EP1721021B1

EP1721021B1 - Recycling of hot-dip zinc galvanizing bath

Info

Publication number: EP1721021B1
Application number: EP05715245A
Authority: EP
Inventors: Michael Gilles; Roger Pankert; Richard Sokolowski; Mathieu Eerdekens
Original assignee: Umicore NV SA
Current assignee: Umicore NV SA
Priority date: 2004-02-26
Filing date: 2005-02-01
Publication date: 2007-06-06
Anticipated expiration: 2025-02-01
Also published as: ES2287903T3; DE602005001323D1; ATE364097T1; WO2005083134A1; EP1721021A1; PL1721021T3; DE602005001323T2

Abstract

The present invention relates to a process to recover zinc from zinc containing residues or scraps, especially those produced by hot-dip galvanizing lines. The process comprises the steps of: - selecting a Si-bearing compound which, at a temperature up to 700 °C, either reacts with zinc, either melts, or dissolves in zinc; - adding the selected Si-bearing compound to an Fe-containing molten zinc bath, whereby the iron in the zinc bath reacts to produce an Fe-Si matte; - allowing the Zn bath to settle, whereby the Fe-Si matte raises to the surface of the bath, and - collecting the Fe-Si matte from the surface of the zinc bath.

Description

The present invention relates to a process to recover zinc from zinc-containing residues or scraps, especially from scraps produced by hot-dip galvanizing lines.
In the past, recycling was performed by distillation or by feeding the zinc-bearing residues to a primary zinc smelter. Although highly pure zinc could be recovered, these processes are now considered to be too costly. Moreover, most of the alloying elements that are present in the scraps are lost.
Nowadays, scraps or residues are typically recycled in a melt furnace were zinc is molten and separated from the remaining solids. An impurity that is only partially removed during this process is iron: due to the solubility of iron in molten zinc, the recovered zinc will still contain at least 0.02 wt% Fe. If such zinc is re-used in a galvanizing process, the iron will lead to the formation of extraneous zinc-iron residues and hence to increased zinc losses. The introduction of e.g. 100 kg zinc containing 0.025 wt% Fe will generate 1 kg of Zn-Fe residue. This illustrates the highly negative impact of even minute amounts of iron contaminating the zinc. Iron removal to 0.01 wt%, or preferably to 0.005 wt% or less, is considered as satisfactory.
Methods were developed to remove iron from zinc to a level far below its solubility in zinc. In US 3,902,894 it was proposed to separate the iron contained in the crystals forming the matte by the addition of aluminium. Aluminium has a high affinity for iron, and forms an aluminium-iron intermetallic compound, which can be separated. However, a specially developed and complicated reactor is needed to perform the purification reaction.
It has now been found that, even in the presence of aluminium in a zinc bath, silicon will preferentially react with the iron to form a floating Fe-Si matte, which can easily be removed. A practical problem however is that it is very difficult to dissolve silicon in molten zinc. Pure silicon has a melting temperature of 1412 °C and does not dissolve in zinc. In US 3,685,985 the use of silicon alloys in general was disclosed, but due to the high content of silicon proposed here, the efficiency of the iron removal is limited, and only relatively pure zinc residues can be treated.
In order to overcome the problems cited before, a new method for removing iron from a molten zinc bath was developed. Such a process comprises the steps of

adding a Ni-Si alloy to the zinc bath, whereby the iron in the zinc bath reacts with said compound to produce an Fe-Si matte,
allowing the Zn bath to settle, whereby the Fe-Si matte raises to the surface of the bath, and
collecting the Fe-Si matte from the surface of the zinc bath.

