Raw material of composite pellets used in kiln phosphoric acid process, and forming method therefor

Abstract

Disclosed is a composite pellet used as a raw material in a kiln process for the production of phosphoric acid, which is of a core-shell structure with an inner ball encapsulated with a shell, the inner ball mainly consists of a inner ball material and a binding agent, and the shell mainly consists of a cladding material and a binding agent; the inner ball material mainly consists of a carbonaceous reductant powder, and phosphate ore powder and silica powder, the inner ball is combined with the shell through the binding agent to form the core-shell shaped structure. The preparation of the composite pellets comprises the steps of preparing the inner ball, preparing the cladding material, forming, drying and solidifying composite green pellets etc. The composite pellets prepared by the present invention have a smaller range of formulation fluctuation, more stable quality, higher strength and better performance.

Claims

1. A forming method of composite pellet, comprises the following steps: (1) Preparing an Inner Ball: a carbonaceous reductant powder, a phosphate ore powder, a silica powder and a binding agent are mixed, feeding the mixed material into a pelletiser for pelleting treatment, and the inner ball is obtained upon completion of the pelleting, wherein in the inner ball CaO/SiO.sub.2 mole ratio is less than 0.6 or greater than 6.5, and the amount of carbonaceous reductant powder is 1.0-2.0 times a theoretical amount of P.sub.2O.sub.5 in phosphate ore powder, the binding agent is 1%-12% of the mixed material by mass; (2) Preparing a Cladding Material: a carbonaceous reductant powder, a silica powder and a binding agent are mixed to obtain the cladding material, wherein a mass ratio of the carbonaceous reductant powder to the silica powder in the cladding material is 1.5-9:1, the binding agent is 1%-12% of the cladding material by mass; (3) Forming a Composite Pellet: the inner ball obtained in step (1) is subjected to screening treatment to obtain inner ball with a granularity meeting the process requirement, feeding the inner ball into another pelletiser for cladding treatment, introducing the cladding material into the pelletiser, composite green pellet is obtained upon completion of the cladding treatment; (4) Drying and Solidifying: delivering the composite green pellet into a dryer for drying and solidifying, and finally forming the composite pellet, wherein the dryer used in the step of drying and solidifying is a scale plate dryer, and the scale plate dryer is divided into three drying stages in a delivery direction of the composite green pellets, including a low-temperature drying stage, an intermediate-temperature drying stage and a high-temperature drying stage.

2. A forming method according to claim 1, wherein the step (1) of the forming method comprises the following steps: adding the carbonaceous reductant powder, phosphate ore powder and silica powder to an intensive mixer or a damp mill, and meanwhile adding the binding agent, feeding the mixed material which is mixed sufficiently and uniformly by a weighing and feeding device into a pelletiser for pelleting treatment, adding the binding agent in an application form of droplets and/or mist, an addition amount of the binding agent is 1%-12% of mass of the mixed material, and the inner ball is obtained upon completion of the pelleting.

3. A forming method according to claim 2, wherein the intensive mixer in step of the preparation of inner ball includes an obliquely rotatable mixing barrel, a rotatable agitator is mounted in the mixing barrel; upon mixing, a rotation direction of the mixing barrel is contrary to a rotation direction of the agitator to allow the mixed materials in the mixing barrel to form a turbulence therein, thereby achieving an effect of agitating sufficiently and uniformly.

4. A forming method according to claim 2, wherein in the step of the preparation of inner ball and forming of composite pellet, the pelletiser is a disc type pelletiser; feeding all the inner balls which is not meeting the process granularity requirement screened out in step (3) into a roller mill or damp mill for milling, and selectively adding the inner ball material according to a requirement for material humidity during milling, and then returning into the intensive mixer or damp mill in step (1) to form closed-loop circulation.

5. A forming method according to claim 1, wherein the step (2) of the forming method comprises the following steps: adding the carbonaceous reductant powder and silica powder to an intensive mixer or a damp mill, meanwhile adding a binding agent, mixing sufficiently and uniformly to obtain the cladding material and feeding the cladding material into a cladding material silo.

6. A forming method according to claim 5, wherein the intensive mixer in step of preparing cladding material includes an obliquely rotatable mixing barrel, a rotatable agitator is mounted in the mixing barrel; upon mixing, a rotation direction of the mixing barrel is contrary to a rotation direction of the agitator to allow the mixed materials in the mixing barrel to form a turbulence therein, thereby achieving an effect of agitating sufficiently and uniformly.

