CA1106789A - Method of reducing the sludge content of a tailings pond - Google Patents

Abstract

METHOD OF REDUCING THE SLUDGE CONTENT
OF A TAILINGS POND
Abstract of the Disclosure A hot water process for processing bituminous tar sands which includes the steps of: A) forming a pulp of the bituminous tar sands and water in a conditioning drum;
B) passing the pulp across screen means and discarding oversize rejected thereby; C) diluting the screened pulp; D) settling the diluted pulp in a separation zone to form an upper primary bitumen froth zone, a middlings layer comprising water and mineral, and a sand tailings layer; and E) separately withdrawing the primary bitumen froth layer and the sand tailings layer;
the improvement which comprises introducing a clay-containing slurry into the conditioning drum such that the pores of clay lumps contained therein become filled to render the lumps resistant to dispersal whereby they are rejected by the screen means and discarded as oversize.

Description

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METHOD FOR REDUCING THE SLUDGE
CONTENT OF A TAILINGS POND

BACKGROUND OF THE INVENTION
This invention relates to the hot water process for treating bituminous sands, such as Athabasca tar sands, and more particularly, to means by which the sludge layer within a tailings pond is diminished, thereby achieving a consequent increase in the clarified water layer.
Tar sands (which are also ~nown as oil sands and bituminous sands) are sand deposits which are impregnated with dense, viscous petroleum. Tar sands are found throughout the world, often in the same geographical areas as conventional petroleum. The largest deposit, and the only one of present commercial importance is in the Athabasca area in the northeast of the Province of Alberta, Canada. This deposit is believed to contain perhaps 700 billion-l trillion barrels of bitumen. For comparison, 700 billion barrels is just about equal to the world-wide reserves of conventional oil, 60% of which is found in the Middle East.
Athabasca tar sand is a three-component mixture of bitumen, mineral and water. Bitumen is the value for the extraction of which tar sands are mined and processed.
The bitumen content is variable, averaging 12 wt.% of the deposit, but ranging from 0 to 18 wt.%. Water typically runs 3 to 6 wt.% of the mixture, increasing as bitumen content decreases. The mineral content is relativ~ly constant ranging from 84 to 86 wt.%.

73~3 l~hile several basic extraction methods have been known for many years for separating the bitumen from the sands, the "hot water process" is the only one of present commercial significance. The hot water process for primary extraction of bitumen from tar sands consists of three major processsteps (a fourth step, final extraction, is used to clean up the recovered bitumen for downstream processing.) In the first step, called conditioning, tar sand is mixed with water and heated with open steam to form a pulp of 70 to 85 wt. % solids.
Sodium hydroxide or other reagents are added as required to maintain pH in the range 8.0 - 8.5. In the second step, called separation, the conditioned pulp is diluted further so that settling can take place. The bulk of the sand-size mineral rapidly settles and is withdrawn as sand tailings. Most of the bitumen rapidly floats ~settles upward) to form a coherent mass known as froth which is recovered by skimming the settling vessel. A third stream may be withdrawn :Erom the settling vessel. This stream, called the middlings drag stream, may be subjected to a third processing step, scavenging. This step provides incremental recovery of suspended bitumen and can be accomplished by conventional froth flotation.
The mineral particle size distribution is particularly significant to operation of the hot water process and to sludge accumulation. The terms sand, silt, clay, and fines are used in this specification as particle size designations wherein sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh, but $~6~

is larger than 2 microns, and clay is material smaller than two microns including some siliceous material of that size.
Conditioning tar sands for the recovery of bitumen consists of heating the tar sand/water feed mixture to process temperature ~180-200F), physical mixing of the pulp to uniform composition and consistency, and the consumption ~by chemical reaction) of the caustic of other reagents added. Under these conditions, bitumen is stripped from the individual sand grains and mixed into the pulp in the form of discrete droplets of a particle size on the same order as that of the sand grains. The same process conditions, it turns out, are also ideal for accomplishing deflocculation of the clays which occur naturally in the tar sand feed. Deflocculation, or dispersion, means breaking down the naturally occurring aggregates of clay particles to produce a slurry of individual particles. Thus, during conditioning, a large fraction of the clay particles become well dispersed and mixed throughout the pulp.
Those skilled in the art will therefore understand that the conditioning process, which prepares the resources (bitumen) for efficient recovery during the following process steps also prepares the clays to be the most difficult to deal with in the tailings disposal operations.

