AU765545B2 - Ammonia recovery - Google Patents

Description

WO 00/41967 PCT/AU00/00014 "AMMONIA RECOVERY" FIELD OF THE INVENTION The present invention relates to ammonia recovery. More particularly, the present invention relates to a process for the recovery of ammonia from ammonium sulphate solutions or ammonium sulphate containing wastewater streams.

BACKGROUND ART In many mineral processing and chemical applications, ammonia is widely used for its mild alkaline characteristics, ease of use, and potential ability to be recycled. For example, in the mineral processing industry, ammonia is used in the ammoniacal leaching, nickel/cobalt hydrogen reduction, and nickel/cobalt solvent extraction processes.

The production of nickel, using downstream nickel solvent extraction technology or hydrogen reduction, can require the use of large quantities of anhydrous ammonia. This requires pressure storage vessels on site and also specialised transportation. The supply of ammonia is further limited by the location of both the metal plant and the ammonia manufacturer. The ammonia added to the process can ultimately be recovered as a by-product fertiliser (ammonium sulphate), but the market for this product is limited.

Presently, ammonia is recovered from solutions of ammonium sulphate by reacting the ammonium sulphate solution with milk of lime (hydrated calcium oxide or hydrated/slaked lime) in a stirred tank, referred to as a "lime boil". The hydrated lime (Ca(OH) 2 is typically produced by reacting quicklime (CaO) with water in a "slaker" or similar device. The milk of lime is then added to the ammonia recovery system.

WO 00/41967 PCT/AU00/00014 -2- The lime utilised in this process must be hydrated prior to addition to the lime boil as gypsum (CaSO,.2H 2 0) is formed during the process. The gypsum thus formed coats the quicklime and consequently there is very poor utilisation of quicklime in the process.

When ammonium sulphate reacts with the milk of lime, ammonia is liberated by the reaction of the ammonium ion with the hydroxide ions in the milk of lime, and the insoluble gypsum is formed as a reaction product. The ammonia is stripped off the slurry by heating to about 90 to 100 0 C by the addition of steam. The offgases from the process contain both ammonia and water vapour. The ammonia is able to be recovered in a relatively pure form as a solution of ammonia in water by cooling the off-gases.

The consumption of lime in this process is often significantly higher than that predicted by the stoichiometry of the chemical reactions taking place, usually to 50% higher. This is a product of the fact that the gypsum formed in the 1,Lobas a tendency to coat any particles of unreacted lime. This restricts further reactions with any underlying lime particles that may not be fully hydrated. The gypsum further tends to precipitate on the inside of the tank, on the agitator, and any internal pipe work such as the steam injection pipes.

These precipitates can be so severe as to interfere with the continuous operation of the process and significant periods can be spent removing precipitate. There have been instances where lime boil plants have been shut down due to the inability to effectively operate this process.

As the process operates at elevated temperatures there is considerable expense incurred in heating the slurry to the required temperature. This problem is exacerbated if the process cannot be run continuously. In hydometallurgical processes that use ammonia as a reagent, ammonia regeneration/recovery can be a critical process both technically and economically.

PCT/AU00/00014 Received 08 September 2000 -3- The process for recovery of ammonia from ammonia sulphate solution of the present invention has as one objection thereof to overcome at least in part the above problems associated with the prior art.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

DISCLOSURE OF THE INVENTION In accordance with the present invention there is provided a process from the recovery of ammonia from an ammonium sulphate solution, the process comprising the method steps of: combining ammonium sulphate solution and quicklime (CaO) in a milling means to provide a reaction slurry; running the milling means whereby the milling action thereof acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means; and injecting air or steam into the reaction slurry to strip liberated ammonia therefrom.

Preferably, the reaction slurry generates heat through the slaking of the quicklime.

Preferably, the milling means is provided in the form of a ball mill.

WO 00/41967 PCT/AUOO/00014 -4- Still preferably, the air or steam is injected continuously into the reaction slurry whilst in the milling means.