7. A forming method according to claim 1, wherein the step (3) of the forming method comprises the following steps: the inner ball obtained in step (1) is subjected to bi-layered roller-type screening treatment to obtain inner ball with a granularity meeting the process requirement, feeding the inner ball into another pelletiser for cladding treatment, meanwhile, introducing the cladding material into the pelletiser, additionally adding the binding agent in an application form of droplets and/or mist during the cladding treatment, the addition amount of the binding agent is 1%-12% of the mass of the cladding material, the composite green pellet is obtained upon completion of the cladding treatment.

8. A forming method according to claim 1, wherein a method of preparing the binding agent used in the present embodiment specifically comprises the following steps: (1) Preparing Raw Material: selecting coal containing humic acid and caustic soda as raw material, mixing caustic soda with water to obtain sodium hydroxide solution; (2) Ball-Grinding and Mixing: ball-grinding and mixing the coal and sodium hydroxide solution in step (1) with a 1:3-10 solid-to-liquid ratio; (3) Synthesizing: feeding the mixed material in step (2) into a reaction tank with an agitator, starting the agitator, heating to 40-95 C. to perform synthesis reaction; (4) Filtering: filtering the reaction product in step (3) to obtain filtrate as the binding agent.

9. A forming method according to claim 8, wherein the time of the ball-grinding in step (2) of the preparing the binding agent is 5 min-120 min.

10. A forming method according to claim 8, wherein the time of reaction in step (3) of the preparing the binding agent is not less than 30 min.

11. A forming method according to claim 8, wherein in step (1) of the preparing the binding agent, the coal material refers to weathered coal, peat and/or lignite containing humic acid above 20%.

12. A forming method according to claim 8, wherein in step (1) of the preparing the binding agent, a mass percentage concentration of the sodium hydroxide solution is controlled in a range of 1%40%.

13. A forming method according to claim 1, wherein low-temperature hot air a range of 100-200 C. introduced into the low-temperature drying stage performs up-down air pumping or down-up wind blowing so that the low-temperature hot air vertically passes through a material layer and performs through-drying for the composite green pellets; the low-temperature hot air originates from exhaust gas discharged at a high-temperature hot air outlet of the high-temperature drying stage; intermediate-temperature hot air a range of 150-250 C. introduced into the intermediate-temperature drying stage performs up-down air pumping or down-up wind blowing so that the intermediate-temperature hot air vertically passes through the material layer and performs through-drying for the composite green pellets; high-temperature hot air a range of 200-350 C. introduced into the high-temperature drying stage performs up-down air pumping or down-up wind blowing so that the high-temperature hot air vertically passes through the material layer and performs through-drying for the composite green pellets.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram of a process flow of preparation of the binding agent in a specific embodiment of the present invention.

(2) FIG. 2 is a schematic diagram of a process flow of a raw material pretreating step in a specific embodiment of the present invention.

(3) FIG. 3 is a structural schematic view of an intensive mixer used in a specific embodiment of the present invention.

(4) FIG. 4 is a view integrating a sectional view taken along A-A in FIG. 3 and a working principle view.

(5) FIG. 5 is a structural schematic diagram of a scale plate dryer used in a specific embodiment of the present invention.

(6) FIG. 6 is a view integrating a sectional view taken along B-B in FIG. 5 and a working principle view.

LISTING OF PARTS

(7) The reference number 1 a driving member; 2 a feed hopper; 3 an agitator; 4 a casing; 5 a material turning plough; 6 a discharge port; 7 a material loading cart; 8 a drying furnace body; 9 a deduster; 10 a low-temperature drying stage; 11 an intermediate-temperature drying stage; 12 a high-temperature drying stage; 13 an air inlet; 14 a temperature-preserving layer; 15 an air outlet; 16 an air vent.

DETAILED DESCRIPTION OF THE INVENTION

(8) The embodiments of the present invention are described more fully hereinafter with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be through and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as systems, methods or devices. The following detailed description should not to be taken in a limiting sense.

(9) Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise. The phrase in one embodiment as used herein does not necessarily refer to the same embodiment, though it may. Furthermore, the phrase in another embodiment as used herein does not necessarily refer to a different embodiment, although it may. Thus, as described below, various embodiments of the invention may be readily combined, without departing from the scope or spirit of the invention.

