GB2437864A

GB2437864A - Rapid and Selective Dissolution of Magnesium Alloy

Info

Publication number: GB2437864A
Application number: GB0714540A
Authority: GB
Inventors: Malcolm Brody; James Reuben Entwistle
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-03-20
Filing date: 2007-07-26
Publication date: 2007-11-07
Anticipated expiration: 2027-07-26
Also published as: GB0705289D0; GB0714540D0; GB2437864B

Abstract

Magnox, including any magnesium hydroxide and magnesium carbonate present, is dissolved in a solution of magnesium hydrogen sulphate (or magnesium and sodium bisulphate). Nimonic alloy and some other alloys do not dissolve thereby allowing separation. Approximately half of the magnesium sulphate produced can be disposed of to sea or by another disposal route. The remaining half is recycled and reacted with sulphuric acid to produce fresh magnesium bisulphate reagent. The process cycle is then repeated and more Magnox dissolved. For sodium bisulphate dissolution the whole of the magnesium sulfate and sodium sulfate produced is discharged to sea.

Description

I

The Rapid and Selective Dissolution of Magnesium Alloy This invention relates to rapid and selective dissolution of magnesium alloy.

BACKGROUND

Magnox nuclear power stations generate large quantities of Magnox waste.

Magnox is magnesium metal alloyed with metals such as aluminium, zirconium and manganese. Magnox waste consists mainly of Magnox itself, magnesium hydroxide, magnesium carbonate and small amounts of Nimonic alloy containing radioactive cobalt-60.

Magnox dissolves in strong acids, initially at a very ist rate, with the very vigorous evolution of hydrogen, e.g. equation (1) Mg + H2S04 = MgSO4 + 112 (1) The reaction can be difficult to control and is potentially hazardous. Magnox metal also dissolves in carbonic acid, according to equation (2) Mg + 2H2C03 = Mg(HCO3)2 + 112 (2) This dissolution process is currently in use at one Magnox power station with sea disposal of the resultant magnesium hydrogen carbonate (magnesium bicarbonate) solution. This process is venr slow, produces an effluent of low magnesium content and does not readily dissolve magnesium hydroxide and magnesium carbonate. Moreover, the manufacture of the carbonic acid reagent can be very inefficient and lead to the discharge of' excess carbon dioxide (a greenhouse gas) to atmosphere. Carbonic acid dissolution is the only process currently available. Due to the large quantities of Magnox waste requiring treatment, a faster dissolution procedure, which also dissolves magnesium hydroxide and carbonate, is more environmentally friendly and produces a smaller volume of effluent, is required, and it is an object of the invention to provide such a process.

SUMMARY OF THE INVENTION

The invention accordingly provides a method for dissolving Magnox, magnesium hydroxide and magnesium carbonate, using bisuiphate solution.

The materials that do not dissolve in bisuiphate solution may be segregated prior to sea discharge.

The preferred bisuiphate solution is that of magnesium or sodium. Alternatively potassium can be used.

The process may dissolve Magnox at a laster rate than the established carbonic acid process, and reduce the overall amount of waste discharged compared to the carbonic acid process.

The process may be able to recycle half of the dissolved Magnox to regenerate fresh magnesium bisuiphate, and may either retain aluminium and zirconium in, or segregate aluminium and zirconium from, the solution for sea discharge.

The bisuiphate ion, HSO4, behaves as an acid with a strength intermediate between that of carbonic acid and strong acids. It dissolves Magnox rapidly and smoothly and does not dissolve Nimonic alloy, e.g. equation (3) Mg + 2HS04 = Mg2' + 2S042 + H2 (3) Bisuiphate solution is thus a faster alternative to carbonic acid. It also readily dissolves magnesium hydroxide and magnesium carbonate.

