AU620822B2

AU620822B2 - Magnesium-calcium alloys for debismuthizing lead

Info

Publication number: AU620822B2
Application number: AU24515/88A
Authority: AU
Inventors: Bernard Closset; Douglas J. Zuliani
Original assignee: Timminco Ltd
Current assignee: Timminco Ltd
Priority date: 1988-05-20
Filing date: 1988-10-28
Publication date: 1992-02-27
Anticipated expiration: 2008-10-28
Also published as: ATE127859T1; YU102489A; DE68924194T2; EP0343012B2; DE68924194T3; EP0343012B1; JPH0270038A; EP0343012A3; JP2714984B2; AU2451588A; EP0343012A2; DE68924194D1; GR3018160T3; ES2076961T3

Abstract

A novel alloy for use in lead refining is disclosed which comprises magnesium and calcium. The preferred ratio on a weight basis of magnesium to calcium is between 1.2 and 5.2. A method of adding this alloy to a lead bath is described which brings calcium into solution with the lead and provides a high recovery ratio.

Description

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AUSTRALIA

PATENTS ACT 195 08 Form COMPLETE SPECIFICATTIO

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: 0e

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TO BE COMPLETED BY APPLICANT Name of Applicant: TIMMINCO LIMITED Address of Applicant: P.O. BOX 1160 STATION A ONTARIO M5W

CANADA

Actual Inventor: Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: MAGNESIUM-CALCIUM ALLOYS FOR DEBISMUTHIZING LEAD The following statement is a full description of this invention including the best method of performing it known to me:-

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The present invention relates in general to calciummagnesium alloys and more particularly to calcium-magnesium alloys for use in the removal of bismuth from lead by the Kroll-Betterton process, or for similar lead related processes which require alkaline-earth metald.

BACKGROUND

In the Kroll-Betterton process, alkaline earth metals are added to the lead melt in order to react with the bismuth therein. One or more alkaline earth metals, usually magnesium and calcium, are added in either a continuous or batch fashion to the unrefined lead. The preferred temperature range for making the addition is between 380 0 C to 450 0 C. Below this temperature range, the reaction is sluggish while excessive oxidation of reactive alkaline earth metals, particularly calcium, occurs at higher temperatures. Oxidation gives rise to bright flaring excessive fume generation and an overall loss of reagent leading to lower reagent recoveries, excessive processing costs and unpredictable final bismuth levels. Furthermore, the addition of calcium metal to the lead bath is often accompanied by an increase in the bulk .temperature of the lead either due to an exothermic release of heat during the reaction and/or to heat generated by the oxidation of calcium metal. This increase in bath temperature results in additional calcium oxidation as well as lengthening the overall processing time since the melt must be cooled to just above its solidification point prior to removing the bismuth rich dross.

-2- Another disadvantage of calcium metal is that it is highly reactive with atmospheric oxygen and humidity. Hence, calcium metal must be packaged, shipped and stored in such a way as to eliminate contact with air and moisture. Excessive contact with water will result in heat and hydrogen evolution which can cause fire and explosion. Mild contamination of the calcium prior to the lead treatment will also result in lower than expected reagent recoveries and unpredictable final bismuth levels.

10, After the lead has been treated with the alkaline 0 0 metals, the melt is then cooled to a temperature near its 6:06 solidification point which causes the alkaline-earth bismuth compounds to float up as a solid dross which may be skimmed from the surface of the melt.

French Patent Application No. 81 19673 assigned to *e Extramet (Publication No. 2514 786, April 22, 1983) discloses i. a process for debismuthizing lead by using a mixture of two types of granules. The first type of granule comprises a 0 calcium-magnesium alloy near the calcium-rich eutectic point (approximately 82 weight calcium) and the second alloy comprises a magnesium-calcium alloy near the magnesium-rich eutectic point (approximately 16.2 weight calcium). These two types of granules are mixed together in the appropriate amounts to give the ratio of the metals for the best result and are injected into the lead melt to react with the bismuth therein. The composition of the individual alloys are chosen to be near the eutectic points so that they have relatively rn/rm I -3lower melting points compared to pure magnesium and calcium metals with respect to the temperature of the lead melt. It is claimed that this speeds up the rate of the reaction at a given processing temperature. The mixture is injected into the lead bath with an inert gas.