(10) In addition, as used herein, the term or is an inclusive or operator, and is equivalent to the term and/or, unless the context clearly dictates otherwise. The term based on is not exclusive and allows for being based on additional factors not described, unless the context clearly dictates otherwise. In addition, throughout the specification, the meaning of a, an, and the include plural references. The meaning of in includes in and on. The term coupled implies that the elements may be directly connected together or may be coupled through one or more intervening elements. Further reference may be made to an embodiment where a component is implemented and multiple like or identical components are implemented.

(11) While the embodiments make reference to certain events this is not intended to be a limitation of the embodiments of the present invention and such is equally applicable to any event where goods or services are offered to a consumer.

(12) In an embodiment of the present invention, the present invention provides a composite pellet used as a raw material in kiln phosphoric acid process, the composite pellet is a core-shell shaped structure with an inner ball encapsulated with a shell, the inner ball mainly consists of a inner ball material and a binding agent, and the shell mainly consists of a cladding material and a binding agent; the inner ball material mainly consists of a carbonaceous reductant powder, and phosphate ore powder and silica powder, the addition quantity of the binding agent in the inner ball is 1%-12% of the mass of the inner ball material; the cladding material mainly consists of a carbonaceous reductant powder and a silica powder, the addition quantity of the binding agent in the shell is 1%-12% of the mass of the cladding material; the inner ball is combined with the shell through the binding agent to form the core-shell shaped structure.

(13) In an embodiment of the present invention, the addition quantity of the binding agent in the inner ball is 1%-10% of the mass of the inner ball material; the addition quantity of the binding agent in the shell is 1%-10% of the mass of the cladding material; the inner ball is combined with the shell through the binding agent to form the core-shell shaped structure.

(14) In an embodiment of the present invention, the addition quantity of the binding agent in the inner ball is 2%-6% of the mass of the inner ball material; the addition quantity of the binding agent in the shell is 2%-6% of the mass of the cladding material; the inner ball is combined with the shell through the binding agent to form the core-shell shaped structure.

(15) In an embodiment of the present invention, the carbonaceous reductant powder is selected from the group consisting of anthracite, bituminous coal, coke, petroleum coke, and combination thereof.

(16) In an embodiment of the present invention, the binding agent is a mixed solution containing sodium humic acid, the sodium humate in the binding agent being 4%-20% mass percentage concentration, the binding agent is obtained after mixing the coal containing humic material with sodium hydroxide solution, reacting and filtering.

(17) In an embodiment of the present invention, the coal is selected from the group consisting of weathered coal, peat, lignite, and combination thereof.

(18) In an embodiment of the present invention, the sodium humate in the binding agent being 4%-16% mass percentage concentration.

(19) In an embodiment of the present invention, the sodium humate in the binding agent being 8%-12% mass percentage concentration.

(20) In other embodiment of the present invention, various binding agent products employed in conventional KPA process also can be the binding agent.

(21) In an embodiment of the present invention, the phosphate ore powder of the composite pellets is a product obtained after homogenizing the phosphate ore powder that grinded and stored in the homogenization silo in a form of gap tape homogenization or continuous homogenization by using compressed air, and the homogenization value greater than 4.

(22) In an embodiment of the present invention, in the inner ball material, CaO/SiO.sub.2 mole ratio is less than 0.6 or greater than 6.5, and the amount of carbonaceous reductant powder is 1.0-2.0 times a theoretical amount of P.sub.2O.sub.5 in phosphate ore powder.

(23) In an embodiment of the present invention, in the inner ball material, CaO/SiO.sub.2 mole ratio is 0.3,

(24) In an embodiment of the present invention, a mass ratio of carbonaceous reductant powder to silica powder in a cladding material is 1.5-9:1.

(25) In an embodiment of the present invention, a mass ratio of carbonaceous reductant powder to silica powder in a cladding material is 1.5-5:1.

(26) In an embodiment of the present invention, a mass ratio of carbonaceous reductant powder to silica powder in a cladding material is 2.5:1.

