FIELD OF THE INVENTION The invention relates to the method specified in the preamble of claim 1 for separating solids from slurry. The invention further relates to the equipment specified in the preamble of claim 9. The invention further relates to the use of the method and/or equipment as specified in claim 17.

BACKGROUND OF THE INVENTION

Oil sand is the term used for bitumen that is bound to grains of sand. Worldwide, the amount of oil in oil sands is of the same magnitude as the amount of normal petroleum. The exploitation of oil sands on a wide scale only began in the last couple of decades of the twentieth century, because it is more difficult to utilise than conventional oil.

Bitumen is formed from petroleum so that the petroleum has lost its lighter hydrocarbons and the remaining heaviest part has been partially decomposed by bacteria. Thus bitumen is partially degraded petroleum. Oil sands contain 1 - 20% bitumen.

The grains of sand are surrounded by a layer of water, including different kinds of particulates such as clay and other minerals. The bitumen film that surrounds the layer of water has to be separated from the sand, water and particulates before it can be processed further . The recovery and processing of oil sands into crude oil requires more energy and money than drilling the equivalent amount of oil from conventional deposits. For this reason, the wide-scale exploitation of oil sands has not previously been profitable, but with the rise in oil price and development in technology the operation has become economically viable. It is possible that in the future, when the current sources of oil are exhausted, people will turn to the use of oil sands as the major source of crude oil. The environmental impacts of refining oil sands are greater than those in conventional oil production.

This proposal focuses on the end of the oil sands refining process, and relates to the issue of what is known as post-treatment of fine tailings, which contain a suspension of water, sand, clay and bitumen whereby the solid particles are made to separate as effectively as possible from the water. Fine tailings are generated in the refining process in large quantities. Traditionally, in the Canadian oil sands area for example, tailings are routed to large artificial ponds for settling. However, clarification in these ponds is very slow and may take years. Currently, there has been an attempt to accelerate the separation of solids and water by flocculating the tailings and separating the solids and liquid with separation equipment, such as thickeners, centrifuges and filters. The solids can be used for instance for earthworks and landscaping and the separated water can be recycled back to the process to save raw water resources .

Some slurry processing methods and equipment for separating solids from slurry containing a suspension of liquid and fine solid particles are known in the prior art in connection with oil sand refining, for example in US patents 4,225,433 and 5,645,714. A quantity of liquid flocculant is added to the slurry that is substantially smaller than the amount of slurry. The slurry and flocculant are mixed to form a contact between the fine solid particles and the flocculant in order to bind the fine solid particles to each other as floes. Then the solids and liquid from the flocculated slurry are separated from one another in a separate process stage following the flocculation stage. The flocculant or flocculation agent is usually a long-chain polymer, which when mixed evenly and electrochemically into the slurry "glues" the fine solid particles to each other in floes that are large enough to be separated from the liquid by conventional separation methods.

In the known processes a static mixer, several blade mixers in series, or a drum mixer have been used for mixing the flocculant into the slurry. A static mixer consists of a flow-through pipe with a series of helical baffles fixed to its inner walls. A blade mixer comprises a rotating blade rotor in the vicinity of the base of the mixing tank. The powerful shear mixing in a blade mixer brings about a large vortex, but the speed difference at the tip of the blade is great and causes turbulence that is too strong and has a large energy intensity. It breaks the floes that have formed. A drum mixer is a rotating drum, in which mixing blades have been attached to the inner surface of the wall. As the drum rotates, the blades mix the mass by lifting and dropping it on each cycle. The problem with all of these mixers is that adverse, severe shear force is directed on the slurry during mixing. This breaks up the floes that have already been generated into smaller pieces, weakening the effectiveness of the post-mixing separation stage and undesirably leaving fine solid particles in the liquid. Attempts are being made to improve the situation by increasing the amount of flocculant, so that a great deal of flocculant is consumed, which is detrimental because flocculation agents are very expensive. A further problem is the fact that a separation facility greater in size is required in the separation stage than in the case where the floes are not broken during mixing. Moreover, the volume flow of the solids-containing slurry to be treated is large in comparison with that of the flocculant to be mixed into it, so that it is difficult to achieve uniform mixing using conventional mixing methods.