Still further preferably, the ball mill has a non-rotary action. The ball mill may be in the form of a vibrating mill. In such a mill the charge therein is mobile at all times and its shell does not rotate, thereby allowing the mill to be gas-tight.

The process of the present invention preferably also comprises the method step of condensing off-gases from the reaction slurry and recovering ammonia as ammonia solution. The off-gases may be recovered from the milling means or from another reaction or stripping vessel depending upon where the stripping of ammonia takes place.

The process of the present invention may further comprise the additional method step of boiling the air or steam stripped discharge slurry or filtrate to recover at least a portion of any remaining ammonia.

It is envisaged that the method steps of air or steam stripping may be combined in a number of ways to achieve varying strength ammonia solutions as desired.

BEST MODE(S) FOR CARRYING OUT THE INVENTION Ammonium sulphate solution, from which it is desired to recover ammonia, is mixed with quicklime (CaO) at ambient temperature in a milling means, for example a ball mill adapted for the purpose, to provide a reaction slurry. A ball mill with a non-rotary action is preferred as the charge is mobile at all times and the shell does not rotate, thereby allowing the mill to be gas-tight.

Air or steam is continuously injected into the reaction slurry in the ball mill during operation and strips liberated ammonia therefrom. It is envisaged that the reaction slurry may be transferred to another vessel, possibly termed a stripping vessel, in which the stripping of liberated ammonia may be conducted. Off- WO 00/41967 PCT/AU00/00014 gases produced by the reaction slurry are condensed and ammonia recovered as ammonia solution.

The action of the charge or balls in the ball mill acts to continuously break up any gypsum (CaSO 4 .2H 2 0) precipitate as it forms during the ammonia recovery process. The breaking up of the gypsum precipitate in this manner exposes fresh quicklime surfaces allowing for further reaction with the ammonium sulphate solution and prevents gypsum scale formation inside the mill and associated pipe work.

The slaking of the quicklime that occurs within the reaction slurry generates heat that is used to heat the reaction slurry. As such, there is less need for additional process heating.

The recovery of ammonia by way of the present invention may be further heightened through the further boiling of the air or steam stripped discharge slurry or filtrate produced from the reaction slurry.

The present invention may be better understood with reference to the following examples. However, it is to be appreciated that the generality of the invention as described above is not to be limited by the following examples.

Example 1 A feed solution of ammonium sulphate was fed with quicklime to a vertical stirred mill with alumina balls and having compressed air injected from the bottom of the mill and bubbled through the slurry. This "wet milling" was conducted over a period of 120 minutes at ambient temperatures, the temperature being recorded as below: min 190C 60 min 190C WO 00/41967 PCT/AUOO/00014 -6min 18 0

C

120 min 18°C The results of the ammonia recovery, may be seen by reference to Table 1.

After conducting the process of the invention in accordance with this example it is to be noted that only a small mass of residue, comprising unreacted lime and any gypsum produced, is formed. The large mass of filtrate, comprising water, ammonium hydroxide and ammonium sulphate, is present as water is used to wash the ammonium hydroxide and ammonium sulphate from the residue so as to produce an accurate residue assay.

Each of the assays was conducted using the Ammoniacal Nitrogen- Neutralisation titration method and a Kjeltec distillation unit. The designation

NH

3 refers to the percentage of nitrogen in each of the ammonium sulphate, residue and solution present as NH 3 The designation Mass NH 3 refers to the mass (in grams) of nitrogen present in each of the noted groups, as NH 3 The designation

NH

3 -N Gas refers to the percentage of gaseous nitrogen present as NH 3 The results indicate a level of ammonia recovery comparable to the prior art lime boil method.

Example 2 This process was conducted in the same manner as that of Example 1 other than additionally boiling the residue filtrate from the ball mill for 30 minutes to strip ammonia. This additional boil step strips off most remaining ammonia and produces a still higher level of ammonia recovery, as may be seen with reference to Table 1.

WO 00/41967 PCT/AUOO/00014 -7- Example 3 A vibrating drum grinding mill was modified to become the reactor for this example. This reactor is a horizontal non-rotary mill which combines drum like rotary particle motion with constant, aggressive high frequency particle agitation to achieve size reduction.

