MILLING PROCESS TO MICRONIZE SULPHUR

FIELD OF INVENTION

This invention relates to apparatus and a method to process sulphur residue so as to produce elemental sulphur as a fertilizer and soil remediation product. This invention also relates to apparatus and a method for wet grinding sulphur feedstock with primary and secondary milling as well as hydrocyclones. The invention also relates to elemental sulphur having a selected micron size for producing sulphur based fertilizers.

BACKGROUND TO THE INVENTION

Elemental sulphur is a byproduct from many processes including the production and refining of hydrocarbons such as natural gas and oil. Elemental sulphur from such refining processes generally comprises substantially elemental sulphur with some impurities. Over the years, various sulphur stockpile areas have become redundant and a residual layer of sulphur has been left, which is contaminated with base ground and soils. These sulphur residues are considered to be environmental liabilities to the oil and gas producers and refinery operators.

Attempts to meet or filter these stockpiles have had poor success leaving landfilling as an undesirable alternative for disposing of the material.

Various prior art apparatuses and methods have heretofore been proposed to recover or separate sulphur. For example, U.S. Patent No. 4,964,981 relates to the recovery or separation of elemental sulphur from contaminated elemental sulphur products by means of froth flotation and use of reagents. A ball mill is used to generate particles less than 300 microns with a substantial amount of particles being finer than 150 or 75 microns.

Moreover U.S. Patent No. 4,952,307 and U.S. Patent No. 4, 871,447 relate to a process for recovery by combined coarse and fine froth flotation of elemental sulphur in the oil and gas industry.

U.S. Patent No. 4,119,699 illustrates a wet metallurgical process on a sulphidic mineral substance with oxygen and sulphuric acid as reagents to form metal sulphates and elemental sulphur.

Also, U.S. Patent No. 7,093,782 shows a process for fine grinding of mineral particles, in particular rocks, ore, sulphide concentrates or other minerals with a high metal content.

Also, U.S. Publication No. 2004/0163433 relates to a wet ground sulphur that is used alone or in combination with a urease inhibitor to coat urea prills or granules.

Finally, U.S. Patent No. 5,788,896 which was assigned to the Alberta Research Council teaches a method of producing micron sized sulphur granules involving heating sulphur until the sulphur becomes molten, tempering water and inducing movement of the water at velocity, injecting an unbroken stream of the molten sulphur under pressure into the heated water moving at a velocity, with an explosive dispersion of the molten sulphur into fine sulphur granules which is capable of producing spherical sulphur granules with a broad range of 70 microns to submicron sizes depending upon the temperature of the water, temperature of the sulphur, pressure of the discharging sulphur, the use of surfactants and the rate of agitation of the target water solution.

It is an object of this invention to process accumulated sulphur residue products from a variety of sources including refinery sites to produce a finely ground product suitable for use as a sulphur rich fertilizer and soil remediation product.

It is another object of this invention to provide a wet grinding apparatus, method and plant to process a crushed sulphur product. In one aspect of the invention the ground sulphur product, in slurry form, will be further processed by a combination of centrifuging, extrusion or pellet pressing or otherwise granulating and drying to produce various marketable products, such as sulphur based fertilizer.

It is another aspect of this invention to convert elemental sulphur (with any impurities) that has been contaminated (for example with soil) into a micronized elemental sulphur mixture or slurry that can be used as a raw material for producing sulphur based fertilizers. Separation of the contaminants from the elemental sulphur is not necessary to achieve the benefits of the invention.

A further aspect of the invention resides in achieving a particle size distribution where

80% of the processed elemental sulphur is less than 20 microns and more preferably less than 10 microns (i.e. less than 635 mesh). This greatly increases the value of the

sulphur particles as a fertilizer raw material because plants have the ability to assimilate sulphur in the form of sulphate but not in the form of elemental sulphur. It is known that the rate of oxidation of elemental sulphur to sulphate is directly dependent on the surface area exposed to specific microbes such as those found in the soil. Accordingly the larger the surface area of the elemental sulphur per unit measurement the faster the rate of oxidation. In other words, the surface area is increased as the particle size is reduced.

It is another aspect of this invention to produce micronized elemental sulphur particles by crushing stockpiled sulphur, wet milling the crushed sulphur to a particle size distribution of 80% elemental sulphur particles being less than 20 microns and more preferably less than 10 microns. Wet milling in one aspect of the invention comprises two stage wet grinding and separation by hydrocyclones.

These and other objects and features of the invention shall now be described in relation to the following drawings.

BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a schematic view of the milling process.

