Patent Application
20190309392
This invention relates to the technical field of the recovery of metals, rare metals and rare earth metals from residues produced during the production of alumina by the Bayer process (known as “red mud”).
More specifically, the invention relates to the extraction of scandium, yttrium, lanthanum, aluminium, iron, titanium, gallium and any other metals, rare metals and rare earth metals present in the so-called red mud, which is a side-product obtained during the Bayer process for the extraction of alumina from bauxite.
The process, in its entirety, according to the invention results in the formation of a concentrate containing the elements of interest and the subsequent selective separation of compounds containing the elements of interest. The entire process is conceived in such a way as to transform a waste to be disposed of in landfill into a series of products which can be reused in the Bayer process or which can be re-inserted in the reference markets, without producing further waste, thus following the principles of zero waste and circular economy. The invention therefore has a strong economic and ecological importance.
As is known, the industrial process most commonly used to obtain alumina from bauxite is called Bayer process. The main steps which characterize the process are:
Although the exact composition of the bauxites used for the production of alumina with the Bayer process can differ according to the mine of origin, they all contain oxides of Al, Ti, Fe and Si in various compositions and percentages; moreover, there are traces of other elements such as zinc, vanadium and some rare and rare earth metals. The extraction efficiency of the Bayer process is very low and this results in the presence of large quantities of metallic elements in the processing waste, generally called “red mud”.
Table 1 indicates a typical composition of a red mud produced by the Bayer process.
Table 2 shows the trace elements and the rare earths present in a NALCO red mud.
In addition to the above-mentioned characteristics, red muds, if they are not neutralized, have an extremely basic pH (approx. pH 12.5).
The red muds are diluted, so that they can be more easily pumped, and are sent to a pressure filter, where some components are recovered; then, in the form of sludge, they are pumped away from the plant to be disposed of in disposal basins, similar to artificial lagoons. This practice has a significant environmental impact since these wastes are not disposed of efficiently and an industrial application has still not been found which is able to absorb the considerable quantity of material produced each year.
Red muds therefore potentially have extremely significant impacts, the management of which still currently constitutes a serious problem. Even though the red muds are currently managed in such a way as to minimize the impacts, they still represent an enormous hazard for human health; moreover, some sites still feel the effects of an incorrect management in the past. The surface deposits where the red muds are stored must be constructed and managed with particular care to avoid contamination of the ground water and the surrounding soils and to prevent dusty material from being dispersed into the air, thus causing harmful effects for the health; in fact, these dusts are of an extremely alkaline nature and cause irritation of the skin, eyes and the respiratory system.
Numerous studies and trials have been promoted over recent years to identify an adequate treatment of these wastes. In some cases, the high content of aluminium in the material has led to the modification of the production cycle in order to reduce the alkaline load, so as to obtain an inert mud which can be used to cover mines that are no longer used as a sub-stratum for planting the original vegetation or for other agricultural purposes or as backfill material for coastal areas. Lastly, the use of red muds has also been tested in the production of construction materials.
On the other hand, the bauxite in the Bayer process contains, as well as the main elements indicated above, also compounds containing rare and rare earth metals, which can be potentially exploited but the concentration of which is too low to promote an economically sustainable extraction process.
The type and concentration of the rare and rare earth metals present varies according to the type of bauxite used; however, amongst those with the highest concentration there is, for example, gallium and scandium. Due to the effect of the extraction of the alumina during the Bayer process, these rare and rare earth metals are also concentrated in the red muds produced, with the result of reaching concentration values which, even though they are still very low, are sufficient to encourage an extraction process. The typical composition data of a red mud is shown in Tables 1 and 2 above. More specifically, even only the exploitation of the scandium and gallium present could, hypothetically, economically support an extraction process, however their concentration in the red mud, as that of the other metals, rare metals and rare earth metals rare present is not yet optimum for rendering their extraction economically sustainable. It is therefore necessary to determine enrichment processes which create “concentrates” of the elements of interest, in order to overcome this qualitative deficit.
Summing up, even though possible alternative paths have been studied for the management of red muds, they are still considered to be wastes with a high environmental impact, the treatment and/or disposal of which represents an enormous cost for society. In addition, the depletion of the available landfill sites constitutes a problem for the continuation of industrial activities: due to the so-called “NIMBY” (Not In My Back Yard) effect, not only in Europe and in the USA, but also in many of the emerging economic powers, the disposal in landfills is seen as the last option, after having implemented the so-called 3 R (Reduce, Reuse, Recycle) principle. Consequently, concessions for new landfill sites are increasingly difficult to obtain.
Therefore, in the specific sector, there is the need to manage red muds with a more advantageous process in both economic and ecological terms.
The need is satisfied by the process according to this invention, which also achieves further advantages which are apparent in the description which follows.
The process, as a whole, allows the elements of interest present in the red muds to be concentrated in smaller fractions, having a chemical composition and physical state such as to render technologically easy their further separation and refinement. The final products produced can be sent to the start of the Bayer process or sold on the reference markets.