This heterogeneous mixture of magnesium-rich and calcium-rich alloy granules is still susceptible to poor reagent recovery because the calcium-rich alloy granules will behave in "much the same way as pure calcium metal. Because of lQ the composition of calcium-rich eutectic alloy granules, the S. eutectic may contain up to almost 2/3 of finely divided calcium metal with the remainder being a Mg 2 Ca intermetallic com- S! pound. The high proportion of calcium metal in the eutectic causes the calcium-rich alloy granules to react with atmospheric oxygen and humidity in much the same way as calcium metal. Tests in our laboratory with ingots cast at the calo* cium-rich eutectic composition shown that this alloy reacts with atmospheric oxygen and humidity and, hence, is not stable in air.

Because of the reactive nature of the calcium-rich granules, the heterogeneous granule mixture of magnesium-rich granules and calcium-rich granules must be packaged under dry, inert gas in a similar fashion to calcium metal. Contamination of the calcium-rich granules with oxygen or moisture prior to treatment will result in lower reagent recoveries and unpredictable final bismuth levels., The calcium-rich granules are also susceptible to oxidation during treatment with the rn/rm lead in much the same way as calcium metal, especially if they float to the surface due to large differences in density between lead and calcium before they have completely reacted.

The injection of the granules into the lead bath with an inert gas carrier adds additional turbulence to the melt, increasing the amount of oxidation and emissions from the lead bath.

In the present invention, the difficulties associated with the use of calcium metal or granular mixtures containing calcium-rich alloy granules are avoided by using a single magnesium-calcium alloy of the desired compositioi.

In this invention, the alloy is primarily made up of the magnesium and calcium but may contain one or more minor amounts of other alloying elements.

*:Ol In the present invention, an alloy for use in lead 9 06 refining is provided which is rich in magnesium and has a magnesium to calcium ratios on a weight basis between 1.2:1 and 5.7:1. Preferably the lower ratio corresponding to the intermetallic compound Mg 2 Ca. In a preferred embodiment of S2" the invention, the alloy has a magnesium to calcium ratio between about 1.9:1 to 3l.

The alloys of the present invention are preferably prepared by melting the appropriate proportions of calcium ooooo S" and magnesium metals under a protective atmosphere and *2 pouring and solidifying the alloy in the same or similar 0:•0 protective atmosphere. The protective atmosphere may comprise nitrogen, argon or any other gases which are protective or non-reactive when in contact with magnesium and 0 calcium. The temperature used to •o ooo In a further aspect of the present invention, a method for achieving the solution of calcium in lead resulting in high recoveries is provided. This method comprises the steps of providing a magnesium and calcium alloy which has a alo S7:/ magnesium to calcium ratio between 1.2 and and adding this alloy to a lead bath.

Since these magnesium-rich alloys consist of eutectic structures which contain mostly finely divided magnesium metal and Mg 2 Ca intermetallic with the complete absence or only minor quantities of finely divided calcium metal, they coo are not subject to the aforementioned difficulties associated S*0@ with calcium metal or calcium-rich alloy granules.

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go In the present invention, these alloys are stable in air. Since the alloy does not oxidize or hydroxylize in air, it does not require special packaging or protective atmospheres. There is no danger of fire or explosion if these oo*o alloys come in contact with moisture.

When added to liquid lead, these alloys react with minimal or no oxidation. The reaction is often accompanied by a minor degree of bubbling; however, there is essentially little or no flaring or fume generation. Since the alloys are not prone to contamination from contact with air prior to treatment and do not excessively oxidize during the treatment even if the alloy floats to the surface of the lead bath, reagent recoveries are higher and mor.e predictable than with other reagents. This reduced variability substantially -6- 10.

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6 increases the predictability of achieving the desired final bismuth level which is particularly important when aiming at low bismuth levels of less than 0.01%.

The alloy is preferably added to the lead bath in the form of large ingots. Under some circumstances, smaller ingots, large chunks, granules or powder may also be used.

When the alloy is added to the lead bath, the bulk temperature of the melt does not increase as is the case with calcium metal additions. Contrary to calcium metal additions which have an upper temperature limit to minimize oxidation and flaring, the dissolution rate of this alloy increases with increasing temperature with the preferred temperature being between 475 to 550 0 C. The dissolution rate is also increased by agitation. Since there is virtually no flaring and related fume with this alloy even at high temperatures and with agitation, no special fume collection system is required to contain emissions which may be given off during the treatment. Agitation is generally avoided when calcium metal is utilized as it increases oxidation and flaring.

