GB2079261A - A process for the production of sintered bauxite spheres - Google Patents

Abstract

High strength sintered bauxite spheres usable as fracture propping agents in oil or gas wells are produced by continuous spray-granulation of an aqueous binder-containing bauxite suspension to form granules which are subsequently sintered to high strength and high density spheres.

Description

SPECIFICATION A process for the production of sintered bauxite spheres Background of the Invention It is well known that the productivity of an oil or gas well may often be increased by a procedure which involves creating a fracture in the subterranean formations surrounding a well and propping the fracture opening by filling it with granular material called propping agent. Methods of this type are disclosed in US patent specifications Nos. 3701 383 and 4068718.

A survey of propping agents and their manufacture is given in the specification to US patent application serial 914 647 filed on the 6th December 1978 (corresponding to DE-OS 2921336).

A granular material must fulfil several conditions to be suitable for use as a propping agent. The material must have high strength to avoid crushing of the particles when exposed to high pressure during their application. The shape of the individual particle should depart as little as possible from the spherical one and the particle size distribution should be within defined, relative narrow limits to insure sufficient gas and oil permeability of fractures propped with the propping agent. Moreover, the particles should be able to resist the corrosive conditions to which they may be exposed at their application.

The material regarded as most suitable for fulfilling these conditions is sintered bauxite pellets.

Severai methods have been proposed for producing sintered bauxite particles. The process which has hitherto found widest commercial success is the one described in the above US application seriai 914647.

According to said methods bauxite spheres are first prepared by agglomeration of a mixture of bauxite, temporary binder and water in an intensive mixer to produce spheres called green pellets, which are afterwards sintered by heating.

In the embodiment exampies of said application the products have a typical density of 3.7 g/cm3, while the crushing loss measured by the method described below was 8.16% and 6.8% resp.

It is however, a drawback of said process that the granulation, which is carried out in an intensive mixer, can only be performed batchwise.

Summary of the Invention It is, however, generally recognized that usualiy a continuous process is to be preferred to a batchwise process when a large-scale industrial production is concerned.

It is therefore an object of the invention to provide a process for the production of sintered bauxite spheres comprising steps all of which may be performed continuously.

Since any crushing of the bauxite spheres by their use as fracture propping agents involves an impaired permeability of the propped fracture it is desired to obtain a strength of spheres corresponding to a crushing loss even lower than the results obtained according to said US application 914647.

Consequently it is another object of the invention to provide a process resulting in bauxite spheres having a crushing loss measured by the below defined method of less that 5%.

In the above prior.art process using an intensive mixer the possibilities are rather limited for controlling the granulometry of the product, e.g.

because a certain minimum mixing intensity will always be necessary to secure homogeneity of the individual particles. This means that a product having a very narrow particle size distribution, as is often required for propping agents, can only be prepared by simultaneous production of a substantial amount of particles being too small or too large, which necessitates extensive sieving operations and impair total process economy.

Therefore it is a further object of the invention to provide a process which may easily be adapted to produce a major fraction of particles complying to varying requirements as to particle size distribution.

The above objects are according to the invention achieved by a process for the production of sintered bauxite spheres having a particle size rangefrom 0.4 to 2.5 mm, or a more narrow particle size range within said limits suitable for use as fracture propping agent in oil wells, which process comprises the steps of (a) preparing an aqueous feed suspension comprising bauxite and a binder, (b) continuously atomizing said feed suspension into a layer of already partly dried bauxite particles fluidized in a stream of drying air, (c) continuously recovering particles from said layer (d) continuously separating said recovered particles in oversize, undersize and product fractions, making allowance for shrinkage in the subsequent sintering operation, (e) continuously recycling material selected from the group consisting of undersize fractions, relative fine product fractions, ground product fractions and ground oversize fractions, to the layer of fluidized particles at a site at substantial distance, measured along the flow path of the particles from the site where said recovering or particles takes place, and (f) drying and sintering the non-recycled product fractions by heating them at a temperature between 1 200 and 1 6500C.

A method involving steps in principle corresponding to the above steps (b)-(e) is called fluidized spray granulation and has been suggested for the granulation of various inorganic and organic products. Spray granulation has, however, not been suggested for the manufacture of green proppant pellets and the suitability of the method for this specific purpose could in no way be predicted. In fact it is rather surprising that a perfect spherical shape of the pellets can be obtained, considering the very rapid evaporations of the atomized suspension in the process.

In the present specification and in the accompanying claims "bauxite" is used in the widest sense of the word comprising also very low grade materials. When high strength propping agents are desired the starting material should, however, preferably have a relative high alumina content.

All steps of the process of the invention may be carried out in a continuous manner, which makes the process especially attractive for large scale proppant manufacture.

