Temp.&PO2 measure |
| Honba No. 2 | 14:25–15:11 | Sagegane 4.31 kg | Oroshigane 2.03 kg | 47% | 14:40 | 14:50 | 2.2 l | Temp.&PO2 meas., Forged |
| Sageba No. 4 | 2006/Nov/13 9:49–10:33 | Pig iron 4.24 kg | Sagegane 4.24 kg | 100% | 10:00 | 10:22 | 2.3 l | |
| Sageba No. 5 | 10:50–11:56 | Pig iron 4.31 kg | Sagegane 4.2 kg | 97% | 11:05 | 11:41 | 3.0 l | Temp.&PO2 measure |
| Honba No. 3 | 12:33–13:50 | Sagegane 3.71 kg | Oroshigane 3.25 kg | 87% | 12:45 | 13:35 | 2.2 l | Temp.&PO2 measure |
As shown in Fig. 7 of Sageba process of No. 5, after 9 min, the temperature above blocks attained at 1200°C and after then “Wakibana” continuously appeared in flame. After 12 min, the temperature attained at 1350°C. Temperature was carefully controlled to keep about 1350°C with pouring water on charcoal. The temperature in front of tuyere attained at 1200°C after 13 min and was kept at 1400°C after 23 min. The temperature behind the blocks gradually increased to attain at 1200°C.
Temperature and oxygen potential changes in front of the tuyer (tuyere), behind (rear) and above (top) pig iron blocks, respectively during Sageba process of No. 5.
Oxygen potential in front of tuyere was almost in air and that above the blocks fluctuated between 10–3 and 10–8 atm, followed by 10–2 atm in the final stage. That behind the blocks gradually increased from 10–15 to 10–12 atm.
Figure 8 shows the aspects of melting of pig iron blocks during Sageba process of No. 5. It was notable that pig iron melted with CO gas bubbling and “Wakibana” in flame.
Aspects of melting pig iron blocks during Sageba process of No. 5.
In Honba process of No. 3, as shown in Fig. 9, after 15 min, the temperature above Sagegane block attained at 900°C and after then Wakibana continuously appeared in flame. After 20 min, the temperature attained at 1300°C and after 50 min at 1400°C. After 70 min, the block started to melt and then blowing increased. The temperatures in front and behind the block suddenly increased and finally attained at 1450°C. During final stage at high temperature, water was poured sometime to prevent temperature from increasing over 1450°C.
Temperature and oxygen potential changes in front of the tuyer (tuyere), behind (rear) and above (top) Sagegane blocks, respectively during Honba process of No. 3.
Oxygen potentials in front and above the block were almost in air and that above the block decreased to 10–10 atm according to temperature rise in the final stage. Also, oxygen potential behind the block increased from 10–15 to 10–12 atm near the equilibrium with Fe and FeO in the final stage.
Figure 10 shows the aspects of melting of Sagegane block during Honba process of No. 3. It was notable that in the final stage, the droplets of molten FeO slag of Noro flowed down on molten steel with Wakibana in flame but without CO gas bubbling.
Aspects of melting Sagegane block during Honba process of No. 3.
The composition of pig iron was 4.033 mass%C, 0.188 mass%Si, 0.175 mass%Mn, 0.245 mass%P. 0.0395 mass%S, 0.001 mass%Al, 0.084 mass%Ti and 0.091 mass%V.
Figures 11 and 12 show the distribution of carbon content in Sagegane of No. 5 and Oroshigane of No. 3, respectively. In Sageba process, the upper part of Sagegane was decarburized to 0.5 to 0.1 mass%C and melted down for tuyere. However, the bottom part was not melted without decarburization. In Honba process, Oroshigane was decarburized to 0.6 to 0.07 mass%C but not uniform.
Cross-section of Sagegane of No. 5 and Carbon distribution.
Cross-section of Oroshigane of No. 3 and Carbon distribution.
