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Pelletized rice hull ash and bentonite clay insulation

Abstrict

The present invention comprises rice hull ash which is pelletized subsequent to its admixture with a bentonite clay binder. A preferred product according to the present invention is pelletized rice hull ash which contains both bentonite clay and organic cornstarch binders. The bentonite clay contains approximately 58-68% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% Cao, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O. Insulator pellets according to the present invention, when placed atop molten steel in ladles or tundishes, insulate well and do not smoke or generate unwanted dust.

Claims

We claim:

1. A rice hull ash pellet product comprising an absorbent pellet consisting essentially of rice hull ash and bentonite clay binder wherein said bentonite clay binder contains approximately 58-64% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% CaO, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O, and further wherein the pelletized product contains about 87.5-96% by weight rice hull ash, about 3-8% by weight bentonite clay, up to 2% by weight cornstarch, and about 1-2.5% by weight water.

2. The rice hull ash pelletized product according to claim 1 wherein said cornstarch is present in the amount of about 1% by weight of the pellet.

3. The rice hull ash pelletized product according to claim 2 wherein said bentonite clay is present in the amount of about 8% by weight of the pellet and the pelletized product contains a maximum of about 15% by weight fines.

4. An absorbent pellet consisting essentially of rice hull ash and bentonite clay binder, wherein said bentonite clay binder contains approximately 58-64% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% CaO, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O, and further wherein the pelletized product contains about 87.5-96% by weight rice hull ash, about 3-8% by weight bentonite clay, up to 2% by weight cornstarch, and about 1-2.5% by weight water.

5. The absorbent pellet according to claim 1 wherein said cornstarch is present in the amount of about 1% by weight of the pellet.

6. The absorbent pellet according to claim 2 wherein said bentonite clay is present in the amount of about 8% by weight of the pellet and the pelletized product contains a maximum of about 15% by weight fines.

7. A method for preparing insulator pellets comprising:

a) admixing ingredients consisting essentially of 87.5-96% by weight rice hull ash, about 3-8% by weight bentonite clay, up to 2% by weight cornstarch and about 1-2.5% by weight water until pellets form, said bentonite clay binder containing approximately 58-64% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% CaO, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O; and

b) drying the pelletized admixture of step a) to yield a product containing a plurality of insulator pellets.

8. The method according to claim 7 wherein step b) further comprises the step of:

b) drying the pelletized admixture of step a) at about 600.degree. F. for 2-6 min.

Description

FIELD OF THE INVENTION

The invention relates to low-cost, lightweight inexpensive pelletized insulation material particularly suitable for insulating molten steel in ladles and tundishes.

BACKGROUND OF THE INVENTION

Rice hull ash has been used widely, in steel mills, as an insulating cover on tundishes and ladles containing molten steel. The rice hull ash is a good insulator because it is inexpensive, it flows over and covers the steel surface well, and it does not get crusty or lumpy during use. As a commodity, rice hull ash is produced by Agrilectric in Lake Charles, La., as the end product of the combustion of rice hulls in a boiler which feeds a power plant.

The major problem with rice hull ash is that, because of its low bulk density (15 to 20 lb/ft.sup.3) and small particle size, some of the ash becomes airborne when it comes in contact with hot metal. The resulting dust is believed not to be a health hazard, but it is annoying to operating personnel.

Previous attempts to improve the properties of rice hull ash for molten steel insulation have met with limited success. Although it is known to produce rice hull ash briquettes with 1) sodium silicate, 2) starch, 3) cement dust/starch and 4) lime/molasses binders, these briquettes do not break down (i.e., spontaneously subdivide into smaller pieces), spread or insulate well when transferred to the surface of molten steel. It is also known to produce rice hull ash pellets with an industrial molasses binder, and such pellets do break down, spread and insulate well on the molten steel surface. However, pellets containing molasses binder smoke excessively when exposed to liquid steel temperatures. Applicants are also aware of other rice hull ash pelletizing attempts which have failed, which failure is probably also attributable to the composition of the binder. These unacceptable binders include lime, lime/lignosol admixtures, starch, starch/lignin admixtures, and wood pulp. (For example, U.S. Pat. No. 4,440,575 to Daussan et al. discloses pelletizing rice hull ash with wood pulp or with admixtures containing a starchy binder and clay. Unfortunately, these and all other organic binders smoke excessively during use, yielding an unsatisfactory insulation material.)

Accordingly, a need remains for a pelletized rice hull ash product which overcomes the disadvantages of the prior art.

SUMMARY OF THE INVENTION

In order to meet this need, the present invention comprises rice hull ash which is pelletized subsequent to its admixture with a bentonite clay binder. A preferred product according to the present invention is pelletized rice hull ash which contains both bentonite clay and organic cornstarch binders. The bentonite clay contains approximately 58-64% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% CaO, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O. Insulator pellets according to the present invention insulate well on the molten steel surface, and do not smoke or generate unwanted dust.

DETAILED DESCRIPTION OF THE INVENTION

Addition of bentonite clay (58-64% SiO.sub.2, 18-21% Al.sub.2 O.sub.3, 2.5-2.8% Fe.sub.2 O.sub.3 /FeO, 2.5-3.2% MgO, 0.1-1.0% CaO, 1.5-2.7% Na.sub.2 O, 0.2-0.45% K.sub.2 O, and 4.5-9.0% free H.sub.2 O) to rice hull ash prior to pelletizing yields a product with outstanding characteristics, yet with low cost comparable to prior art rice hull ash insulating materials. A preferred product according to the present invention is a pelletized rice hull ash which contains both bentonite clay and organic cornstarch binders. On a final pellet weight basis, pellets according to the present invention contain about 87.5-96% by weight rice hull ash, about 3-8% by weight bentonite clay, about 2% or less by weight cornstarch, and about 1-2.5% by weight retained water.

High speed, high impact agglomeration process equipment is necessary to prepare the final pelletized rice hull ash product. Two suitable mixing processes are identified in the Examples, below. Batch or continuous mixing may be used, and a fluidized bed dryer, turbo-dryer or other equipment can remove the water necessary to yield the final product. However, mixing of the constituents must take place with an adequate (but not excessive) amount of water. During mixing, between 40 and 50%, preferably 41-42%, by weight of the rice hull ash/bentonite clay/cornstarch/water admixture must be water. With the correct amount of water, pellets form with an average pellet size of about 8 mesh (0.093 inch, 2380 microns) and a pellet size distribution of about 6-12 mesh.

Pellets are ordinarily packed in 20 lb. bags, which can be transported to the ladle or tundish site. Insulator pellets according to the present invention also have utility in other short- and long-term insulation applications. In addition to insulation applications, the present pellets are highly absorbent to liquids. The present compositions are therefore well suited to toxic and hazardous waste cleanup applications, because they do not create dust problems.

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