By using the steps (a)-(e) it is possible to obtain pellets of spherical shape and of sufficient strength for handling prior to and during the final drying and sintering process. After sintering these pellets form propping agents of higher strength than described in the prior art.

Description of the Drawing The invention is further elucidated with reference to the drawing, which is a schematic flow sheet illustrating an embodiment of the process.

Description of the Invention On the drawing a feed tank is designated 1. In this tank an aqueous feed suspension comprising bauxite and a binder is prepared. Preferably the feed contains 40-60, more preferably approximately 50, % by weight bauxite and preferably 1-5, more preferalbly 1--2.5, % by weight binder. The bauxite should preferably have a particle size below 20 micron which is conveniently achieved by a wet grinding process which is less energy consuming than dry milling prescribed in connection with some of the prior art processes. The preferred binders are polyvinyl acetate, polyvinyl alcohol, methylcellulose, dextrin and molasses.

The function of the binder is to provide green strength to the pellets until the sintering thereof.

During the sintering most of the binders coming into consideration will decompose. This means that a relatively high amount of binder will impair the strength of the final sintered product, for which reason binders are preferred which exhibit a sufficient temporary binding ability even when used in small amounts.

Also further auxiliary agents may be added to the feed, such as dispersing agents, e.g.

ammonium citrate.

From the tank 1 the feed is led to a pump 2 feeding atomizing nozzles 3 arranged in a fluidized bed unit 4.

Between the feed tank 1 and the nozzles 3 may be inserted a grinding mill and/or sieve (not shown), to prevent that too coarse particles reach the nozzles and the fluid bed.

The atomizing nozzles 3 are pressure nozzles of conventional design, or two-fluid nozzles. The design of such nozzles is well known e.g. from K. Masters: "Spray Drying Handbook", John Wiley and Sons, New York (1979).

The fluid bed unit 4 is of conventional design as described in e.g. US patent specification 3 533 829 and in British patent specification 1 401 303.

In the illustrated embodiment a fluidized particle layer 5 is supported by a perforated plate 6 through which hot fluidizing gas is flowing. Said hot gas is introduced in the bottom part of the fluid bed unit by means of a fan 7 and an air heater 8.

The distance from the atomizing nozzles 3 to the perforated plate 6 is adjustable and the nozzles are preferably positioned a rather short distance above the surface of the fluidized particle layer 5. The exact position of the nozzle will in each individual case be fixed with due regard to the fact that too long distance from the nozzles to the surface of the fluidized layer caused undesired dust formation, because the atomized feed droplets will be dried to too high an extent, before they reach the fluidized layer, while a too short distance on the other hand results in formation of irregular and too coarse particles. Therefore, the position of the nozzles is adjusted on the basis of analyses of powder taken out from the fluid bed unit.

The velocity of the fluidizing and drying air passing the layer 5 is preferably 0.9-1.5 m/sec and the height of the fluidized particle layer will typically be 20-60 cm.

The temperature of the drying- and fluidizing air when introduced at the bottom part of the fluid bed unit 4 will preferably be 250--6500C, more preferably 400--6000C.

When leaving the fluid bed unit the temperature of said air is preferably below 1 000C, more preferably approximately 700 C.

From the fluid bed unit the air entraining dust consisting primarily of fine bauxite particles are led to a collector unit 9, which may for instance be an electrostatic precipitator, a cyclone, a bag filter or a wet scrubber or a combination thereof. The collector unit 9 may e.g. comprise a cyclone wherein coarse particles are collected and a wet scrubber for collecting fine particles. This enables recycling of the coarse particles to the fluidized layer 5 and of the fine particles to the feed tank 1.

it is essential that the fluid bed unit 4 is designed to give a long and uniform residence time for the particles to obtain a desired particle size distribution and the desired spherical shape of the product. Therefore, the flow of particles in the fluidized layer should be of the type conventionally termed plug flow, which is a flow pattern wherein very little back mixing takes place. Thereby an equal treatment of all particles is secured.

In a fluidized particle layer plug flow of the particles may be obtained by various measures. In the embodiment shown on the drawing the desired flow pattern is obtained by introducing powder particles serving as seeds or nuclei through a powder inlet 10 in one end of the fluid bed unit 4 and removing particles from the fluidized layer 5 through an outlet 11 situated at the opposite end of the fluid bed unit. Alternatively plug flow may be obtained by using guide walls in the fluid bed as is well known in the art.

The seed or nuclei particles introduced through powder inlet 10 consist of recycled material as will be explained below.

Alternatively to the illustrated embodiment the fluid bed unit may comprise two or severai compartments in which different conditions prevail as to fluidizing air velocity, temperature and slurry feeding conditions. Such fluid bed unit having more than one compartment are well known in the art, and may e.g. have a circular perforated plate and radian partitions preventing back mixing.