Fig 2 is another schematic view of the milling process including a process for the production of sulphur granules from sulphur base-pad materials.

BEST MODE FOR CARRYING OUT THE INVENTION

In the description which follows, like parts are marked throughout the specification and the drawings with the same respective reference numerals. The drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order to more clearly depict certain features of the invention.

Sulphur Feedstock

Generally speaking the stockpile recovery and crushing operations will be at the respective sulphur storage sites and that the crushed and screened product will be transported to a central milling facility. The sulphur feedstock used may comprise elemental sulphur (that may have less than 0.5% impurities) as byproduct from the refinery process. Moreover the stockpiled sulphur feedstock is generally contaminated with minerals from the soil and organic matter.

- A -

The contaminated elemental sulphur used in the milling process generally contains from 75% to 95% (dry weight basis) elemental sulphur, with most of the contamination being minerals (from soil) and organic matter. Contamination of the sulphur includes large river boulders to fine clay, muskeg soil and small evergreens. The percentages of each will vary depending upon the source of the contaminated sulphur but organic matter contaminants are usually 1/3 or less the level of mineral contaminants. Heavy metal impurities need to be monitored to ensure quality control.

Sulphur feedstock known as block or base-pad (sulphur feedstock which is produced as a byproduct from the refining of natural gas and oil) is an example of sulphur feedstock that is used, as the elemental sulphur concentration is relatively high. However, other feedstock can be used.

The milling process to be described can use either contaminated sulphur, non- contaminated sulphur, or a mixture of the two as a feedstock. Mixing the contaminated sulphur with non-contaminated sulphur will very likely be required to maintain a consistent level of sulphur in the fertilizer product.

The Process

The process plant consists of a conventional two-stage grinding circuit as shown in Fig. 1 and Fig.2. In one broad aspect this invention relates to grinding crushed product to a selected size comprising two stages of hydrocyclones where the first stage cyclone overflow product is received in a second stage to remove the minus 10 μm fraction. The primary mill product is further ground in a fine grinding operation using vertical milling to produce a product sizing of 80% less than 10 microns. In one example to be described herein, a 20 tonnes per hour ball mill, grinding minus 25.4 mm crushed product to approximately 80 percent passing 45 μm, operates in closed circuit with two stages of hydrocyclones. The first stage cyclone overflow product being classified in a second stage to remove the minus lOμm fraction. The primary mill product will be further ground in a fine grinding operation using a Vertimill to produce a final product sizing of 80% passing 10 μm.

The feedstock sulphur material can be recovered by a variety of means including recovery by front-end loader and crushed. In one example the feedstock sulphur material can be crushed to approximately 80% passing 25 mm in a rotating breaker

fitted with a trammel screen of approximate 30 mm apertures. The screen undersize product, being the crushed sulphur together with some soil and small stones, will discharge onto a conveyor feeding a stockpile ahead of the proposed grinding circuit.

The coarse (for example + 25 mm) product from the trammel screen, consisting of larger rocks and some unbroken sulphur lumps, will be deposited to a small stockpile area at the discharge end of the trammel screen. Subsequent recycling of this oversize product to recover as much of the sulphur as possible will depend on the nature and mix of the rock/sulphur combination and process economics. It is anticipated that the larger size rocks recovered with the sulphur from the original stockpiles will act as a source of grinding media to break up the lump sulphur in the rotating trammel. The crushing operation is expected to be done on a campaign basis, with the trammel screen undersize product being transported to a central storage facility and processing plant.

Fig. 1 and Fig. 2 schematically illustrates a wet grinding plant which comprises a standard ball mill as the primary grinding stage 10 and a Vertimill as the second grinding unit 80. In one example, the primary grinding stage can produce a size of 80% passing 45-50 microns, and the secondary grinding unit can produce a final product of 80% passing 10 microns. Hydrocyclones 30 and 60 are used as separators.

The crushed product 12 will be discharged from the stockpile by an electro-magnetic vibrating feeder on the mill feed conveyor 14 and into the primary ball mill 10. A weightometer 16 will be installed on the conveyor 14 to monitor and control the feed rate to the mill 10.

A two stage cyclone configuration is utilized to achieve a final product sizing of 80% passing 10 μm. In the example described herein the cyclone configuration consists of two 250 mm primary cyclones and twenty six 50 mm secondary cyclones operating at 13 psi and 20 psi respectively. However, other configurations can be used to achieve the desired results .