Magnesium bisuiphate solution is one of two preferred reagents. The only products from the dissolution of Magnox are then magnesium sulphate and hydrogen, see equation (4) Mg + Mg(HSO4)2 = 2MgSO4 + H2 (4) Importantly the magnesium sulphate product is more soluble than magnesium bicarbonate, so reducing the volume for sea disposal. Magnesium sulphate, like magnesium bicarbonate, is also suitable for sea discharge.

Magnesium bisuiphate only exists in solution and is not available in solid form. It is easily pmduced by reacting sulphuric acid and magnesium sulphate, see equation (5).

H2S04 + MgSO4 = Mg(HSO4)2 (5) ifa stoichiometric amount, or an excess, of magnesium is used in equation (4), half of the magnesium sulphate produced from the bisulphate dissolution can be recycled and mixed with sulphuric acid to generate fresh bisuiphate reagent (equation 5). The remainder of the magnesium sulphate can be discarded and discharged to sea. The overall process is shown in the diagram below: r---a Mg + Mg(HSO4)2 = H2 + I 2MgSO I 1 MgSO4 I,. .1 t I H2S04 + I MgSO4 (regeneies bsi4phate) If a known excess of bisuiphate is used in the reaction, the known fraction of magnesium sulphate solution available for discharge is less than one half Also neutralization may be required prior to discharge. Thus the only consumable using magnesium bisulphate dissolution, after preparation of the initial solution, is sulphuric acid.

The other preferred reagent is sodium bisuiphate solution: Mg + 2NaHSO4 MgSO4 + Na2SO4 + H2 (6) Sodium bisuiphate, as a solid, is much more expensive than sulphuric acid, but may be commercially prepared, relatively cheaply, in solution. It has the advantage over the magnesium bisuiphate route that division of the product into two streams, one being for recycle, is not required.

As an alternative, potassium bisulphate can be substituted for the sodium bisuiphate but the effluent stream may then be less suitable for sea discharge.

Nimonic alloy does not dissolve in bisuiphate solution. Aluminium and zirconium both dissolve when alloyed with magnesium.

When excess Magnox is present towards the end of dissolution, the pH increases towards neutrality.

With sodium bisuiphate, precipitation of aluminium and zirconium hydroxides occurs at pH 2 to 4, the pH then increasing to approximately 7.

With magnesium bisuiphate, as the pH increases, precipitation of aluminium and zirconium does not occur and the pH stays in the range 2 to 4. However, the addition of strong alkali which raises the pH to 7, results in precipitation.

When excess bisulphate is present, aluminium and zirconium both remain in solution at the end of the reaction. They can be precipitated by neutralizing the excess acidity.

DESCRIPTION OF PREFERRED METBOD.

A batch process may be used for the dissolution of Magnox, magnesium hydroxide and magnesium carbonate in magnesium bisuiphate solution. The process may be performed in a single vessel, as shown in Figure 1 below, which is an example of a diagrammatic cross-section of a processing vessel. The start-up batch of magnesium sulphate, which effectively primes the procedure, is prepared from commercially obtained solid magnesium sulphate. Addition of dilute sulphuric acid then forms the magnesium bisuiphate reagent (equation 5). Magnox, contained in an inert basket of suitable size and ofsuitable mesh to retain undissolved Nimonic alloy, is lowered into the magnesium bisuiphate reagent. The Magnox dissolves in the stirred or sparged reagent (equation 4) and hydrogen is produced as an off-gas. When the reaction is complete, the basket is raised, drained and removed from the vessel. A half portion of the magnesium sulphate product is removed for discharge to sea. The requisite amount of sulphuric acid is then added to the remaining magnesium sulphate product to regenerate fresh reagent. The process starts again with a fresh basket of Magnox. Undissolved Nimonic alloy can be removed after each dissolution, if required.

A simplified procedure is used for sodium bisuiphate dissolution. Priming is not required, the reagent being added directly to the dissolution vessel. After dissolution of the Magnox all of the product solution is available for sea discharge.

