SINTERED SPHERES, PROCESS FOR THEIR PRODUCTION AND USE THEREOF

Abstract

Described herein are sintered spheres, obtained from red mud, comprising at least aluminium oxide, iron oxides, silicon oxide, and titanium oxide, characterized in that the roundness and the sphericity of the sintered spheres is higher than 0.6.

Described is further a process for the production of sintered spheres, comprising the following steps: a) providing red mud, being a residue from alumina production, b) optionally adjusting the pH value of the red mud to a value lower than 9, c) granulating the red mud from step b) under continuous drying, d) sintering the granulate from step c).

Described herein is also the use of sintered spheres as proppant in fracking processes or as aggregate or lightweight fine aggregate (LWFA) for construction purposes or for geological solidification processes.

Claims

1. Sintered spheres, obtained from red mud as starting material, comprising at least aluminium oxide, iron oxides, silicon oxide, and titanium oxide, characterized in that the starting material contains at least 70% (w/w) red mud, calculated on the dry mass, and that the roundness and the sphericity of the sintered spheres is higher than 0.6.

2. Sintered spheres, according to claim 1, characterized in that the size of the sintered spheres is in the range of from 0.2 mm to 1 mm, and/or that the water absorption is in the range up to 10% (w/w) and/or in that the bulk density is in the range from 1,100 to 1,400 kg/m.sup.3.

3. Sintered spheres, according to claim 1, characterized in that the spheres further comprise additives and/or binders, wherein the additives are selected from the group consisting of feldspar minerals, alumina minerals, calcinated alumina minerals, clay minerals, or silicate minerals, or mixtures thereof, and wherein the binder is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, methyl cellulose, dextrine and molasses.

4. Process for the production of sintered spheres, according to claim 1, comprising the following steps: a) providing red mud, being a residue from alumina production, b) optionally adjusting the pH value of the red mud to a value lower than 9, c) granulating the red mud from step b) under continuous drying, d) sintering the granulate from step c).

5. Process, according to claim 4, characterized in that a step b1) is performed after step b), wherein additives and/or binders are admixed to the red mud, wherein red mud is present in a quantity of at least 70% (w/w), the rest being additives and/or binders, and wherein the additives are selected from the group consisting of feldspar minerals, alumina minerals, calcinated alumina minerals, or silicate minerals, or mixtures thereof, and wherein the binder is selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, methyl cellulose, dextrine and molasses, or a mixture thereof.

6. Process, according to claim 4, characterized in that in step b) a slurry is formed form the red mud by adding water, and wherein the ration of red mud and water is the range from 35/65 to 65/35 (w/w).

7. Process, according to claim 4, characterized in that after step b) the red mud is dried.

8. Process, according to claim 4, characterized in that after step c) the process is stopped and the obtained product isolated as an intermediate product.

9. Intermediate product for the preparation of sintered spheres comprising non-sintered spheres, comprising at least iron oxides, titanium oxide, and aluminium oxide, characterized in that the roundness and the sphericity of the non-sintered spheres is higher than 0.6.

10. Intermediate product, according to claim 9, characterized in that the density of the non-sintered spheres is in the range of from 800 to 1,000 kg/m.sup.3.

11. Use of an intermediate product, according to claim 9, for the production of sintered spheres, obtained from red mud as starting material, comprising at least aluminium oxide, iron oxides, silicon oxide, and titanium oxide, characterized in that the starting material contains at least 70% (w/w) red mud, calculated on the dry mass, and that the roundness and the sphericity of the sintered spheres is higher than 0.6.

12. Use of an intermediate product, according to claim 9, for geological solidification processes or as landfill material.

13. Use of sintered spheres according to claim 1, as proppant in fracking processes or as aggregate or lightweight fine aggregate for construction purposes or for geological solidification processes.

Description

[0050] The present invention is explained in detail by the attached figures.

[0051] FIG. 1 shows photomicrographs of a first intermediate product according to the invention in different image magnifications;

[0052] FIG. 2 shows photomicrographs of a second intermediate product according to the invention in different image magnifications;

[0053] FIG. 3 shows photomicrographs of a third intermediate product according to the invention in different image magnifications;

[0054] FIG. 4 shows, for comparison purposes, photomicrographs of an intermediate product not according to the invention in different image magnifications;

[0055] FIG. 5a shows a photomicrograph of sintered spheres according to the invention from wet admixture, and

[0056] FIG. 5b shows a photomicrograph of sintered spheres not according to the invention from dry admixture as known in the art.