After the alloy has been added to the lead melt and the dissolution is complete, the lead melt is allowed to cool in the customary fashion of the Kroll-Betterton process to separate out the solid bismuth rx.ch dross.

The following-examples are given to demonstrate the high reagent recoveries that are possible with this alloy.

Refined lead low in bismuth was used in all tests to enable investigation of the effects of process conditions on alloy rn/rm II_ f i 1 I *0 0 6000* 0 00 0 00 a 00

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EXAMPLE 1: Approximately 98.8 grams of a magnesium-calcium 2.7 :I alloy with a magnesium to calcium ratio of.2-- was plunged into a 20 kilogram quiescent lead melt at 419°C. No flaring, oxidation or fume was observed. Approximately 45% of the alloy dissolved after 30 minutes with essentially 100% reagent recovery. Final magnesium and calcium analyses were 0.16% and 0.06% respectively.

EXAMPLE 2: Approximately 98.7 grams of a magnesium-calcium

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alloy with a magnesium to calcium ratio of,:4 4 was plunged into a 20 kilogram agitated liquid lead melt at 415"C. No flaring or fume was observed. Approximately 98% of the alloy dissolved after 23 minutes of stirring with essentially 100% reagent recovery. The final magnesium and calcium analyses were 0.33% and 0.11%, respectively.

EXAMPLE 3: Approximately 98.8 grams of a magnesium-calcium 'alloy with a magnesium to calcium ratio of 2.7 was plunged into a 20 kilogram quiescent lead melt at 432 0 C. Approximately 90% of the sample had dissolved after 30 minutes with essentially 100% reagent recovery. No flaring or fume was observed during the treatment. The final magnesium and calcium anaylses were 0.32% and 0.12% respectively.

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EXAMPLE 4: Approximately 97.7 grams of a magnesium-calcium alloy with a magnesium to calcium ratio of 4 was plunged into a 20 kilogram quiescent liquid lead melt at 500*C. The reaction was characterized by heavy bubbling; however, no flaring, oxidation or fume was evident. The alloy was completely dissolved after 12 minutes with essentially 100% recovery at 0.38% magnesium and 0.13 calcium. Black dross was observed S to form on top of the melt after 22 minutes which was accompan:ed by a 13 15% fade in the dissolved magnesium and calcium after 30 minutes to 0.33% magnesium and 0.11% calcium.

see 0 In summary, this application has disclosed an *o invention which improves the dissolution characteristics of magnesium and calcium in lead thereby improving the efficiency of bismuth removal from lead. This alloy is stable in atmospheric air and humidity and requires no special protective packaging as does calcium metal. When added to liquid lead, t the alloy dissolves with essentially no oxidation, flaring and fume generation. This results in higher and more consistent 0 20. reagent recoveries and more predictable final bismuth levels c which are pazticularly important when aiming for final bismuth levels less than about 0.01%. The virtual absence of fume precludes the need for special fume collection systems. The absence of flaring and oxidation enables the alloy to be added with agitation and at higher processing temperatures than is customary with calcium metal.

The present invention has been described using pre- -9ferred ratios of magnesium to calcium. Cleatly, minor variations in these ratios may be made within the scope of the invention. The alloy may contain other constituents, such as different alkali earth metal, which do not E.ffect the essential nature of the metallurgical process herein disclosed.

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Claims

2. A method for achieving the solution of calcium in lead with eventual high recovery comprising the steps of: providing an alloy comprising primarily magnesium and calcium having a ratio of magnesium to calcium between 1.2:1 and 5.7:1 on a weight basis with any balance of the alloy being incidental impurities; and acding said alloy to a lead bath and permitting the alloy to dissolve in the lead bath.
3. A method for achieving the solution of calcium in lead with eventual high recovery comprising the steps of: providing an alloy comprising primarily magnesium and calcium having a ratio of magnesium to calcium between 1.9:1 and 3:1 on a weight basis with any balance of the alloy being incidental impurities; and Sadding said alloy to a lead bath and permitti.ig 0 the alloy to dissolve in the lead bath. 00 6 4. A method according to claim 2 or 3 wherein said 0 alloy is added to the lead in the form of solid ingots. S 5. A method according to claim 2 or 3 wherein said see*$: 0 lead bath is at a temperature of 475 0 C to 550 0 C.
06. A method according to claim 3 or 4 further 00 including the step of agitating said lead bath. 00 DATED THIS 28th DAY OF November 1991 TIMMINCO LIMITED By Its Patent Attorneys GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia