Technology Discussion

February 17, 2012

Talking about glazed clay roofing tile production technology form North Korean tile production line

1 Preface

WEIDA cooperate a Shandong kiln company supplied a complete clay roofing tile production line for North Korean in 2007,this article mainly introduce and discuss its clay roofing tile machines, kiln, related production technology and tile formula.

2 Production technological process and main production equipments

2.1Production technological process:

Mainly process as below:

Raw material—-15m3 box feeder—- cage crusher—-double shaft mixer—- roller crusher—-double shaft mixer—- roller crusher—- disc screen feeder—- material aging—- forklift carry material—-5m3 box feeder—-de-airing extruder—-automatic tile cutting machine—- auto tile press—-auto loading car machine— -tunnel dryer—- auto glazing machine—-tunnel kiln

2.2 Main production equipments:

2.2.1 Cage crusher:

The model is LA-1000, big rotating cage diameter is 1000mm, machine motor power is 55Kw, rotation speed is 500rpm, capacity meet 8~10T/H.

Working principle: small and big two cages high speed countermotions, semi-hard and block raw material fall down into the cage and impact each other, the crushed material discharge from bottom outlet.

2.2.2 Double shaft mixer

The model is SJ240X80, screw blade diameter is 240mm, motor power is 22kw, rotation speed is 60RPM, capacity is 20~25T/H.

Double shaft mixer is mainly used for fully mixing the material, in this production line, it is installed behind of cage crusher and firstly roller crusher for material kneading, make the water and clay composition more uniformity.

2.2.3 Roller crusher

The model is GS80X60, roller diameter is 800mm. width is 600mm, total motor power is 52Kw, capacity is 20~35T/H.

Roller crusher is the key equipments of raw material preparation machines. Its working principle is two rollers high speed rotation face to face, material fall down into the clearance between two rollers, material is crushed along with high speed rotation of rollers.

2.2.4 Disc screen feeder

The model is WD1200, screen diameter is 1200mm, screen hole is 22~25mm, motor power is 30kw, capacity is 10~30T/H.

Disc screen feeder is manly used for high-strength knead material again, make the material composition and water more uniformly, improve its plasticity.

2.2.5 De-airing extruder

The model is TCJ350, structure use twin stage three shafts. Down main shaft screw diameter is 350mm, TCJ 350 de-airing extruder is available used for extruding split tile, glazed clay roofing tile, high temperature roller kiln. The machine de-airing ability is good, the vacuum degree keep with above -0.092 when production.

The maximum rotation speed of main shaft is 19.8RPM, circuit configure frequency converter enable it can regulating speed according to production requirement.

2.2.6Automatic tile cutting machine

The model is WDQ-2, working principle: extruded green tile move forward by carrier roller, carrier roller is installed on the trolley of cutting machine. There have damper plate in the front of carrier roller, when extruded green clay roofing tile reach damper position, it will push the trolley move forward. When trolley move forward to required position, the cutting wire will performance cutting, then damper open, the tile after cutting will move forward to fast speed conveyor, and then trolley and carrier roller will reset and waiting for next cutting signal. As the extruded tile fully keep pace with wire cutting, it realized synchronized cutting, cutting face is smooth and straightly, cutting accuracy is high.

2.2.7 Fully automatic tile press

The model is WYJ100-1, main motor power is 7.5kw, capacity is 15~17pcs/min, fully automatic tile press is the key equipments of clay roofing tile production line.

Fully automatic tile press mainly consist of machine body, pressing platform, six side rotating wheel, back and forth manipulator, suction system and other components. The up mould is installed on the pressing platform, and the down mould is installed on the six side rotating wheel respectively.

When working, the front manipulator sucks the green tile up and put down into one of the six side rotating wheel which is backward titling 60°,then six side  rotating wheel spinning 60° on axis, make the green tile is on horizontal position, now the up mould move downward for tile pressing, at the same time, the front manipulator sucks the next green tiles up and put down into one of the six side rotating wheel which is backward titling 60°,green tile after double pressurizing press, up mould move upward, rotating wheel continuous backward titling 60°and the green tile after press is sucks by back manipulator and put down into conveyor.

2.2.8 First tunnel drying kiln

First tunnel drying kilnFirst tunnel drying kilnhe upperaccording to production requirment.ean  have three channels, each channel length is 90m, width is 3M, and every channel have 2 orbits, total 6 orbits, every channel have inside kiln car 60 sets. Thermal resources is come from residual thermal of cooling section of tunnel kiln and waste smoke of smoke exhaust fan, use air blower dehumidifying and Negative pressure humidity discharging. Maximum adjusting temperature is 50-150℃, design drying period is 20~24 hours.

2.2.9 Auto glazing line

Auto glazing line is consisting of auto hang tile machine, auto glazing machine, tile conveyor and transmission device. Compare to manual glazing, auto glazing machine glazing is smoother, texture is better and productivity is higher.

Working principle: tiles are hang up on the tile conveyor in turn and move forward, when pass to glazing machine, glazing head according locating device spray glaze on the tiles, tiles continuous move forward and become dry gradually, when tiles reach to setting position, put the tiles on the kiln car manually.

2.2.9 Second drying kiln

Thermal resource is come from hot blast stove (diameter 1.2m), kiln width is 2.6m, and width is the same with tunnel kiln. It mainly used to double drying and pre-heating with the temperature on 150-180℃, after 8~10 hours drying, the moisture of green tile is below 1%. Thus can improve the temperature of tunnel kiln head to 200-250℃ and realize fast heating, avoid tile crack.

2.2.10 High temperature tunnel kiln

This tunnel kiln use generator gas as fuel, inner width is 2.6m, effective width is 0.65m, and total length is 126m: pre-heating section is 39m, firing section is 36m, and cooling section is 51m. Kiln has total 120 nozzles: pre-heating section have 12 pairs, firing section up and down rows had 48 pairs. Nozzles left and right sides separation, Kiln maximum design firing temperature is 1200℃, design firing period is 21 hours, daily output clay roofing tile 25,000pcs.


3. Raw material and formula

3.1 Tile material and formula

3.1.1 Tile material

Kaolin: come form north area of North Korean, appearance is gray, blocky and soft clay, plasticity is good, contain a lot of charcoal impurity, iron and titanium content is a little higher. It appear ash black after firing at 1200℃. It mainly used for improve green tile plasticity and avoid tiles crack when in the process of pressing and drying. According to North Korean people introduction, this kaolin were use for daily ceramic production since 15th century, is the most famous clay mineral locally, and mineral reserves is large, but the distance between factory is 800Km thus need railway transportation. Considering to the rail freight capacity is nervous, North Korean side required use as less as it, even do not use it at all.

PAQIN clay: Come form a place called “PAQIN” Where the distance between with factory is 25Km, it have hundred years for clay roofing tile making, is traditional clay roofing tile material in North Korean. The material is blocky and soft clay, organic content is higher, and aluminum content is lower, plasticity is poor, the tiles single using this material is easy to crack when nature drying, it can not single forming to tiles.

Bao hill red clay: Come from farm where the distance between with factory is 20Km, appearance is red, it also is soft clay and have good calenderability, but plasticity is poor. It mainly used for improve calenderability of green clay roofing tile.

White Sea sand: come form west coast, mineral reserve is rich, and only 20Km distance between with factory, it is white quartz fine sand, granularity is 20 mesh. It mainly used for lower the clay roofing tile firing and drying shrinkage, reduces tile distortion.

Above 4 kinds clay material chemical composition as below:                                Chart 1































Bao hill red clay










White Seasand


3.1.2 Tile body formula

After repeated analysis and experiment, we confirmed the last production tile body formula, it show as below:

Tile body formula(%)                                  Chart 2




Bao hill red clay

White Seasand







3.2 Glaze material and formula 

Glaze material as below:

Lead transparent frit: Origin come formChina, chemical composition as chart 3. Using Lead frit is used for make glaze have good gloss when generate on lower temperature.

Potassium feldspar powder: Origin come form North Korean, granularity pass 120 mesh, white degree is good, it mainly worked as fluxing agent. At the same time, it can improve glaze hardness and other performance. The chemical composition is as chart 3.

Chinese kaolin: Origin isJiangsu province,China, it mainly used for improve suspension property, avoid glaze precipitation and increase glaze generation temperature width.

Calcite powder: Origin is from China, it is mainly used form composite fluxing agent, lower glaze high temperature viscosity and improves glaze gloss, usage too many is not suitable, otherwise it will cause crystallization devitrification.

Barium carbonates powder: Origin isChina. It is mainly used form composite fluxing agent with other fusing agent composition, lower the glaze maturing temperature and high temperature viscosity, improve glaze gloss.

Zinc oxide: Origin is China, it appears yellow in color, as high-strength flux of glaze, it can well lower glaze maturing temperature, high temperature viscosity and surface tension, improve glaze gloss degree and covering power. As it also is a crystallizing agent, glaze surface will be crystalline when tile drying is improper.

   After repeated test and try, confirmed below formula as chart 4:

Glaze material chemical composition                           Chart 3























Potassium feldspar










Lead  frit











Glaze formula                                              Chart 4






Barium carbonates

Zinc oxide


water reducing agent














4. Mainly production technological parameters:

4.1 Raw material preparation and forming

a. Roller crusher clearance: First roller crushing clearance: 1.8-2mm

                      Secondary roller crushing clearance: 0.8-1.0mm

b. Tiles material aging period:  ≥24h

c. Extruding moisture of green tile: 18-19%

d. Extruder vacuum degree: ≤-0.090Mpa

e. Extruding pressure: ≥1.0-1.5Mpa

f. Auto tile press forming pressure: ≥100t

4.2 Glaze preparation and

a. Material: ball: water=1:1.5:0.8

b. Ball mill period: Charge amount 1.0T, ball mill time is 12 hours

c. Ball mill granularity fineness: passing 325 mesh

d: Glaze proportion: 1.70-1.75

e: Glazing weight: 85~95g(clay roofing tile size: 315X315)

4.3 Drying and firing

a. Clay roofing tile nature drying time: ≥24h

b. First drying kiln max. temperature is 45-50℃, drying period is 24 hours

c. Green tile moisture after first drying: 4~6%

d. The second drying kiln max. temperature is 180-200℃

e. Green tile moisture after second drying: <1%

.  f. Firing temperature: 17-20h

5. Clay roofing tile property

After mass production, according to TC709~1998《China building materials industry standard products sintering watts》inspect clay roofing tiles, the inspection result fully up to standard. The main inspection result as below:

  a. Water absorption: 6~8%

  b. Ridge tile bending failing load is 2800N, main tile bending strength is N.

  c. 130±2℃~-15±5℃, thermal exchange 3 times no crack happens.

  d. Frost resistance: -20±3℃~15-25℃, repeated freezes 31 hours 15 times no layering and disintegration happens.

6. Problem analysis and discussion

6.1 1 should pay more attention to raw material test on the early stage.

According to normal procedure, it should do project feasibility study firstly; the most important work of project feasibility study is raw material analysis and test. Due some reasons, North Korean side neither sent material for us test and invite our engineer inspect material on site, they simply suppose their local material (PAQIN) was suit for clay roofing tile producing and placing a large order on this material. Our engineer fined this material plasticity is not good and organic content is higher when arrives their factory, it impossible forming tiles singly by this material. We have to re-test and research material thus waste a lot of time.

6.12 Production technology design and machine selection

For saving investment, this clay roofing tile production line canceled Raymond grinder on original proposal, and cause a lot of problem and limitation to late works of production technological formula R&D. Our ceramic engineer deny the possibility of forming roofing tile singly by PAKIN, and find Kaolin produce in north have good plasticity, but this Kaolin have a lot of wooden organism, it will cause bubbles and black heart if can not crush the material below 1mm. Due to there is no other good plasticity materials, at last North Korean side have to grinding the material by ball mill then use sprayer for granulating. Besides, we found hard terracotta stone which is suite for clay roofing tile nearby their factory, due to there is no proper fine crushing material, we have to abandon using them.

When design a production line, it can not just simple save investment, should fully consider technological requirement based on raw material analysis and test, necessary equipment can not be canceled.

6.1.3 Kiln design should according to clay roofing tile specialty and raw material property 

Glazed clay roofing tile is seems a simple terracotta product, but it have special requirement for kilns. It can not reach perfect result by using other kiln’s design and construction indiscriminately. The kiln for this production line is supplied by a Shandong kiln company, the kiln design and construction have not consider the actual material condition, thus make great troubles on production line commissioning.

a. Kiln 3 area length proportion problems: Regarding clay roofing tile, the structure is simple but thickness is a little thicker(15~20mm) and due to is formed by de-airing extruder, green clay tile density is high, organism oxidation and carbonate not easy to decompose when firing, the condition will be more serious if organic content is high, thus must make sure it have enough time for oxygenolysis when produce clay roofing tile, it required longer pre-heating area and firing area accordingly.

For cooling area, as tile structure is simple, tie size is small, it also can realize fast and uniform cooling even thickness is thicker. And due to water absorption is a litter higher(《12%), glass phase is few so that tile is not easy crack, the cooling area can be shorter.

But the cooling area of this kiln is 51 M when total lengths is 126M, account around 40% of total length, it easily cause black heart and bubbles when firing. If can extend 10~15M for length of firing area and shorten the length of cooling area accordingly, the tile quality will be better.

   b, Kiln air curtain problem: The tunnel kiln head did not set kiln door but airlock air curtain, and set mixing air curtain on pre-heating area. We do not know the reasons, all air curtain thermal resource is use indoor room temperature air. We think this is a failure design, kiln head air curtain is used for stop cold air of outside kiln enter pre-heating area which is under negative pressure condition, use cold air as air resource means cold air enter kiln head and air curtain is unuseful. Besides, mixing air curtain use cold as air resource lower the kiln head temperature, after second drying, the green tile temperature is above 150 ℃, but it be cooling when enter kiln, it waste energy and lower the heating curve. We suggest the air resource should be changed to residual thermal air form cooling area. After this production line, WEIDA established its own kiln Dept for better serve customers.

Glaze formula                                              Chart 4

Clay Roofing Tile