[0057] The manufacturing process for sintered spheres according to the invention involves three key steps: [0058] a) Mixing of raw materials; [0059] b) Granulation; and [0060] c) Sintering.

[0061] The granulation process is performed as a granulation from a wet admixture. Before granulation, sieving was performed to remove any coarser particles exceeding 100 m which would be detrimental to the granulation process.

[0062] For sintering, a gas fired direct heated kiln was used for the trial. The maximum temperature of the kiln can reach up to 1,400 C.

[0063] The process according to the invention described herein provides a great range of products that can be produced. The process according to the invention involves some essential parameters which allow the tailoring of the product in respect to the requested needs.

[0064] Granulation from slurry can be performed with and without binder. Without a binder, granules of smaller size (<300 m) were produced. In order to increase granules size and granulation (growth) process a binder was used. With 2% (w/w) binder, the granular growing and the continuous new formation of granular particles were observed. However, with further increase in binder to 5% (w/w), rough particle surface granules were produced (see FIG. 4).

[0065] Granules produced from wet admixture have a bulk density of 800-900 kg/m.sup.3. But, the resulting bulk density from the granulation process can be adjusted by the parameters used in the granulation process, i.e. an intensive granulation process produces compact granules resulting in higher bulk density.

[0066] Granules produced from slurry are up to 30% lighter than conventional frac sand and ceramic proppants. The lightweight granules (proppants) will help to avoid premature settling in the down-hole placement. The additional benefits from lightweight proppants are, i.a. reduction of consumption of costly gels or polymers; lower consumption of high viscosity gels allows to use low viscosity proppant carrier frac fluid; lower viscosity carrier fluid allows to use slower pump rate, which minimises pipe friction and disturbance of lower lying fluid; minimise equipment, time and personnel required for chemical mixing; low transportation costs, both on land and offshore.

[0067] Granules produced from the slurry were having an initial mean grain size in the range from 500-600 m. By varying physical parameters of the granulating process and by the use of binders, grain size could be improved to the desired upper range (600-900 m). The process used also allows to divide the process into two steps: from fine up to 300 m and 300 m to 900 m. This shows that any present demand of users can be fulfilled.

[0068] Surprisingly it was found that slurry based granules are very round and spherical. The sphericity and roundness are very high (both >0.9) compared to commercially available ceramic proppants in the market. This is one of the critical factor in shale gas recovery economics. This makes the products according to the invention very useful as proppant.

[0069] In order to demonstrate the advantages of the wet admixture for granulation according to the invention a comparative example has been performed. Due to high plasticity of red mud granulation was possible even with 100% red mud, when using the wet admixture according to the invention. This is shown in FIG. 5a. In contrast, granulation from dry admixture, as known in the art, was relatively easy. However, fine stones inside the admixture they were destroying granules and at the same time generating less spherical granules.

[0070] The resulting granules are shown in FIG. 5b. This comparison shows the advantages of the wet admixture granulation process according to the invention. This wet admixture granulation process according to the invention makes it possible to use 100% red mud for the granulation process and convert the red mud into granules with high sphericity and roundness.

[0071] Another parameter is the amount and the type of additive to be used for admixing with red mud. In the presence of additives (up to 30% (w/w)), no significant granular effect was observed. Comparing with 0% additives, a marginal reduction of mean grain size and smooth surface was observed. This is due to the fact that additives have a lower plasticity which results in slow granular growth. Very fine nature of additives gives a smoother surface. Additives that can be used according to the present invention are also known in the art. These additives include different types of minerals. Useful for performing the teaching of the present invention are minerals that are selected from the group consisting of feldspar minerals, alumina minerals, calcinated alumina minerals, or silicate minerals, or mixtures thereof. It is believed that the used minerals improve the strengths of the produced sintered spheres. The addition of minerals can also influence the granulation process according to the invention. It is therefore possible, without deviating from the teaching of the present invention, to adjust the sintered spheres produced to the needs of the consumer in respect to different purposes.

[0072] Furthermore, the amount and the type of binder to be used is also an essential parameter. Binders are used in the art for converting red mud into fracturing propping agents. Binders have the effect to support and enhance the granulation process. Useful binders as known in the art are for example polyvinyl alcohols, polyvinyl acetates, methyl cellulose, dextrin, and molasses. Further binders may also be used and are part of the present invention, as far as they support and enhance the granulation process.