The ball mill 10 discharges into a pump box 18 and dilution water 20 added to produce the slurry to be pumped to the primary cyclone 30. The slurry density in the example described is selected at a slurry density of approximately 26% solids by a

8x6x18 SRL slurry pump PPOl, installed with a 22 kilowatt (30 hp) motor. However, other slurry densities and motor horsepower can be used.

The primary cyclone underflow 32 will gravitate to the mill inlet hopper and the primary cyclone overflow product 34 will gravitate into a second pump box 40 for delivery to the bank of secondary cyclones 60 using a slurry pump PP02. In the example described herein a 5x4x14 SRL slurry pump PP02 is installed with a 19 kilowatt (25hp) motor. The cyclones can be selectively operated to achieve the desired size separations. In the example described herein the primary cyclone 30 is operated at a feed density of 25.9% solids and the secondary cyclones 60 at 15% solids to achieve the desired size separations. A circulating load of 150% has been assumed for the ball mill.

Process water is added to both the mill inlet and mill discharge pump box to maintain the required slurry densities (of approximately 60-65% solids for the combined mill feed and 26% solids for the feed to the primary cyclones).

The secondary cyclone underflow product 62 will gravitate to a settling tank 64 ahead of the Vertimill fine grinding unit 80.

Another aspect of this invention resides in the utilization of a portion of the process stream to the Vertimill as a saleable product without any additional grinding.

The secondary cyclone overflow product 66 with an approximate size of 80% passing 10 μm can combine with the final Vertimill product stream 82 and then be directed to the dewatering centrifuge operation 90.

Fine grinding can be achieved by utilization of a vertical grinding mill 80 to produce elemental sulphur having a particle size distribution of 80% less than 20 microns or more preferably 80% less than 10 microns. In one aspect the particles can be generally spherical such that the elemental sulphur has a size distribution of 80% less than 20 microns in diameter and preferably 80% less than 10 microns in diameter. However, the particles do not need to be spherical.

The Vertimill 80 can be used where grinding media, such as steel balls, is stirred by an overhung double helix screw and an external recycle pump provides an upflow velocity which causes a classification of particles in the upper portion of the mill.

Pre-classification and removal of product sized feed (minus 10 μm material) reduces over grinding and increases the mill efficiency. Accordingly, the secondary cyclone is utilized.

Pulp overflows from the mill body into a splitter box 90 with a dart valve and control devices which split the pulp into a product stream 82 and a recycle stream 84. The recycle stream 84 is controlled to produce an optimum upflow velocity in the mill body for the specific grinding application.

A suitable Vertimill for:

Maximum capacity = 10 tonnes per hour Feed sizing, 80% passing (F ₈₀ ) = 50 μm Product sizing, 80% passing (P go) = 10 μm

Power requirement = 41 kWhr/t

is a VTM 400WB, which has installed power of 298 kW (400 hp).

The secondary cyclone overflow product 66 can combine with the Vertimill discharge stream 82 for delivery to a proposed centrifuge operation 90.

Optionally an intermediate slurry holding tank (not shown) can be installed for surge capacity between the Vertimill 80 and centrifuges 90. In one example, assuming the Vertimill product is at a density of 25% solids, the combined feed to the centrifuges will be approximately 15% solids and at a flowrate of 118 mVhour.

The specifications of the centrifuge can be selected. In one example, four centrifuge units, each with an installed power of lOOhp (75kW), will meet the process requirements.

The overall process described herein will be a net user of water, the water being discharged with the various sulphur slurry and centrifuge products.

The process described above can be used to produce a sulphur based fertilizer. One example includes the production of sulphur fertilizer pellets as follows:

1. Sulphur base-pad materials were wet ground to 80% less than 13 microns in diameter, which can be stored in a storage bin 93.

2. The base-pad sulphur slurry was dried to 20% moisture content in for example a thermal screw dryer 93.

3. Bentonite clay (5%) and sodium lignosulfonate (3%) and water can be added to the base-pad sulphur to bring the moisture content up to 26.0%.

4. The mixture was then formed into pellets using a Kahl pellet press at 20Hz fitted with a 3 mm (diameter) die or extruder 95. The cutter was placed immediately under the die leading to the production of the shortest possible pellet (~3 mm length).

5. The pellets were then placed in a "Spheronizer" 97 for 3 min

6. The rounded elemental sulphur granules coming out of the spheronizer were dried in a dryer 99 at 7O ⁰ C to less than 1% moisture content.

Various embodiments of the invention have now been described in detail. Since changes in and/or additions to the above-described best mode may be made without departing from the nature, spirit or scope of the invention, the invention is not to be limited to said details.