Froth flotation method for recovery of bitumen from aqueous suspensions of tar sands

This invention relates to a froth flotation method for the recovery of bitumen from aqueous suspensions of tar sands.

The invention is particularly useful in 5 conjunction with the Hot Water Process for the extraction of bitumen from tar sands, because in this process a number of flotation operations are or can be performed.

Tar sands deposits, also known as oil sands and bituminous sands, have been identified in many areas of the world including four major deposits in Alberta,

Canada.

The Alberta tar sands typically contain between 70 and 90 percent by weight of mineral solids, 5 and 20 per cent of viscous petroleum having a 6° to 10° API gravity and specific gravity of about 1.0 commonly referred to as bitumen, and from about 1 to 10 percent of water. The mineral solids usually referred to as ''coarse'· are mostly quartz sand over 45 microns in particle size, whereas those referred to as fines are mostly clay, silts and fine sands having particle size less than 45 microns. The content of fines has been generally found to increase with the decrease in bitumen content in the tar sands. The usually accepted physical arrangement of the sand-bitumen1^37689

-2water mixture that constitutes the tar sands depicts an aqueous phase which envelops the hydrophilic sand grains and separates them from the bitumen phase. The key requirement for the production of bitumen is a fast and complete separation of the bitumen from the mineral solids. Various techniques for separating the bitumen from mined tar sands have been developed to different degrees in the last several decades. These include: direct coking, solvent extraction, cold water separation, spherical agglomeration and a sand reduction process just to mention a few. These methods are not being used on commercial scale due to a variety of reasons, such as material handling problems, low bitumen recoveries, high energy requirements, solvent losses, etc.

The present commercial plants, Suncor and Syncrude, are based on the Hot Water Process which was initiated in the 1920’s by the Research Council of Alberta led by K. A. Clark.

An excellent write-up on tar sands and on bitumen recovery technology is presented by Donald Towson in KirkOthmer Encyclopedia of Chemical Technology, Third Edition, pages 602-607.

In the Hot Water Process, the mined tar sands are treated in a tumbler with hot water, steam and additives, usually caustic, to break down the lumps and produce a slurry at approximately 8O’C-9O°C. The slurry is diluted with hot water to approximately 50% solids, then pumped into a gravity separation cell where entrained air causes the bitumen to float. The flotation process produces the primary froth, middlings and an underflow, which constitutes the primary tailings.

The secondary froth, which is produced from the middlings by air flotation, contains approximately twice the amount of solids and water as compared to the primary froth. Tailings from the secondary circuit join the primary tailings to form the extraction plant tailings. Following de-aeration and heating, the combined froth stream is further treated by dilution and centrifuging to remove the solids and water from the bitumen in preparation for the upgrading or coking process. The solids and water removed together with residual hydrocarbons constitute the froth treatment plant tailings.

Since the early stages of development of the Hot Water Process, it has been recognized that lower grade ore, with its higher percentage of fines and lower bitumen content, results in lower bitumen recovery in the primary separation stage and increased load (total middlings flow and solids content) in the secondary air flotation circuit. Since the secondary separation produces froth with high, mainly fine solids content, the total solids and water content may surpass the handling capability of

-4the froth treatment plant. Consequently, large amounts of bitumen are lost in the extraction plant tailings. A more efficient separation of bitumen from the middlings stream is highly desirable if a high level of bitumen recovery is to be achieved. This may also allow for reduced cut-off grade of tar sands ore at the mine and increase the total tar sand resource available for processing.

Many improvements of the Hot Water Process have been proposed which could be classified under two headings:

1. Improved control to increase bitumen recovery and separation efficiency of the existing plants;

2. Development of methods for the recovery of the residual hydrocarbons from the tailings and for improving the settling characteristics of the sludge.

Some of these proposals seem to be more effective than others; however, most are not cost effective because they require major modifications to the existing plants.

It is, therefore, the object of the present invention 20 to provide a novel flotation method whereby a high quality froth with high bitumen recoveries is obtained, and this without requiring major modifications to the existing installations. This object is achieved by carrying out flotation with CO₂ in the presence of a dispersing/sequestering agent.

C0₂ gas flotation is already known and is disclosed in

-5~

Canadian Patent Nos. 949,910 and 1,023,677. Carbon dioxide flotation reduces the pulp pH and enhances destruction of the mineral-hydrocarbon bond thus increasing froth grade ((Bitumen/{Bitumen + solids + water) over that with air flotation. However, bitumen recovery to the concentrate remains about the same or lower compare to air flotation.

It is also known to perform C0₂ flotation with acid addition to the pulp to initially reduce the pH to about pH5 as is disclosed in Canadian Patent No. 1,022,098. This was found to increase bitumen recovery over C0₂ flotation but the froth grade is drastically reduced.

On the other hand, it is also known to perform air flotation with addition of dispersing agents such as sodium phosphate compounds or soda ash to the pulp as disclosed in Canadian Patents 775,088 and 914,093 and U.S. Patents 3,846,276 and 4,425,227. Although the addition of a dispersing agent generally increases froth grade, it has been found to have a devastating effect on the overall Bitumen recovery.