PURPOSE OF THE INVENTION

The purpose of the invention is to eliminate the aforementioned drawbacks.

In particular, the purpose of the invention is to present a method and equipment whereby a very gentle mixing is achieved that does not break up floes. A further purpose of the invention is to present a method and equipment through which the amount of flocculant required can be reduced and cost savings achieved . Yet another purpose of the invention is to present a method and equipment through which the size of the equipment demanded by the separation stage subsequent to the post-flocculation stage can be reduced and separation accelerated.

SUMMARY OF THE INVENTION

The features of the method accordant with the invention are characterized by what is disclosed in claim 1. Furthermore, the features of the equipment accordant with the invention are characterized by what is disclosed in claim 9. Further, the features of the use of the method and equipment accordant with the invention are characterized by what is disclosed in claim 17.

In the method accordant with the invention, a flocculant is mixed gently into the slurry with a helical rotor mixer at a low mixing intensity, imparting to the mixture in the cylindrical mixer tank a vertical circulation flow, while simultaneously forming smaller turbulent flows in it essentially without directing shear forces on the mixture in order to avoid breaking up the floes.

Correspondingly, the mixer belonging to the equipment accordant with the invention is a helical rotor mixer, which is adapted for gently mixing flocculant into slurry at a low mixing intensity and without directing shear forces on the mixture in order to avoid breaking up the floes. The method and equipment accordant with the invention are especially useful in separating solids from slurry in the oil sand refining process, in which the slurry to be treated is the fine tailings generated in the oil sand refining process, in which the fine solid particles are mostly sand and clay, and the liquid is water, in which there are small amounts of residual bitumen .

However, the invention is not restricted solely to the oil sand application but is useful in the separation of solids and liquid in any slurry, where a flocculant is mixed into the slurry prior to separation.

The benefit of the invention is that a very gentle and homogenous mixing of flocculant into slurry is obtained with a helical rotor mixer at a low and uniform intensity so that the mixture is subjected to as little floc-breaking shear force as possible and the powerful turbulences it causes. A helical rotor mixer provides a large surface area capable of moving slurry, whereby the energy is distributed evenly into the mixture. Helical rotor mixers have no tips that cause powerful turbulences and the breaking up of floes. A helical rotor mixer enables coverage of the entire volume of the slurry, ensuring that it rotates the slurry around thoroughly in every part of the tank. When using a helical rotor mixer a smaller amount of flocculant is required than when mixing with conventional mixers, so that large cost savings can be made. Furthermore, the size of the post-mixer separation equipment can be reduced, which also results in substantial cost savings.

In one embodiment of the method, a vertical circulation flow and turbulent flows are formed with a helical rotor mixer, and in its tank there is at least one helical rotor (known as a helix rotor) equipped with a spiral member rotating at a constant radius around a vertical rotation shaft.

In one embodiment of the method, the flocculant is mixed into the slurry with a helical rotor mixer in which there are several helix rotors. The bigger the slurry volume it is intended to mix and the bigger the mixing tank, the more advantageous it is to use several helix rotors in the same tank. In a small tank it is possible to use only one helix rotor, but if only one large helix rotor is used in a large tank, the peripheral speed of the rotor may become too great and start to form floc-breaking turbulence. Additionally, strength issues will arise. It is beneficial to increase the number of helix rotors so that the peripheral speeds can be kept low. In one embodiment of the method, the flocculant is mixed into the slurry with a mixing intensity of the magnitude of 0.04 - 0.3 k /m ³ , preferably a maximum of 0.10 kW/m ³ .

In one embodiment of the method, polymers of long- chain anions or cations are used as the flocculant, such as polyacrylamide (Ρ7ΛΜ) and/or polyethylene oxide (PEO) .

In one embodiment of the method, the discrete separation stage subsequent to the flocculation stage, in which the solids and liquid of the flocculated slurry are separated from each other, is accomplished using a mixer/settler, centrifuge or by vacuum filtration .

In one embodiment of the method, the flocculant is mixed into the slurry in a volumetric flow ratio of the order of magnitude of 1:40.

In one configuration of the equipment, the helical rotor mixer consists of a tank, whose interior is bounded laterally by the cylindrical sidewall and below by the base; at least one helix rotor, which is arranged in the interior; a power device to rotate the helix rotor, and a group of elongated vertical baffles projecting from the sidewall towards the central axis of the tank.