Through the powder outlet 11 is withdrawn a powder having a moisture content of 15% which powder via a rotary valve 12 is conducted to a sieving unit 1 3 wherein it is separated into three or more fractions, viz. an oversize fraction, one or more product fractions (in the embodiment shown: two fractions) and an undersize fraction.

The oversize fraction is conducted to a grinding unit comprising a mill 14 and a sieve 1 5 which may possibly be combined. Oversize material are recycled from the sieve 15 to the mill 14 and fractions having preferably a size of app. 0.5 mm are, in the embodiment shown, led to the powder inlet 10 of the fluid bed unit together with the fine fraction. In case the quantity of material of these two fractions is not sufficient to form seed or nuclei material for the fluid bed a part of the product fraction or of one of the product fractions may be added thereto as indicated by the dotted line in the lower part of the drawing. A part of or the total amount of recycled product fraction may be ground before being introduced to the fluid bed, as indicated on the drawing.On the other hand, if the amount of material in the oversize fraction together with the undersize fraction is higher than what is required as seed or nuclei material, a part thereof may be added to the feed tank 1, as illustrated by the dotted line in the left upper part of the drawing.

Non-recycled product fraction or fractions are led to a drying oven 1 6 wherein residual moisture and organic additives are evaporated and thereafter to a kiln 17, e.g. a rotary kiln, wherein the particles are sintered to form high strength spheres suitable as propping agents. The firing process taking place in the kiln 1 7 is conducted under the same conditions as those used in the prior art processes in which an agglomeration has been performed in a mixing apparatus.

The dust recovered from the collector unit 9 may suitably be recycled to the feed tank 1 as shown, but it might also be possible to use at least the coarse fraction thereof as seed in the fluid bed.

The size limits for the product fractions separated in the sieving unit 1 3 must be fixed with due regard to the fact that in the subsequent firing process in the kiln 1 7 a substantial shrinkage takes place. The extent of this shrinkage depends on the origin of the original bauxite and may typically amount to app. 25% on linear basis.

As it appears the process may be performed on a continuous basis and it is very suitable for being automatically controlled using a minimum of manpower.

As mentioned above the resulting propping agent has higher density and higher crushing strength than obtained in the embodiment examples of the above mentioned US-application.

The crushing strength is evaluated by a method in which the fraction between app. 600 microns and app. 700 microns is placed in a 158 inch diameter steel cylinder, and pressure is applied to the sample through a plunger fitting the top of the cylinder according to the following schedule: 1 minute to 700 kg/cm2, 3 minutes hold at this pressure and 1 minute down to 0 pressure.

Afterwards the amount of material having a particle size below 600 microns is measured and expressed as % of the total amount. The result is recorded as the weight % crushing loss.

The invention is further elucidated by means of the following embodiment example.

EXAMPLE 1 The process is carried out in a plant corresponding to the one illustrated on the drawing.

In the feed tank a feed suspension is prepared from water, fresh bauxite, recycled bauxite dust from the collector unit and the below indicated auxiliary agents in amounts giving a total solids content of the feed suspension of app. 509/0 by weight, which solids consist of ca. 98% bauxite, 1% polyvinyl alcohol and 0,3% ammonium citrate.

This feed is in an amount of 4000 kg/hour atomized through the pressure nozzles 3 in a fluid bed unit in principle designed as shown on the drawing and having a fluidizing area of 3 m2.

The velocity of the fluidizing air is 1.2 m/sec and the inlet temperature of the air is 5500C while the outlet temperature is 700 C. Recycled material introduced through the powder inlet 10 amounts to 1 700 kg/hour. The height of the fluidized particle layer 5 is app. 35 cm.

The average residence time of the particles in the fluidized layer may under these conditions be estimated to 1 5 minutes.

Through the outlet 11 material is withdrawn in a quantity of 3400 kg/hour, which by sieving is separated in an oversize fraction having a particle size above 2.1 mm (50 kg/hour), a coarse product fraction having a particle size between 1.2 and 2.1 mm (300 kg/hour), a fine product fraction having a particle size between 0.6 and 1.2 mm (2450 kg/hour) and an undersize fraction having a particle size below 0.6 mm (600 kg/hour).

In the collector unit 9 which is a bag filter 300 kg/hour entrained particles are collected and recycled to the feed tank 1.

The total amount of the oversize fraction together with 400 kg/hour of the fine product fraction is ground in a grinding unit having a sieve of mesh size 3000 microns, and together with the undersize fraction led to the fluid bed unit as seed or nuclei particles. 650 kg/hourfine product fraction is recycled to inlet 10 without prior grinding.

The remaining material from the product fractions is led through an oven in which the remaining moisture and organic additions (app.

totally 4% by weight) are removed and afterwards the sintering is performed in a rotary kiln at a temperature of app. 1 5000 C. The residence time of the particles at this temperature is app. 10 minutes.

The sintered particles is subject to a further sieving operation to assure that substantially all the product has a particle size between 0.4 and 1.5 mm. The sphericity of the particles is excellent and their density app. 3.8 g/cm3. The crushing strength according to the above method is 1.5% by weight, for which reason the product must be regarded as being excellently suitable as a propping agent.

EXAMPLE 2 In this embodiment the process is performed in a plant which only departs from the one used in Example 1 by having a sieving unit 13 which separates the particles in only three fractions (viz.

an oversize fraction, a product fraction and an undersize fraction) and by having a collector means 9 comprising a cyclone collecting particles coarser than app. 100 microns and a wet scrubber collecting the finer particles as an aqueous slurry.

Also in this embodiment the total solids content of the feed suspension is app. 50% by weight and the quantity atomized is 4000 kg/hour.

Inlet air temperature is 5300C and the outlet air temperature 680C.

That part of the material recycled as seed to the powder inlet 10 which consists of the undersize fraction, the ground oversize fraction and a part of the product fraction (ground) amounts to 300 kg/hour.

As in Example 1 the fluidizing air velocity is 1.2 m/sec and the height of the fluidized layer is app. 35 cm. The average residence time is app. 20 minutes.

The material withdrawn from the fluid bed through 11 amounts to app. 2100 kg/hour and is let to the sieving unit 13 having two screens of mesh width 1.5 and 0.6 mm resp. Thereby is obtained 30 kg/hour oversize fraction, 2030 kg/hour product fraction and 40 kg/hour undersize fraction.

230 kg/hour of the product fraction is together with the oversize fraction ground to a particle size less than 600 microns and recycled together with the undersize fraction as mentioned above.

In the above mentioned cyclone 100 kglhour particles having a size above 100 microns are collected and recycled to fluid bed inlet 10, while in the wet scrubber connected to the cyclone 200 kg/hour particles of an average size below 100 microns are collected and recycled as a slurry to the feed tank 1.

The product fraction not recycled which amounts to 1800 kg/hour is dried and sintered as described in Example 1.The crushing strength is 1.7% by weight and the density app. 3.8 g/cm3.

Claims (8)

1. A process for the production of sintered bauxite spheres having a particle size range of from 0,4 to 2.5 mm or a more narrow particle size range within said limits, suitable for use as fracture propping agent in oil wells, comprising the steps of (a) preparing an aqueous feed suspension comprising bauxite and a binder, (b) continuously atomizing said feed suspension into a layer of already partly dried bauxite particles fluidized in a stream of drying air, (c) continuously recovering particles from said layer (d) continuously separating said recovered particles in oversize, undersize and product fractions making allowance for shrinkage in the subsequent sintered operation, (e) continuously recycling material selected from the group consisting of undersize fractions, relative fine product fractions, ground product fractions and ground oversize fractions, to the layer of fluidized particles at a site at substantial distance, measured along the flow path of the particles, from the site where said recovering of particles takes place, and (f) drying and sintering the non-recycled product fractions by heating them at a temperature between 1200 and 1 6050C.

2. A process according to claim 1, wherein the aqueous feed suspension contains 4060% by weight bauxite having a particle size below 20 micron and 1-5% by weight binder selected from the group consisting of polyvinyl alcohol, polyvinyl acetate, methyl cellulose, dextrine and molasses.

3. A process according to claim 1 , wherein the material recycled in step (e) has been ground to a controlled particle size distribution.

4. A process according to claim 1, wherein the stream of drying air fluidizing the bauxite particles has a velocity of 0.9-1.5 m/s.

5. Sintered bauxite spheres produced by a process according to claim 1 and having less than 5% by weight crushing loss, measured as defined herein.

6. A process for the production of bindercontaining bauxite spheres for use in the manufacture of sintered bauxite spheres comprising (a) preparing an aqueous feed suspension comprising bauxite and a binder, (b) continuously atomizing said feed suspension into a layer of already partly dried bauxite particles fluidized in a steam of drying air, (c) continuously recovering particles from said layer, (d) continuously separating said recovered particles in oversize, undersize and product fractions, and (e) continuously recycling material selected from the group consisting of undersize fractions, relative fine product fractions, ground product fraction and ground oversize fractions, to the layer of fluidized particles at a site at substantial distance, measured along the flow path of the particle% from the site where recovering of particles takes place.

7. Binder-containing bauxite spheres produced by a process according to claim 6.

8. A process substantially as described herein with reference to the drawing.