Applicant has surprisingly found in accordance with the present invention, that the addition of a dispersing/ sequestering agent in the tar sands pulp during CO₂ gas flotation not only results in a significant improvement in froth quality (i.e. lower solids content) but also increases bitumen recovery. It is believed that the prime function of the dispersing/sequestering agent during CO₂

-6flotation is to sequester (form soluable complexes) or precipitate undesirable cations, such as calcium, which contribute to the mineral-bitumen bond and enhance mineral flotation. Once this bond is destructed the mineral 5 flotation is depressed while the bitumen and the surfactants present in the pulp are readily attached to the CO₂ gas bubbles and form the froth. The dispersing/sequestering or precipitating agent may, for example, be a phosphate compound, such as sodium tripolyphosphate or soda ash. The amount of dispersing/sequestering agent to be added may vary depending on the agent used and on the actual process or treatment in which it is employed. When using sodium tripolyphosphate and soda ash, it has been found that dispersant additions of up to 0.7 kg/t of pulp are sufficient to improve recovery to the point that no further addition is beneficial. The dispersing/ sequestering agent may be added prior to or during flotation or in stages during flotation.

The invention will now be further described with reference to a non-1imitative example, and with reference to the appended drawing in which Figure 1 shows comparative values of bitumen recovery versus froth grade using various flotation methods.

-7EXAMPLE

Tests were conducted on primary separation vessel middlings containing 1.1% bitumen and 33% solids. The flotation tests were conducted with air, with CO₂, with C0₂ and H₂SO₄, with air and a dispersing/sequestering agent and finally with C0₂ and a dispersing/sequestering agent. The dispersing/sequestering agent dosages were varied. The froth was collected after 6 and 12 minutes of flotation. The results of the tests are presented in Figure 1 and in the following Table.

-8TABLE

MINUTE CONCENTRATE 12 MINUTE OCNCEHERME

<td> METHOD</td><td> GRADE</td><td> BITUMEN RECOVERY</td><td> GRADE RECOVERY</td>

<td></td><td> B,%wt</td><td> %wt.B.</td><td> B,%wt % wt.B.</td>

<td></td><td> (wet. basis)</td><td></td><td> (wet basis)</td>

<td> C0₂</td><td> 14.7</td><td> 73.3</td><td> 11.3</td><td> 81.0</td>

<td> CO2 + H2SO4</td><td> 4.8</td><td> 71.8</td><td> 4.5</td><td> 84.0</td>

<td> AIR</td><td> 12.5</td><td> 76.8</td><td> 9.1</td><td> 81.8</td>

<td> AIR + AGENTS*</td><td></td><td></td><td></td><td></td>

<td> STPP 0.084</td><td> 24.4</td><td> 72.8</td><td> 16.1</td><td> 78.1</td>

<td> STPP 0.168</td><td> 30.7</td><td> 66.9</td><td> 21.4</td><td> 73.8</td>

<td> STPP 0.672</td><td> 40.9</td><td> 57.5</td><td> 31.0</td><td> 65.9</td>

<td> Soda ash 0.168</td><td> 15.4</td><td> 72.9</td><td> 15.4</td><td> 78.6</td>

<td> Soda ash 0.336</td><td> 21.0</td><td> 70.9</td><td> 13.8</td><td> 78.2</td>

<td> Soda ash 0.705</td><td> 18.9</td><td> 69.9</td><td> 18.9</td><td> 76.1</td>

<td> OO₂⁺ AGENTS*</td><td></td><td></td><td></td><td></td>

<td> STEP 0.168</td><td> 19.4</td><td> 80.2</td><td> 15.6</td><td> 84.3</td>

<td> Soda Ash 0.336</td><td> 16.4</td><td> 77.5</td><td> 12.7</td><td> 84.2</td>

<td> STEP 0.504</td><td> -</td><td> -</td><td> 19.5</td><td> 82.3</td>

*Dosages are in Kg/ton middlings pulp.

-9The above Table and Figure 1 depict the bitumen recovery and concentrate grade after 6 and 12 minutes of flotation time at 80'C using C0₂, C0₂ + H₂SO₄,

Air, Air + dispersing/sequestering agent and finally CO₂ + dispersing/sequestering agent. From the above results, the following observations can be made;

a) the application of sodium tripolyphosphate or soda ash with air or carbon dioxide as flotation gas substantially improves the concentrate bitumen grade over the standard air flotation.

b) all levels of sodium tripolyphosphate and soda ash when used with air had the effect of reducing the rate of bitumen recovery. This phenomenon was not observed with carbon dioxide where the rate of bitumen recovery was increased by about 3% over the standard air flotation.

Applicant has also found that the addition of a dispersing/sequestering agent during co₂ flotation raises the initial pulp pH from 8.3 to 8.5 while flotation with Co₂ alone as disclosed in Canadian Patents 949,910 and 1,023,677, or with CO₂ + H₂SO₄ as disclosed in Canadian Patent 1,022,098 reduces the initial pH.

The invention is not limited to the specific embodiments described above and any modifications obvious to those skilled in the art are also included therein. The novel method can be used in any system that utilizes induced or forced CO₂ gas flotation but it is particularly

-10useful in conjunction with the Hot Water Process where it can be employed, with advantage, at various stages such as, middlings streams, tailings streams, tailings ponds and any combinations of the above.

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