In one configuration of the equipment, the helix rotor consists of a vertical rotation shaft, which is connected to a power device, and the helix member, which is fixed to the rotation shaft with a support arms that are a radius away from the shaft. In one configuration of the equipment, the helix member is circular in cross-section, like a tube.

In one configuration of the equipment, the helix member is non-circular in cross-section, like a flat rectangle .

In one configuration of the equipment, the helical rotor mixer comprises several helix rotors.

In one configuration of the equipment, the helical rotor mixer comprises 2 - 8 helix rotors.

In one configuration of the equipment, the rotating shafts of the helix rotors are arranged radially with regard to the vertical centre axis of the tank and at a constant distance from one another.

LIST OF DRAWINGS

Below the invention is described in detail by means of example embodiments, with reference to the appended drawings, in which

Figure 1 presents the part of the process to which the invention relates in diagrammatic form, namely the mixer and separation equipment.

Figure 2 presents a vertical section of a helical rotor mixer of one embodiment accordant with the invention,

Figure 3 presents section III-III from Figure 2,

Figures 4 - 6 present sections of three other helical rotor mixer embodiments corresponding to Figure 3. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 is a process diagram of the part of the process which comprises the mixer 1 and its subsequent separation equipment 2.

Slurry containing a suspension of liquid and fine solid particles is conducted to the mixer 1. A quantity of liquid flocculant is added to the slurry that is materially less than the quantity of the slurry. The slurry and flocculant are mixed with the mixer 1 to bring about contact between the fine solid particles and the flocculant in order to ' bind the fine solid particles to one another as floes. The flocculated slurry is routed from the mixer to the separation equipment 2, in which the solids and liquid from the flocculated slurry are separated from each other .

The flocculant is mixed into the slurry for instance with a helical rotor mixer 1 accordant with Figure 2 with a low mixing intensity, for example a mixing intensity of the order of magnitude of 0.04 - 0.3 k /m ³ , preferably a maximum of 0.10 k /m ³ , gently providing a vertical macrocirculation flow in the mixture in the cylindrical mixer tank and simultaneously forming smaller turbulent microflows there without materially directing shear forces on the mixture, in order to avoid the floes from being broken up. The vertical macrocirculation flow and the turbulent microflows are formed with the helical rotor mixer 1.

As an example, Figures 2 and 3 show a configuration in which the helical rotor mixer 1 comprises three double helix rotors 7. The helical rotor mixer 1 consists of a tank 3, whose interior 4 is bounded laterally by a cylindrical inner wall 5 and beneath by the base 6. The double helix rotors 7 are arranged in the interior 4 in a triangular formation a radius away from the central axis of the tank, as shown in Figure 3. Each double helix rotor 7 is rotated by a power device 8. Some baffles 9 protrude from the inner wall 5 of the tank 3 towards the central tank axis. Each double helix rotor 7 comprises a vertical rotating shaft 10, to which a power device 8 is connected, and two identical helical tubes 11, which are circular in cross-section and are attached to the rotation shaft 10 by support arms 12 symmetrically opposite each other at a distance of one radius from the shaft.

Figures 4 - 6 depict further potential mixers in which the number of double helix rotors 7 is different. In the mixer 1 of Figure 4 there is only one double rotor 7 situated centrally in the tank. In the mixer 1 of Figure 5 there are five double helix rotors 7 arranged in a pentagonal configuration symmetrically with regard to the vertical central tank shaft. In the mixer 1 of Figure 6 there are eight double helix rotors 7 arranged in an octagonal configuration symmetrically with regard to the vertical central tank shaft. There may be any suitable number of double helix rotors 7 whatsoever.

Double helix rotors with two helical members are referred to in the examples above only by way of illustration. There may even be only one helical member in the helix rotor depending on the requirements of the application. Two helical members in one rotor make the rotor more robust structurally and obtain a more even distribution of mixing power compared with a single helical member. As for the helical member, its cross-section may be a shape other than circular. It may be a flat bar bent into a spiral, in which case the cross-section is a flat rectangle .

The invention is not restricted to refer only to the example embodiments presented above, but many variants are possible while remaining within the framework of the concept of the invention as specified in the patent claims.