The mill was charged to 40% by volume with steel grinding media, together with 12 L of 150g/L ammonium sulphate solution. Quicklime was added directly to the mill. Air or steam (as required) were injected into the slurry inside the mill as it operated to strip the ammonia from the solution.

The test of Example 3 was run at ambient temperature with no air or steam stripping. Upon adding quicklime, ammonia evolution started immediately, and the mill became hot due to the reaction heat of lime slaking. Ammonia gas continued to be emitted from the mill after 4 hours of milling.

Example 4 Further tests 4a and 4b were conducted in accordance with Example 3, although air and steam respectively were added to strip ammonia out of the slurry.

Samples were taken to determine the reaction rate and the stripping rate.

Samples taken from the mill were heated on a hot plate to drive any liberated ammonia from the solution. The elapsed time was recorded. An overdose of caustic soda was then added to the solution to react with any remaining ammonium sulphate. The results of tests 4a and 4b are shown in Table 2.

It can be seen from Table 2 that after approximately 10 minutes of milling, there was no further ammonia evolution after adding caustic soda to the solution. This suggests that the liberation of ammonia by quicklime can be completed within minutes.

WO 00/41967 PCT/AUOO/00014 -8- Due to the milling action of the reactor and its effective hydration of the quicklime the hydration step of prior art processes may be avoided. This produces savings in equipment and operating costs. Further, the safety of the process of the present invention is an improvement over prior art processes in that there is a reduction in the number of transfers of alkaline materials.

The heat generated by the slaking of the lime is used to heat the reaction slurry.

Further, as no extra water is added, in the form of milk of lime which would act to dilute the ammonia concentration in the reaction slurry, a more concentrated ammonia product solution may be obtained, whilst also leading to lower steam consumption.

The milling action employed in the process of the present invention both provides high utilisation of quicklime through the continual abrading of the gypsum surface of the particles, and substantially prevents fouling of the reaction vessel and other equipment by gypsum precipitates.

It is envisaged that the method steps of either air or steam stripping of ammonia from the reaction slurry may be combined in a number of ways to achieve varying strength ammonia solutions as desired. It will be apparent to the skilled addressee that the characteristics of the feedstock, the ammonium sulphate solution, and any downstream requirements may have an impact upon the specific manner in which the process of the present invention is carried out, whilst not departing from the scope of the present invention.

Modifications and variations such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims (10)

1. A process for the recovery of ammonia from an ammonium sulphate solution, the process characterised by the method steps of: combining ammonium sulphate solution and quicklime (CaO) in a milling means to provide a reaction slurry; running the milling means whereby the milling action thereof acts to break up any gypsum precipitate as it forms in the reaction slurry or milling means; and injecting air or steam into the reaction slurry to strip liberated ammonia therefrom.

2. A process according to claim 1, characterised in that the reaction slurry generates heat through the slaking of the quicklime.

3. A process according to claim 1 or 2, characterised in that the air or steam is injected continuously into the reaction slurry whilst in the milling means.

4. A process according to any one of the preceding claims, characterised in that the milling means is provided in the form of a ball mill.

A process according to claim 4, characterised in that the ball mill has a non- rotary action.

6. A process according to claim 5, characterised in that the ball mill is a vibrating mill. PCT/AU00/00014 Received 08 September 2000

7. A process according to any one of the preceding claims, characterised by the additional method step of condensing off-gases from the reaction slurry and recovering ammonia as ammonia solution.

8. A process according to claim 7, characterised in that the off-gases are recovered from the milling means or from another reaction or stripping vessel depending upon where the stripping of ammonia takes place.

9. A process according to claim 8, characterised by the further method step of boiling the air or steam stripped discharge slurry or filtrate to recover at least a portion of any remaining ammonia.

10. A process for the recovery of ammonia from an ammonium sulphate solution substantially as hereinbefore described with reference to any one of Examples 1 to 4. i