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.. . . . ~ , '7&'~3 The second process step, called separation, is actually the bitumen recovery step, ~the separation having already occurred during conditioning). The conditioned tar sand pul~ is screened to remove rocks and unconditionable lumps of tar sands and clay. The reject material, "screen oversize", is discarded. The screened pulp is further diluted with water to promote two settling processes: globules of bitumen, essentially mineral-free, settle (float) upward to form a coherent mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly the sand size mineral, settle down and are removed from the bottom of the separation cell as tailings. The medium through which these two settling processes take place is called the middlings. The middlings consists primarily of water, with suspended fine material and bitumen particles.
The particle sizes and densities of the sand and of the bitumen particles are relatively fixed. The parameter which influences the settling processes most is the viscosity of the middlings and viscosity is directly related to fines content. Characteristically, as the fines content rises above a certain threshold (which varies according to the composition of the fines), middlings viscosity rapidly reaches high values with the effect that the settling processes essentially stop. In this operating condition, the separation cell is said to be "upset". Little or no oil is recovered, and all streams exiting the cell have about the same composition as the :Eeed.

As ~eed ~ines content increases, more water must be used in the process to maintain middlings viscosity within the operable range.
The third step of the hot water process is scavenging.
The feed fines content sets the process water requirement through the need to control middliings viscosity which, as noted above, is governed by the c~Lay/water ratio. It is usually necessary to withdraw a drag stream of middlings to maintain the separation cell material balance, and this stream of middlings can be scavenged for recovery of in-cremental amounts of bitumen. Air flotation is an effec-tive scavenging method for this middlings stream.
Final extraction or froth clean-up is typically ac-complished by centrifugation. Proth from primary extrac-tion is diluted with naptha, and the diluted froth is then subjected to a two stage centrifugation. This process yields an oil product of an essentailly pure (diluted) bitumen. Water and mineral removed from the froth, during this step constitute an additional tailing stream which must be disposed of.
In the terminology of extractive processing, tailings is the throwaway material generated in the course of ex-tracting the valuable material from an ore. In tar sands processing, tailings consist of the whole tar sand ore body plus net additions of process water less only the recovered bitumen product. Tar sand tailings can be `