(27) The present invention also provides a forming method of composite pellet, comprises the following steps:

(28) (1) Preparation of Inner Ball: a carbonaceous reductant powder, a phosphate ore powder, a silica powder and a binding agent are mixed according to a proportional requirement, feeding a mixed material into a pelletiser for pelleting treatment, and inner ball is obtained upon completion of the pelleting;

(29) (2) Preparation of Cladding Material: a carbonaceous reductant powder, a silica power and a binding agent are mixed according to proportional requirements to obtain a cladding material;

(30) (3) Forming of Composite Pellet: the inner ball obtained in step (1) is subjected to screening treatment to obtain inner ball with a granularity meeting the process requirement, feeding the inner ball into another pelletiser for cladding treatment, introducing the cladding material into the pelletiser, composite green pellet is obtained upon completion of the cladding treatment;

(31) (4) Drying and Solidification: delivering the composite green pellet into a dryer for drying and solidification and finally forming to obtain the composite pellet.

(32) In an embodiment of the present invention, the step of preparation of inner ball in the forming method of composite pellet, comprises the following steps: adding the carbonaceous reductant powder, phosphate ore powder and silica powder to an intensive mixer or a damp mill according to a proportional requirement, and meanwhile adding a binding agent, feeding the mixed material which is mixed sufficiently and uniformly by a weighing and feeding device into a pelletiser for pelleting treatment, adding the binding agent in an application form of droplets and/or mist, an addition amount of the binding agent is 1%-12% of mass of the mixed material, and inner ball is obtained upon completion of the pelleting;

(33) In an embodiment of the present invention, the step of preparation of cladding material in the forming method of composite pellet, comprises the following steps: adding the carbonaceous reductant powder and silica power to an intensive mixer or a damp mill according to proportional requirements, meanwhile adding a binding agent, mixing sufficiently and uniformly to obtain a cladding material and feeding the cladding material into a cladding material silo;

(34) In an embodiment of the present invention, the step of forming of composite pellet in the forming method of composite pellet, comprises the following steps: inner ball obtained in step (1) is subjected to bi-layered roller-type screening treatment to obtain inner ball with a granularity meeting the process requirement, feeding the inner ball into another pelletiser for cladding treatment, meanwhile, introducing (by an electronic weighing and feeding device in said pelletiser according to a corresponding proportion set with respect to the inner ball material) the cladding material into the pelletiser, additionally adding the binding agent in an application form of droplets and/or mist during the cladding treatment, the addition amount of the binding agent is 1%-12% of the mass of the cladding material, composite green pellet is obtained upon completion of the cladding treatment;

(35) In an embodiment of the present invention, a method of preparing the binding agent used in the present embodiment specifically comprises the following steps:

(36) (1) Preparation of Raw Material: selecting coal containing humic acid and caustic soda as raw material, mixing caustic soda with water to obtain sodium hydroxide solution;

(37) (2) Ball-Grinding and Mixing: ball-grinding and mixing the coal and sodium hydroxide solution in step (1) with a 1:3-10 solid-to-liquid ratio;

(38) (3) Synthesis Reaction: feeding the mixed material in step (2) into a reaction tank with an agitator, starting the agitator, heating to 40-95 C. to perform synthesis reaction;

(39) (4) Filtering: filtering the reaction product in step (3) to obtain filtrate as the binding agent.

(40) In an embodiment of the present invention, the coal in step (1) of preparation the binding agent generally with a granularity 20 mm.

(41) In an embodiment of the present invention, the time of the ball-grinding in step (2) of the preparing the binding agent is 5 min-120 min.

(42) In an embodiment of the present invention, the time of reaction in step (3) of the preparing the binding agent is not less than 30 min.

(43) In an embodiment of the present invention, the time of reaction in step (3) of the preparing the binding agent is 30 min-180 min.

(44) In an embodiment of the present invention, in step (1) of the preparing the binding agent, the coal material refers to weathered coal, peat and/or lignite containing humic acid above 20%.

(45) In an embodiment of the present invention, in step (1) of the preparing the binding agent, the coal material refers to weathered coal, peat and/or lignite containing humic acid above 40%.

(46) In an embodiment of the present invention, in step (1) of the preparing the binding agent, a mass percentage concentration of the sodium hydroxide solution is controlled in a range of 1%-10%.

(47) In an embodiment of the present invention, in step (1) of the preparing the binding agent, a mass percentage concentration of the sodium hydroxide solution is controlled in a range of 2%-10%.

(48) In an embodiment of the present invention, the intensive mixer in step of the preparation of inner ball and preparation of cladding material includes an obliquely rotatable mixing barrel, a rotatable agitator is mounted in the mixing barrel; upon mixing, a rotation direction of the mixing barrel is contrary to a rotation direction of the agitator to allow the mixed materials in the mixing barrel to form a turbulence therein, thereby achieving an effect of agitating sufficiently and uniformly.