This invention therefore relates specifically to a process in which the metals, rare metals and rare earth metals present in the powdery by-products coming from the processing of the bauxite (red muds) are concentrated, through a multistage process, until reaching values such as to allow an extraction and separation which is industrially efficient. The invention also relates to the process for separating the elements of interest, through a multistage process, transforming them into single products to be re-used in the Bayer process and/or sending them to the respective reference markets.
The process is divided into the following passages.
The basicity index, in the conditions of the melting process in question, can vary from 0.1 a 2.0, according to the type of red mud to be treated.
With the variation in the content of calcium, magnesium, silicon and aluminium oxides, there can be variations, for example, in the melting temperature, the modes of separation of some phases during the melting process, and their chemical composition.
In this invention, unlike the prior art processes on red muds, the addition of the components necessary to modify the basicity index of the mixture is achieved by adding in suitable proportions a waste coming from other processes, known as “fly ash”.
In a preferred application, these fly ashes come from the combustion of coal (Coal Fly Ash) and have the further advantages of containing, in turn, rare and rare earth metals.
The following table summarises the typical values for the main components in a coal fly ash:
At the end of the first concentrating step the rare and rare earth metals present are drawn into the fraction operatively indicates as “slag”, which is subjected to the subsequent operations. The metallic iron produced, which is of poor quality, equivalent to a pig iron, may be directed to the reference markets.
The liquid obtained during the basic leaching contains mainly aluminium in the form of hydrated salt of an alkaline or alkaline earth metal, but it also contains hydrated silica; this liquid is firstly treated with milk of lime, to eliminate the silica; subsequently, the liquid is treated with CO2. The buffering effect of the carbon dioxide brings the pH to a value such as to precipitate Al(OH)3. The solid obtained in this way is separated and calcined, producing Al2O3. The separated liquid returns to the start of the alkaline leaching section, whilst the separated hydrated silica is characterized in order to direct it to the correct recovery.
The ratio (weight/volume) between the solid to be treated and the acid solution is between 1/2 and 1/50.
The temperature of the system during the leaching is between 40° C. and 95° C. at atmospheric pressure. The duration of the leaching reaction time can range from 15 minutes to 120 minutes.
The solid separated at the end of the leaching reaction time can, if necessary, be subjected to a new leaching step, similar to the previous one.
In the reaction conditions, the solution extracts the rare and the rare earth metals present (plus the aluminium, the titanium and the other elements present in smaller concentrations), whilst the majority of the iron present in the slag remains in the insolubilized residue; the latter is separated from the acid leaching liquid, by known means, such as, for example, decantation and filtration, and may be treated for the extraction of aluminium and silicon compounds, according to the methods described in the case of alternative A of the second concentrating step.
The liquid resulting from the acid leaching is sent, after filtration, to the next third concentrating step.
The liquid coming from the acid leaching, and which contains, in solution, rare and rare earth metals, as well as a not insignificant quantity of other elements, is treated on selective ion exchange resins, of cationic type.
The resins “capture” the rare and the rare earth metals present in the alkaline solution (such as, for example, scandium, yttrium and lanthanum), and block them on the active sites of the resin, thereby concentrating them.
Together with the cations of the metals and rare metals, other cations of any undesired elements present are also captured, if present.
The separation of the undesired elements is achieved by processing the resins with an extracting solution of HNO3 with a concentration of between 1.25 N and 1.75 N; in fact, with this concentration, only iron, aluminium, calcium, titanium and sodium are selectively brought into solution, whilst the rare and rare earth metals of interest remain blocked on the resins.
Subsequently, by treating the resins with HNO3 with a concentration of between 3 N and 10 N, the rare and rare earth metals of interest are extracted and concentrated in acid solution, simultaneously regenerating the resins and making them available for a subsequent concentration cycle on resins.
The acid solution contains an enriched mixture of rare and rare earth meals, which can be selectively separated, according to one of the prior art techniques. By way of example, but without limiting the scope of the invention, the extraction can be carried out with organic solvents such as DEHPA (di-(2-ethylhexyl) phosphoric acid).
Concentrates of rare and rare earth metals are obtained with the process described above.
The sole FIGURE appended shows the simplified block diagram of the invention, in terms of its most extensive definition.
The description of the invention given above is of a general nature. A more detailed description of a relative embodiment will now be given, with the help of the example, aimed at achieving a better understanding of the objects, features and advantages of the invention.
The example illustrate an application of the process according to the invention.
A sample of red mud is dried to 250° C. and then ground. The following table shows the elementary analysis of the main elements of interest present on the sample of pre-treated red mud thus obtained, used in this example.
during this step the aim is to drastically reduce, selectively, the iron content present in the matrix, both to obtain a concentration effect of the elements of interest, and because the iron is an important interfering element for the processes used in the subsequent concentration and separation steps.