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subdivided into three categories; viz: ~1) screen oversize, ~2) sand tailings (the fraction that settles rapidly), and ~3) tailings sludge ~the fraction that settles slowly).
Screen oversize is typically col]ected and handled as a separate stream.
Recently, in view o-F the high level of ecological consciousness in Canada and the llnited States, technical interest in tar sands operation has begun to focus on tailings disposal. The concept of tar sands tailings disposal is straightforward. Visualize mining one cubic foot of tar sands. This leaves a one cubic foot hole in the ground. The ore is processed to recover the resource ~bitumen) and the remainder, including both process material and the gangue, constitutes the tailings; tailings that are not valuable and are to be disposed o-f. In tar sands processing, the main process material is water, and the gangue is mostly sand with some silt and clay.
Physically, the tailings ~other than oversize) consists of a solid part ~sand tailings) and a more or less fluid part ~sludge). The most satisfactory place to dispose of these tailings is, of course, in the existing one cubic foot hole in the ground. It turns out, however, that the sand tailings from the one cubic foot of ore occupy just about one cubic foot. The amount of sludge is a variable, depending on ore quality and process conditions, but may run up to 0.3 cubic feet. The tailings simply will not fit into the hole in the ground.
The historical literature covering the hot water process for the recovery of bitumen from tar sands contains little in the way of a recognition that a net accumulation of liquid tailings or sludge would occur. Based on 7~3~
analysis of field test unit operations which led to the Great Canadian Oil Sands plant design near Ft. McMurray, Alberta, the e~istence of sludge accumulation was predicted. This accumulation came to be called the "pond water problem". Observations during start-up and early commercial operations at Ft. McMurray ~1967-1969) were of insufEicient precision to confirm the predlction.
Since 1969, commercial operating data haveconfirmed the accumulation of GCOS~s tailings disposal area of a layer of fine material and water (sludge) which settles and compacts only very slowly, if at all, after a few years.
At the GCOS plant, for dike building, tailin~s are conveyed hydraulically to the disposal area and discharged onto the top of a sand dike which is constructed to serve as an impoundment for a pool of liquid contained inside.
On the dike, the sand settles rapidly, and a slurry of fines, water, and minor amounts of bitumen flows into the pond interior. The settled sand is mechanically compacted to build the dike to a higher level. The slurry which drains into the pond interior commences stratification in settling over a time scale of months to years. As a result of this long-term settling, two layers form. The top 5 to 10 feet of the pool are a layer of relatively clear water containing 0 to 5 wt. % solids. Below this clarified water layer is a discontinuity in solids content. Over a matter of a few feet, solids content increases to 10-15 wt. %, and thereafter, solids content increases regularly toward the pond bottom. In the deepest parts of the pond, sclid contents of over 50 wt. % have been recorded. This second layer is called the sludge layer. The solids content of the sludge layer increases regularly from top to bottom by a factor of 4-5. The clay-water ratio in this layer increases also, but by a lower factor of 1.5-2.5. The clays, dispersed during processing, apparently have partially re-flocculated into a very fragile gel network. Through this gel, fines of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are discharged over the top of the sand dike directly into the liquid pool. A rapid and slow settling process occur but their distinction is not as sharp as in dike building and no mechanical compaction is carried out. The sand portion of the tailings settles rapidly to -Eorm a gently sloping beach extending from the discharge point toward the pond interior. As the sand settles, fines and water drain into the pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals, ~2) in the hot water extraction process, most of the clays become dispersed in the process streams and traverse the circuit, exiting in the tailings, (3) the amount of process water input is fixed by the clay content of the feed and the need to control viscosity of the middlings stream, ~4) the amount of water required for middlings viscosity control represents a large volume relative to the volume of the ore itself, and (5) upon disposal, clays settle only very very slowly; thus, the process water component of tailings is only partially available for reuse -via recycle. That which can't be recycled represents a net accumulation oE tailings sludge.

The pond water problem is then: to devise long-term economically and ecologically acceptable means to eliminate, minimize, or permanently dispose o-f, the accumulation of liquid tailings or sludge.
It is therefore a broad object of this invention to control the size of the sludge layer in a tailings pond.
In another aspect, it is an object of this invention to provide means for withdrawing the sludge from the sludge layer and discarding the clay constituents as agglomerated masses.
In a more specific aspect, it is an object of this invention to provide, in the hot water process for processing bituminous tar sands into synthetic crude oil, means for controlling the sludge layer of an associated tailings pond by withdrawing sludge therefrom and introducing sludge into the conditioning drum where it agglomerates into clay masses of sufficient size for discard with the oversize.
The manner in which these and other objects of the invention are achieved is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, may best be understood by reference to the following description taken in conjunction with the drawing which is a somewhat simplified block diagram of a hot water process for converting bituminous tar sands into synthetic crude oil.

DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawing, bituminous tar sands are fed into the system through a line 1 and pass -to a conditioning drum or muller 18. Water and steam are introduced to the muller through another line 2. The total water so introduced in liquid and vapor form is a minor amount based on the weight of the tar sands processed.
The tar sands, heated and conditioned with steam and water, pass through a line 3 to a screen 29. The purpose of the screen 17 is to remove from the pulp any debris such as rocks or over-sized lumps of tar sand as indicated generally at 30. This oversize material is discarded at a suitable site. The screened pulp passes through a line 31 to the feed sump 19 which serves as a zone for diluting the pulp with additional water before passage to the separation zone 20.
The pulp tar sands are continuously flushed :Erom the feed sump l9 through a line ~ into a separator 20. The settling zone within the separator 20 is relatively quiescent so that bituminous froth rises to the top and is withdrawn via line 5 while the bulk of the sand settles to the bottom as a tailings layer which is ithdrawn through line 6.
A relatively bitumen rich middlings stream is withdrawn through line 8 and transferred to a flotation scavenger zone 21. In this scavenger zone, an air flotation operation is conducted to cause the formation of additional bituminous :Eroth which passes from the scavenger zone through line 9 in mixture with the primary froth from the separation zone 20 to a froth settler zone 22. A