(49) In an embodiment of the present invention, in the step of the preparation of inner ball and forming of composite pellet, the pelletiser is a disc type pelletiser; feeding all the inner balls which is not meeting the process granularity requirement screened out in step (3) into a roller mill or damp mill for milling, and selectively adding the inner ball material according to a requirement for material humidity during milling, and then returning into the intensive mixer or damp mill in step (1) to form closed-loop circulation.

(50) In an embodiment of the present invention, in the forming method, the dryer used in the step of drying and solidification is a scale plate dryer, and the scale plate dryer is divided into a total of three drying stages in a delivery direction of the composite green pellets, including a low-temperature drying stage, an intermediate-temperature drying stage and a high-temperature drying stage.

(51) In an embodiment of the present invention, low-temperature hot air a range of 100-200 introduced into the low-temperature drying stage performs up-down air pumping or down-up wind blowing so that the low-temperature hot air vertically passes through a material layer and performs through-drying for the composite green pellets; the low-temperature hot air originates from exhaust gas discharged at a high-temperature hot air outlet of the high-temperature drying stage.

(52) In an embodiment of the present invention, intermediate-temperature hot air a range of 150-250 introduced into the intermediate-temperature drying stage performs up-down air pumping or down-up wind blowing so that the intermediate-temperature hot air vertically passes through the material layer and performs through-drying for the composite green pellets.

(53) In an embodiment of the present invention, high-temperature hot air a range of 200-350 introduced into the high-temperature drying stage performs up-down air pumping or down-up wind blowing so that the high-temperature hot air vertically passes through the material layer and performs through-drying for the composite green pellets.

EXAMPLES

(54) The embodiment provides a composite pellets used as a raw material in kiln phosphoric acid process in the present invention is a core-shell shaped structure with an inner ball encapsulated with a shell, the inner ball mainly consists of a inner ball material and a binding agent, and the shell mainly consists of a cladding material and a binding agent; the inner ball material mainly consists of a carbonaceous reductant powder, and phosphate ore powder and silica powder, the addition quantity of the binding agent in the inner ball is 6% (or 1%-10%) of the mass of the inner ball material; the addition quantity of the binding agent in the shell is 6% (or 1%-10%) of the mass of the cladding material; the inner ball is combined with the shell through the binding agent to form the core-shell shaped structure.

(55) The binding agent in the present embodiment is a mixed solution containing sodium humate, the sodium humate in the binding agent being 8% mass percentage concentration, the binding agent is obtained after mixing weathered coal (or peat or lignite) containing humic acid with sodium hydroxide solution, reacting and filtering. The phosphate ore powder of the composite pellets in the present embodiment is a product obtained after homogenizing the phosphate ore powder thar grinded and stored in the homogenization silo in a form of gap tape homogenization or continuous homogenization by using compressed air, and the homogenization value greater than 4; In the inner ball material, CaO/SiO.sub.2 mole ratio is 0.3 (less than 0.6 or greater than 6.5), and the amount of carbonaceous reductant powder is above 1.5 times a theoretical amount of P.sub.2O.sub.5 in phosphate ore powder; a mass ratio of carbonaceous reductant powder to silica powder in a cladding material is 2.5:1 (in a range of 1.5-5:1).

(56) A method of preparing the binding agent used in the present embodiment specifically comprises the following steps:

(57) (1) Preparation of Raw Material: selecting weathered coal (or peat or lignite) containing humic acid as a raw material, content of the humic acid contained in the decomposed coal in the present embodiment being above 40%; mixing caustic soda (93% sodium hydroxide) with water to obtain sodium hydroxide solution with 2% mass percentage concentration;

(58) (2) Ball-Grinding and Mixing: ball-grinding and mixing the above decomposed coal and sodium hydroxide solution with a 5:1 liquid-to-solid ratio for 20 min;

(59) (3) Synthesis Reaction: feeding the mixed material into a reaction tank with an agitator, starting the agitator, heating to 90 to perform synthesis reaction for 30 min;

(60) (4) Filtering: filtering the reaction products to obtain filtrate as the binding agent.