A basicity index corrector, containing silica and calcium oxide, is added to the sample of red mud, suitably pre-treated. In this test, the basicity corrector is added in order to obtain a binary basicity index value IB2 of approximately 0.6.
Moreover, an appropriate quantity of carbon is added, to give a suitable reducing potential to the load. A quantity of carbon equal to 11% of the weight of the sample of red mud is added in this test.
It should be noted that the quantity of the reducing agent and the basicity index corrector added is determined, each time, on the basis of the red muds used.
The mixture formed by the pre-treated red mud, the basicity index corrector and the carbon is loaded in a plasma transferred arc reactor, in which the plasmogenic gas is nitrogen. The system must be maintained in the reaction conditions, that is, at a temperature greater than 1300° C., until completion of the reduction reactions. In the case of the reactor used, this phenomenon occurred in approximately 60 minutes, but this time may vary on the basis of the type of technology used to reach the reaction conditions, the type of load (depending, for example, on the content of iron oxides and interfering elements present), the geometry of the reactor etc.
The slag and the pig iron produced are collected separately at the end of the reaction time. The slag, compared with the calcined red mud, is 55% by weight; this means that the rare and the rare earth metals of interest, in the slag, should have a concentration of approximately double, with respect to the pre-treated red mud. The following table shows the elementary analysis of the main elements of interest present on the sample of slag produced:
As may be seen from the data shown in Table 4 and Table 5, the concentration di Yttrium, Lanthanum and Scandium has almost doubled compared with the sample of pre-treated red mud. As expected, the content of iron in the slag has considerably reduced, compared with the pre-treated red mud, falling from approximately 16.5% to approximately 2.5%. The iron removed has resulted in a ferrous-based metallic phase (iron content>92%), which is similar to pig iron in terms of quality. The content of aluminium before and after the treatment has a less regular trend, because the compounds containing this element also undergo reactions with the development of volatile compounds (flow managed separately and not included in the invention), so the concentration factor of this element in the slag is approximately 30%.
The ratio (weight/volume) between the solid and the acid liquid, used in the test, was 1/50; the reaction system was also maintained at 90° C. and with atmospheric pressure.
At the end of the reaction time, the liquid was separated from the solid by mechanical decantation, by centrifuging; the clarified liquid was then filtered and analysed. The solid treated was recovered for any extraction of aluminium and silicon compounds, according to the methods described in the case of alternative A of the second concentrating step.
The following table shows the percentage of extraction of the rare and rare earth metals of interest, calculated from the elementary analysis of these elements in the 0.6 N nitric acid solution, at the end of the leaching reaction:
The liquid resulting from the acid leaching is sent, after filtration, to the next third concentrating step.
Third Concentrating Step, by Separating Compounds Containing Aluminium:
the liquid coming from the acid leaching, and which contains, in solution, rare and rare earth metals, as well as a not insignificant quantity of other elements, is treated on selective ion exchange resins, of cationic type.
The resins “capture” the rare and the rare earth metals present in the alkaline solution (such as, for example, scandium, yttrium and lanthanum), and block them on the active sites of the resin, thereby concentrating them. The concentration factor of the rare and rare earth metals on the resins depends on the ratio between the volume of leaching liquid treated and the volume of the bed of selective resins. A typical concentration ratio is approximately 100 times.
The concentration ratio on the resins in the test in question was 20 times. A volume of 2 litres of acid solution has been treated with a volume of 0.1 litres of selective resins.
Together with the cations of the metals and rare metals, other cations of any undesired elements present, such as aluminium and iron, are also captured, if present, in the test described.
The 0.6 N nitric acid solution, after the passage on resins, is sent to the second step, for re-use in the process, if necessary with the addition of new solution.
After having captured the rare and rare earth metals of interest, together with the undesired elements, the bed of selective resins, of the cationic type, is treated with a solution of nitric acid with a concentration of between 1.25 N and 1.75 N. A nitric acid solution with this concentration frees from the resin only the undesired elements, leaving the rare and rare earth metals on the bed.
The bed of selective resins is then washed with a nitric acid solution with a concentration of between 3 N and 10 N; in these conditions, the rare and rare earth metals which were linked to the resins are removed from the bed. In this way, an acid solution is generated containing a concentrate of a mixture of rare and rare earth metals. The concentration of the rare and rare earth metals in the resulting acid solution depends on the content (in grams) of each element in the filtering bed (degree of saturation of the filtering bed) and on the ratio between the volume of the bed of resins and the total volume of the 3 N to 6 N nitric acid solution used. In the example in question, for instance, the 0.1 litre filter bed, containing 1.8 mg of scandium, has been washed with a volume of 0.5 litres of 6 N nitric acid solution, obtaining a concentration of 3.6 mg/litre of scandium in solution.
The rare and rare earth metals present in the nitric acid solution obtained in this way can be separated selectively by means of one of the prior art techniques.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102016000101809 | Oct 2016 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IT2017/000213 | 10/3/2017 | WO | 00 |