bitumen-lean water stream is removed from the bottom of the scavenger zone 21 through line 10 to be further processed as described below. In the froth settler zone 22, some further bitumen-lean water is withdrawn from the froth and removed through line 11 to be mixed with the bitumen-lean water stream from the flotation scavenger zone and the sand tailings stream from the separation zone.
The bitumen from the settler 22 is removed through line 12 for further treatment, typically upgrading to synthetic crude oil.
Bitumen-lean water from the froth settler 22, the scavenger zone 21, and the separation zone 20, all of which make up an effluent discharge stream carried by line 7, are discharged into a settling pond 15 having a clarified water layer 26 and a sludge layer 27. The sand included in the tailings stream quickly settles in the region 14, and the fines-containing water flows into the body of the pond 15 where settling takes place. As previously noted, in an actual settling pond, the demarcation between a clarified water layer 26 and a sludge layer 27 is ill-defined and variable, and the characteristics of the sludge layer 27 change from top to bottom. Water from the clarified water layer 26 may be withdrawn by a pump 28 for recycle by a line 17 to be mixed with fresh water and charged into the hot water process.
According to the present invention, sludge from the layer 27 is withdrawn by a pump 23 and is transferred through a line 2~ to the conditioning drum 18. Those skilled in the art will recognize that the introduction of clay-laden sludge into the conditioning drum 1~ is totally contrary to the prior art teachings. However, it is believed that clay lumps which already have their pores filled, or partly filled with water are less and less easily dispersed in a clay slurry as the solids content of the slurry increases. As a result, rather than dispersing, the clay lumps tend to "grow" such that a large proportion of them are discarded as oversize when subjected to the screen 29. Thus, the sludge layer 27 is diminished with a consequent increase in volume of the clarified water layer 26 of the tailings pond.
While the principles of the invention have now been made clear in an illustrative embodiment, there will be immediately obvious to those skilled in the art many modifications of structure, arrangements, proportions, the elements, materials and components, used in the practice of the invention which are particularly adapted for specific environments and operating requirements without departing from those principles.

Claims (2)

THE EMBODIMENTS OF THE PRESENT INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:

1. In a hot water process for processing bituminous tar sands which includes the steps of:
A) forming a pulp of the bituminous tar sands and water in a conditioning drum;
B) passing the pulp across screen means and discarding oversize rejected thereby;
C) diluting the screened pulp;
D) settling the diluted pulp in a separation zone to form an upper primary bitumen froth zone, a middlings layer comprising water and mineral, and a sand tailings layer; and E) separately withdrawing the primary bitumen froth layer and the sand tailings layer;
the improvement which comprises introducing a clay-containing slurry into the conditioning drum such that the pores of clay lumps contained therein become filled to render the lumps resistant to dispersal whereby they are rejected by the screen means and discarded as oversize.

2. In a hot water process for processing bituminous tar sands which includes the steps of:
A) forming a pulp of the bituminous tar sands and water in a conditioning drum;
B) passing the pulp across screen means and discarding oversize rejected thereby;
C) diluting the screened pulp;
D) settling the diluted pulp in a separation zone to form an upper bitumen froth layer, a middlings layer comprising water, mineral including clay, and bitumen, and a sand tailings layer;
E) separately removing the primary bitumen froth layer and the sand tailings layer;
F) withdrawing the clay-containing middlings stream and subsequently transferring at least a fraction thereof as effluent into a tailings pond;
and G) allowing the effluent to settle in the tailings pond into a sludge layer and a clarified water layer;
the improvement which comprises withdrawing sludge from the sludge layer and introducing the sludge into the conditioning drum such that the pores o-f clay lumps contained therein become filled to render the lumps resistant to dispersal whereby they are rejected by the screen means and discarded as oversize.