(61) As shown in FIG. 2, a forming of composite pellets method according to the present embodiment, specifically comprises the following steps:

(62) 1. Preparation of Inner Balls

(63) The carbonaceous reductant powder (the present embodiment selects coal powder above 200 mesh, e.g., coke powder, anthracite powder or petroleum coke), phosphate ore powder (above 150 mesh) and silica powder (above 150 mesh) are added to an intensive mixer according to a proportional requirement in composite pellets, and ingredients may be weighed by an electronic scale, and meanwhile a binding agent according to the present embodiment is added in the above-mentioned amount.

(64) As shown in FIG. 3, the intensive mixer used in the present embodiment comprises an obliquely rotatable mixing barrel, the mixing barrel comprises a casing 4 and a rotatable agitator 3 mounted in the barrel, a feed hopper 2 and a driving member 1 are disposed above the mixing barrel, a material turning plough 5 is disposed on one side of the barrel, a discharge port 6 is disposed on a bottom of the barrel; a working principle of the intensive mixer is as follows: upon mixing, a rotation direction of the mixing barrel is contrary to a rotation direction of the agitator (see FIG. 4); after the above raw materials are fed in the oblique rotatable mixing barrel, they rotate relative to the agitator which rotates inversely so that disperse mixed materials therein form a circulating material flow and thereby function to mix intensively; inverse rotation of the agitator and the mixing barrel may enable mixed materials to form a turbulence therein, thereby achieving an effect of agitating sufficiently and uniformly; materials are fed continuously into and discharged continuously out of the intensive mixer to ensure continuity of the production procedure.

(65) The mixed material which is mixed sufficiently and uniformly is fed into the material bin whose lower portion is mounted with a weighing and feeding device which weighs with an electronic scale, such weighing and feeding device may be a feeding device combining a disc feeding machine with an electronic scale, a weight weighed by the electronic scale is compared with a predetermined feed quantity; when a deviation occurs, a computer control system automatically adjusts a rotation speed of the disc of the disc feeding machine to make the quantity of the fed material equal to the predetermined feed quantity (other weighing and feeding devices having an electronic scale may be used directly).

(66) The mixed material which is mixed sufficiently and uniformly is fed by the weighing and feeding device into a disc pelletiser for pelleting treatment, the above-mentioned binding agent in the embodiment is additionally added in an application form of droplets and/or mist, the addition amount of the binding agent is 4%-6% of mass of the mixed material, and inner balls are obtained upon completion of the pelleting.

(67) 2. Preparation of Cladding Material:

(68) The aforesaid carbonaceous reductant powder and silica power are added to another intensive mixer according to proportional requirements, meanwhile the binding agent of the present embodiment is added according to the addition quantity of the composite pellets of the present embodiment, they are mixed sufficiently to obtain a cladding material; the intensive mixer in this step is identical with the intensive mixer used in step 1 in respect of the working principle and functional structure. The intensive mixer may also be replaced with a roller mill or damp mill which continuously feeds material and continuously discharges material.

(69) 3 The Forming of Composite Green Pellets:

(70) Inner balls getting out of a ball disc in step 1 are subjected to bi-layered roller-type screening treatment by using a bi-layer roller-type screening machine to obtain inner balls with a granularity meeting the process requirement, said inner balls are fed into another disc pelletiser for cladding treatment, meanwhile the cladding material obtained in step 2 is introduced in said disc pelletiser, the above binding agent is additionally added in an application form of droplets and/or mist during the cladding treatment, the addition amount of the binding agent is 4%-6% of the mass of the cladding material, an isolation layer separating a reduction zone from an oxidization zone is formed externally the inner balls upon completion of the cladding treatment to obtain the composite green pellets.

(71) After the by-layered roller-type screening treatment, unqualified inner balls greater than or smaller than a predetermined particle diameter are screen away and fed into the roller mill (or damp mill) for milling, during milling the inner ball material in the previous ingredient-formulating step is selectively added according to the roller mill's requirement for material humidity, and then returns into the intensive mixer in the above step 1 to form closed-loop circulation to sufficiently use the process raw material and reduce discharge and waste of the waste material during the process.

(72) The composite green pellets fabricated in the present embodiment has an anti-pressure strength 10N per ball, and a falling strength of about 10 times per 0.5 meter, and furthermore, a fluctuation range of CaO/SiO.sub.2 mole ratio in the composite green pellets in the present embodiment can be controlled less than 5%.

(73) 4 Drying and Solidification:

(74) The composite green pellets obtained after step 3 are delivered into a scale plate dryer for drying and solidification.