In Sageba process, pig iron started to melt after 20 min when the temperature in front of and above pig iron blocks attained at 1350 to 1400°C. Pig iron melted in the initial stage was hardly decarburized. After then, pig iron melted and flowed down on the surface of block to the direction of tuyere. Molten pig iron was oxidized by air to make molten FeO slag layer of Noro on the surface of pig iron. Oxygen gas in air reacted with positive holes (h) in molten FeO slag to be O2– ion at the slag surface;
| (1) |
The hole transferred from the surface of slag to the interface with molten pig iron and reacted with iron and with carbon in pig iron and O2– ion;
| (2) |
| (3) |
The mobility of hole in FeO slag is larger by a few order than the diffusion rates of O2– and Fe2+ in slag as well as the slag layer was stirred by CO gas bubbling. Thus, the oxygen potential at the interface was high as 10–3 to 10–8 atm. On the other hand, the oxygen potential in equilibrium with δ-Fe and molten FeO is about 1 × 10–10 atm at 1400°C6) and that in equilibrium with carbon in pig iron and 1 atm of CO gas is about 1 × 10–18 atm at 1400°C. There was large difference of oxygen potential at the interface to proceed both decarburization and oxidation of iron with active bubbling of CO gas simultaneously.
5.2. Decarburization Mechanisms of “Honba” ProcessIn Honba process after 50 min, the temperature of Sagegane block rapidly increased to 1450°C and then the block melted. In the initial stage of melting, the decarburization of molten steel took place with moderate CO gas bubbling and after then generated FeO droplets on steel without gas bubbling, as shown in Fig. 10(c). Oxygen potential above the block attained to 10–10 atm in the final stage and iron was violently oxidized directly by air to generate heat with increasing the temperature of molten steel. This oxidation is realized from the yield of iron by 60 to 70%. During decarburization of molten steel, oxygen content in steel increased to near 0.15 mass% of eutectic point at 1528°C, as shown at the point B in Fig. 13(a), and carbon content in liquid steel was about 0.1 mass% in equilibrium with δ-Fe as shown at the point D in Fig. 13(b). The decarburized molten steel was solidified near the melting point of iron.
Reaction paths of decarburization and oxidation of pig iron and Sagegane on the phase diagrams of (a) Fe–O and (b) Fe–C systems.
As mentioned above, water played very important role that the temperature of steel block was suppressed under 1400°C for Sageba process and 1450°C for Honba process in order to prevent steel block from oxidizing by air during decarburization. Pouring water in flame on charcoal was to control the temperature of steel blocks. However, water put in charcoal bed cooled hearth by vaporization. In the present experiments, the lower part of Sagegane did not melted because of vaporization of water. Thus, a Sage craftsman always stirred molten pig iron to check the viscosity of steel with Hodotsuki and controlled temperature by blowing rate in order to prevent pig iron from solidifying.
White sparks of Wakibana were fine iron particles that were generated from molten iron by strong force when CO gas bubble over the critical radius suddenly appeared at the interface of molten slag and molten steel.
“Okaji” works produced low carbon steel of Hocho-tetsu with about 0.1 mass%C from pig iron and low grade of high carbon steel. The decarburization process was composed of two processes of Sageba and Honba processes. In the former process, pig iron blocks were heated by charcoal burning and decarburized by the reaction with molten FeO slag of Noro at the temperature near 1400°C. Sagegane blocks with the carbon content of average 0.7 mass% were produced. In the later process, the blocks were heated to keep at 1450°C in hearth with the same manner as the former process. The blocks were oxidized to generate heat themselves and to increase the temperature over 1528°C. The melted blocks were agglomerated in a lump using a iron bar of Sokotsuki. The lump was decarburized by oxygen in air directly and the carbon content decreased to average 0.1 mass%, while the oxygen content increased to about 0.2 mass%. Thus, Hocho-tetsu was over-saturated with oxygen and the distribution of carbon and oxygen content was not uniform.
Authrs thanks Mr. Sumihira Manabe, Japanese sward master, for his help with our experiments.