In practice it may be advantageous to: Use a deficiency or an excess of Magnox in equations (4)01(6). This ensures that dissolution is completed more rapidly.

To avoid excess bisuiphate in the effluent for disposal, excess Magnox is preferred; otherwise neutralization of the excess bisuiphate may be required.

Illustrative weights and volumes are used in the following example. These perform well when scaled down to laboratory levels. Using the magnesium bisuiphate procedure a 2m3 process volume is assumed. The process is primed by adding 2 kmol (493 kg) MgSO4.7H20 (magnesium sulphate) to approximately 1.5m3 water and 2 kmol (222 litres wI) sulphuric acid in the dissolution vessel. The solution is then mixed to produce magnesium bisuiphate solution, (equation 5). 96 kg Magnox, which is I OO% in excess, is then added and the mixture and agitated until dissolution is completed as indicated by pH measurement or some other means. At this point all the magnesium bisulphate will have been consumed (equation 4), and the excess Magnox will remain undissolved. The resultant solution is diluted to give a total volume of 2m3 by the addition of water, I m3 of which is transferred to a holding vessel prior to sea discharge. To the remaining I m3 of magnesium sulphate solution, an additional 48 kg Magnox is then added, to maintain the 100% excess, prior to the addition of 0. Sm3 water and the sulphuric acid, as above, which is then mixed resulting in the dissolution of more Magnox. After dissolution, the solution is diluted to 2m3 as before, 1 m3 is again discharged to the holding vessel and the cycle repeated.

Using sodium bisuiphate solution the procedure is similar but simpler. Weights and volumes are less critical. Approximately 2m3 sodium bisulphate solution (2 molar) is transferred to the dissolution vessel. About 96 kg Magnox, 100% in excess, is then added and dissolved. Once the reaction is complete with the consumption of the sodium bisulphate (equation 6), the whole solution is then transferred to the holding vessel prior to sea disposal. The process is then repeated but after the initial 96 kg of Magnox addition to the first batch, 48 kg is added to subsequent batches to preserve the approximate 100% excess On a laboratory scale, using magnesium metal, the temperature increases from ambient to 40-70 degrees C, depending on the surtlice area of the magnesium, and then subsides. The temperature may need to be controlled in a larger system. A suitable operating temperature between 40 and 90 degrees C may be chosen to complete the reaction.

The laboratory trials indicate that the dissolution of 50 to 100kg of Magnox will take approximately 3 hour to complete assuming a 2m3 dissolution vessel.

Process control is required to: a) Confirm Magnox dissolution has finished. For example, the "excess of magnesium" procedure, the pH of the magnesium bisulphate solution, initially about 1, increases to about 7 as the Magnox and any magnesium hydroxide and carbonate present dissolve and all the magnesium bisuiphate converts to magnesium sulphate. This process is probably best monitored by continually measuring the pH. For the "deficit of magnesium" procedure, conductivity is the preferred technique for monitoring the process and for confirming that the reaction is completed, as the pH change is much less.

b) Confirm that the radioactivity of the solution for disposal Ihils within the required limits.

Claims

CLAJMS

1. A method for dissolving Magnox, magnesium hydroxide and magnesium carbonate, in bisuiphate solution which results in a solution suitable for sea disposal.

2. A method according to Claim I wherein materials that do not dissolve in bisulphate solution are segregated prior to disposal of the resulting solution.

3. A method according to Claim I or 2 wherein the bisuiphate is present as potassium bisulphate.

4 A method according to Claim 1 that dissolves Magnox at a faster rate than the established carbonic acid process.

5. A method according to Claim 1 that reduces the overall amount of waste discharged in comparison with the carbonic acid process.

6. A method according to Claim I comprising recycling half of the magnesium from the dissolved Magnox to generate fresh magnesium bisuiphate reagent.

7. A method according to Claim 1 wherein aluminium and zirconium are retained in, or segregated from, the solution for sea discharge.