[0073] Surprisingly it has been found that the amount of binder should be below 5% (w/w). Advantageously an amount of up to 2% (w/w) of binder should be used in the process according to the present invention.

[0074] It has to be pointed out that the parameters chosen determine the kind of product (lightweight fine aggregate or proppant) and their properties (bulk density, sphericity, roundness) of the product.

[0075] As already described herein, one main object of the present invention is to provide an intermediate product that can be used for the production of the final material, the sintered spheres according to the invention. It has surprisingly been found that the intermediate product already shows all important parameters of the sintered spheres. These parameters are especially sphericity and roundness. Depending on the process parameters used during the process, intermediate products can be produced and tested during the production process in respect to the parameters that are essential for the final product. The intermediate product can be stored and handled after being produced and therefore serve as an intermediate product. Using the intermediate product the final product, the sintered spheres can easily be prepared by a sintering process. This means that in the case that red mud lagoons have to be rehabilitated the process of mixing and granulating may be performed close to the lagoons, while the further process of sintering may be performed elsewhere. Therefore, respective plants like kilns have not to be positioned near the lagoons to be rehabilitated. The intermediate product is therefore a key feature of the present invention. The intermediate product comprises the same compounds as the sintered spheres according to the invention. Intermediate products are non-sintered spheres that differ from sintered spheres in that they are not yet sintered.

[0076] The teaching of the present invention is illustrated by examples described herein. Table 1 shows the composition of intermediate products according to the invention that are described in the respective examples.

TABLE-US-00001 TABLE 1 Composition of intermediate products Example Amount Amount no. Additive % (w/w) Binder % (w/w) 2 (FIG. 1) none 0 none 0 3 (FIG. 2) none 0 PVA 2 4 (FIG. 3) clay mineral 9.8 PVA 2 5 (FIG. 4) none 0 PVA 4.8

[0077] Essential physical parameters and properties of the sintered spheres according to the invention are presented herein already. Based on the type of admixture the sintered spheres according to the invention can be produced in a wide range in respect to their bulk density. The type of admixture has also an impact on the roundness and sphericity of the sintered spheres. It is therefore possible to prepare sintered spheres with different physical and/or chemical properties by simply modifying essential process parameters.

[0078] In order to determine if the physical parameters of the sintered spheres according to the invention are sufficient in order to fulfil the requirements for us as frac sand or proppant, the same have to be compared with the specification given in the art. API (American Petroleum Institute) has laid out specifications that have to be fulfilled. Some of the physical properties are defined in ISO 13503-2. The definition of sphericity and roundness can be found in those specifications as well.

[0079] The teaching of the present invention provides a great range of advantages in respect to environmental challenges. According to the teaching of the present invention it is possible to use red mud for the production of different types of materials for use in fracking technology, as aggregate or sand for building purposes and for landfill use. By using the process according to the invention it is possible to produce a wide range of products that can be used in many applications.

[0080] The following examples explain the invention in more details. It is not intended that the examples restrict the scope of the invention.

[0081] It is clear, that the given examples do not limit the scope of the invention to the presented examples. The examples are only mentioned for demonstrating the scope of the invention. Without deviation from the principle idea of the invention it is possible for a person skilled in the art to perform the invention in order to obtain further embodiments that are also within in the scope of the invention.

General Procedures

[0082] For the moisture analysis the moisture detector MA100 (Sartorius AG) with halogen rays at 105 C. in automatic modus was applied. The bulk density was measured in accordance to DIN ISO 697 and EN ISO 60 using a 100 ml vessel. For the visual investigation of the granules the light optical microscope Technival 2 (Carl Zeiss Jena) was used. The data of the sieving analysis were measured by using the particle size analyzer Camsizer XT (Retsch Technology GmbH, Germany).

EXAMPLE 1

[0083] Preconditioning of Red Mud as Starting Material

[0084] Red mud sample was supplied which has moisture of >30%. The red mud sample was treated with sodium hydroxide solution to a pH value of 9 and then was dried in oven at 120 C. overnight to remove the moisture. After drying it was gently crushed and ground to less than 1 mm in size. Following this, the sample was sieved using a 1 mm mesh. This product served as a starting raw material for both wet admixture based granulation as well as dry admixture based granulation for comparison purposes (Example 7). The bulk density of the material after crushing and drying (<1% (w/w) moisture) was 950100 kg/m.sup.3.