(75) As shown in FIG. 5 and FIG. 6, the scale plate dryer in the present embodiment comprises a drying furnace body 8, the drying furnace body 8 comprises a low-temperature drying stage 10, an intermediate-temperature drying stage 11 and a high-temperature drying stage 12, a hot air inlet 13 is disposed on top of the drying furnace body 8, an air outlet 15 is disposed on a bottom, a temperature preserving layer 14 is cladded peripherally, material loading carts 7 are disposed in a chamber of the drying furnace body 8, several material loading carts 7 are connected consecutively to form a ring, an air vent 16 is provided on the material loading cart 7, chain transmission is employed, the material loading carts 7 are dragged via a chain with seats to rotate cyclically to achieve a purpose of continuous drying and delivery. The bottom of the drying furnace body 8 is provided with a deduster 9 to collect and process fume and dust generated during the drying. During delivery of the composite green pellet materials, dry hot air is introduced from up to down in a direction perpendicular to the movement of the material to achieve the purpose of drying.

(76) A specific working principle of the scale plate dryer in the present embodiment is that a total of three drying stages, namely, low-temperature, intermediate-temperature and high-temperature drying stages are divided along a movement and delivery direction of the composite green pellets material loading carts 7. The composite green pellets first enter the low-temperature drying stage 10, low-temperature hot air of 130-200 is introduced in the low-temperature drying stage 10 through up-down air pumping or down-up air blowing so that the low-temperature hot air vertically passes through the composite green pellet layer and performs through-drying for the composite green pellets; the low-temperature hot air originates from exhaust gas discharged at the high-temperature hot air outlet of the high-temperature drying stage, and is introduced to the low-temperature drying stage 10 via a blower; the low-temperature drying stage 10, on the one hand, uses residual heat of low-temperature hot air discharged out of the high-temperature drying stage 12, and on the other hand, can effectively prevent damages to the pellets caused by burst of wet composite green pellets and ensure the quality of composite green pellets entering the kiln subsequently due to a lower gas flow temperature of the low-temperature drying stage 10. The composite green pellets after being dried in the low-temperature drying stage 10 then enter the intermediate-temperature drying stage 11 for drying, intermediate-temperature hot air of 220-250 is introduced in the intermediate-temperature drying stage 11 through up-down air pumping or down-up air blowing so that the intermediate-temperature hot air vertically passes through the material layer and performs through-drying for the composite green pellets; intermediate-temperature hot air not carrying moisture is introduced into the intermediate-temperature drying stage to form a higher humidity difference, which quickens the drying of the pellets in the case of ensuring the pellets do not burst. The composite green pellets after being dried in the intermediate-temperature drying stage 11 then enter the high-temperature drying stage 12 for drying, moisture content in the composite pellets finally entering the high-temperature drying stage 12 already falls below 4%, high-temperature hot air of 250-350 is introduced in the high-temperature drying stage 12 through up-down air pumping or down-up air blowing so that the high-temperature hot air vertically passes through the material layer and performs final drying for the composite green pellets. The high-temperature hot air of the high-temperature drying stage 12 preferably originates from use of residual heat of the exhaust gas in subsequent discharge-cooling stage of the rotary kiln, or a hot air furnace may be additionally arranged to supply hot air. The exhaust gas discharged by the low-temperature drying stage 10 and intermediate-temperature drying stage 11 may be collected by a fan, is dedusted by a deduster 9 and exhausted into the atmosphere after reaching the environment protection requirement.

(77) Moisture content in the composite pellets obtained after drying and solidification is controlled 1.0%, an average anti-pressure strength of the pellets reaches 250 KN per ball, and a falling strength reaches 20 times per meter, and it can be ensured that the composite pellets are not destroyed during subsequent rotation in the reducing rotary kiln so as to ensure smooth performance of the reduction procedure of the composite pellets.

(78) Among the composite pellets getting out of the dryer in the present embodiment, the composite pellets damaged during drying (those pellets with a diameter less than 5 mm) are screened via a vibrating screen to reduce the amount of powder material subsequently entering the rotary kiln and thereby delay a ringing cycle of the material in the high-temperature stage in the rotary kiln. The composite pellets getting out of the vibrating screen are fed via an air lock valve through a discharge duct from the kiln tail box of the rotary kiln to the rotary kiln for subsequent high-temperature reducing treatment.