EXAMPLE 2

[0085] Production of Intermediate Product Directly from Red Mud Slurry

[0086] Preconditioned red mud from Example 1 was used. The granulation was performed using a fluid bed technology and by a continuous spray granulation.

[0087] This spray liquid used for granulation contained about 50% solid material, the rest of the liquid being water.

[0088] The process yielded a round shaped product with a measured bulk density of 1023 kg/m.sup.3 and a residual moisture of approximately 2% (w/w).

[0089] The photomicrograph of the product is shown in FIG. 1.

EXAMPLE 3

[0090] Production of Intermediate Product from Red Mud by Addition of Binder

[0091] Preconditioned red mud from example 1 was used. The granulation was performed in the same manner as given in Example 2. Polyvinyl alcohol (PVA) in a concentration of 2% (w/w) was added to the spray liquid used. The spray liquid used contained about 50% (w/w) solid material the rest of the liquid being water.

[0092] The process yielded to a round shaped product with a measured bulk density of 805 kg/m.sup.3 and residual moisture of 3.1% (w/w).

[0093] The photomicrograph of the product is shown in FIG. 2

EXAMPLE 4

[0094] Production of Intermediate Product from Red Mud by Addition of Binder and Additive

[0095] Again, preconditioned red mud from Example 1 was used. Clay mineral in an amount of 9.8% (w/w) was blended with the preconditioned red mud. Polyvinyl alcohol in a concentration of 2% (w/w) was added to the spray liquid used. The spray liquid used contained about 50% (w/w) solid material the rest of the liquid being water.

[0096] The process yielded to a round shaped project with a measured bulk density of 813 kg/m.sup.3 and residual moisture of 3.7% (w/w).

[0097] The photomicrograph of the product is shown in FIG. 3.

EXAMPLE 5 (COMPARATIVE EXAMPLE)

[0098] Production of Intermediate Product from Red Mud by Addition of Binder in Higher Concentration

[0099] The present example is performed in the same manner as given in Example 3, but polyvinyl alcohol (PVA) was used in a concentration of 4.8% (w/w).

[0100] The process yielded to a round shaped project with a measured bulk density of 650 kg/m.sup.3 and a residual moisture of 5.3% (w/w).

[0101] The photomicrograph of this product is shown in FIG. 4.

[0102] It is apparent from the photomicrograph that the roundness and the sphericity of the product according to Example 5 is not in the same range as in the Examples 2 to 4. This shows that the amount of the binder added during the granulation process is critical in respect to the properties of roundness and sphericity.

EXAMPLE 6

[0103] Production of Sintered Spheres from Intermediate Products

[0104] The sintering process for the intermediate products as given in Examples 2 to 4 has been done in a gas fire direct heated kiln. The maximum temperature of the kiln was 1400 C.

EXAMPLE 7 (COMPARATIVE PRODUCT)

[0105] Production of Sintered Spheres Using Dry Admixture

[0106] Preconditioned red mud from example 1 was used and optionally admixed with additives. Granulation by conventional means could easily be achieved. However, due to fine stones inside the mixture they were destroying granules and at the same time generating less spherical granules. Due to high plasticity of the red mud granulation was possible even with 100% red mud.

[0107] This sintering process is performed in the same way as given in Example 6. In FIGS. 5a and 5b granules from wet admixture and dry admixture are presented as photomicrographs. It is apparent that roundness and sphericity are depending on the type of the admixture.

[0108] The inventor likes to point out that the properties of the final products can be adjusted by the use of additives and/or binders according to the invention.

[0109] The use of the binder is essential to achieve a high value of roundness and sphericity. But, the amount of binder used has a maximum, which can be easily found out be a few number of experiments, as the optimum amount is also depending from the origin of the starting material, the red mud.

[0110] On the other hand, the use of additives is essential for the physical properties like hardness and bulk density. The amount of additives used for the production of sintered spheres according to the invention may also to be determined by experiments.

[0111] This means that the prospected properties of the sintered spheres and the intermediate product according to the invention can easily be achieved by carrying out a small number of experiments in order to find the optimum composition of red mud, additives and binders, wherein the amount of red mud is at least 70% (w/w), based on the dry mass, and the amount of bounders and additives form the rest to yield 100%.

[0112] The inventor provides a simple and comprehensive method to convert red mud, being an environmental harmful waste material, into sintered spheres with valuable properties, useful as proppant, as aggregate or for landfill purposes.