This process is typically applied to a secondary zinc product, such as top dross from hot-dip galvanizing baths, preferably after the removal of any solids remaining on zinc bath upon melting.
The binary system Al-Si has an eutectic composition with 11.7 wt% Si, having a minimum melting temperature of 577°C. At higher or lower Si concentrations, the melting temperature increases rapidly. In practice, it shows that only binary alloys close to this eutectic composition melt easily in molten zinc. Other binary Al-Si alloys only dissolve after very long mixing time or must be melted at high temperature. Besides higher energy consumption, higher temperatures are very inconvenient since the vapor pressure of zinc becomes significant above 600°C, leading to considerable evaporation of zinc. Therefore, in practice, from all binary Al-Si alloys, only those alloys containing between 10 wt% Si and 15 wt% Si are appropriate to remove Fe from zinc or zinc alloy. It follows that, when using an Al-Si alloy, the molten zinc temperature should preferably be above the melting point of the alloy, which is about 580 °C, as this ensures rapid mixing with the zinc.
In particular cases however, the introduction of aluminium in the zinc may be considered as a drawback, e.g. when the de-ironed zinc is to be used in a specific low aluminium hot-dip galvanization process.
In DE3911060 A1 the use of a silicon and aluminium containing alloy was disclosed, but the advantage of using an alloy of 85-90 wt% Al and 10-15 wt% Si was not recognized.
A Ni-Si alloy composed of 60-70 wt% Ni and 30-40 wt% Si is preferred. For example, use can be made of the commercially available 65 wt% Ni - 35 wt% Si alloy, which has a melting temperature of 992 °C. The use of the eutectic alloy with 62 wt% Ni and 38 wt% Si, which melts at a slightly lower temperature, is also possible. It has been found that these alloys dissolve rapidly in molten zinc, even at temperatures well below their melting point. When a Ni-Si alloy is used to precipitate the iron, some nickel will dissolve in the alloy. This is entirely acceptable since nickel is a valuable alloying element which is commonly used in general galvanizing.
SiCl₄ is also suitable as a silicon source. This volatile compound can be injected as a gas through the zinc melt where it immediately reacts to form very finely dispersed elemental silicon which dissolves and reacts readily with iron, thereby forming ZnCl₂.
The process is preferably carried out at a bath temperature between 480 and 700 °C. Too low a temperature results in prohibitively slow reaction kinetics, while too high a temperature results in increasing zinc losses through evaporation. A bath temperature limited to 600 °C is even more advisable to render the process more energy efficient.
After addition of the silicon compound, the reaction can be accelerated by mixing the melt by hand, with a mixer, or by the electromagnetic stirring effect typically generated by an induction furnace. The latter increases the reaction rate of silicon in a significant way. When stirring is stopped, the Fe-Si matte formed will rise and float on the surface of the zinc bath, from where it can easily be removed. The invented process allows for the removal of iron down to 0.002 wt%, or even down to 0.001 wt% in the zinc bath. Such results are to be considered as excellent.
The following examples illustrate the advantages of the use of a Ni-Si alloy in the removal of iron.

Example 1

In a crucible, 10 kg of zinc with 0.024 wt% Fe is heated until a homogeneous melt is obtained. Finely divided (<3 mm) 65 wt% Ni - 35 wt% Si alloy is then added. After stirring for 15 minutes, a 5 minutes settling period is inserted, during which the Fe-Si particles that are formed raise to the surface of the melt. By spreading NH₄Cl on top of the bath, an exothermic reaction occurs and zinc retained in the Fe-Si matte is allowed to melt out, leaving a powdery layer on the surface. The surface is then skimmed and the iron concentration in the melt is determined. This experiment is repeated at different bath temperatures and with different amounts of Ni-Si alloy.
A summary of the results is given in Table 1. In this Example, 2.2 g Ni-Si per kg zinc is found to be sufficient to lower the iron concentration in the zinc from 0.024 wt% to 0.002 wt% at a temperature of 500 °C. At the same time, the nickel concentration went up from 0.014 wt% to 0.036 wt%. Table 1: Residual iron in zinc melt using Ni-Si alloy

Ni-Si added Temperature of the melt (°C)

(g/kg Zn) 450 500 550

Residual Fe in Zn (wt%)

0 0.024 0.024 0.024

1.5 - 0.015 -

2.2 0.017 0.002 0.002

2.6 0.010 0.002 0.002
In other experiments it was shown that by optimising the stirring, e.g. by using an induction furnace, the Ni-Si quantities added could be lowered to less than 1 g/kg zinc. Satisfactory de-ironing could be performed at temperatures as low as 480 °C.

Example 2 (comparative example)

In a crucible, 10 kg of zinc with 0.024 wt% Fe is heated to 600 °C. The 88.3 wt% Al - 11.7 wt% Si eutectic alloy is added. After stirring for 15 minutes, a 5 minutes settling period is inserted during which the Fe-Si particles that are formed raise to the surface of the melt. The surface is then skimmed and the iron concentration in the melt is determined. This experiment is repeated with different amounts of Al-Si alloy. A summary of the results is given in Table 2. In this example, 5 g Al-Si per kg zinc is sufficient to lower the iron concentration in the zinc from 0.024 wt% to 0.005 wt% at a temperature of 600 °C. At the same time, the aluminium concentration went up to 0.35 wt%. Table 2: Residual iron in the zinc melt using Al-Si alloy

Al-Si added (g/kg Zn) Residual Fe in Zn (wt%)

0 0.024

1.0 0.009

5.0 0.005

10.0 0.001

Example 3 (comparative example)

In a crucible, 10 kg of zinc with 0.024 wt% Fe is heated to 600 °C. Gaseous SiCl₄ is blown at a rate of 2 g/min through the bath using an immersed tube reaching the bottom of the crucible. The end of the tube is equipped with a porous ceramic piece, ensuring the production of finely dispersed gas bubbles. The zinc bath is thoroughly mixed during gas injection. Table 3 summarises the results, showing that nearly complete iron removal is again achieved. Table 3: Residual iron in the zinc melt using SiCl₄

SiCl₄ added (g/kg) Residual Fe in Zn (wt%)

0. 0.024

0.56 0.016

0.98 0.010

1.96 0.001

Example 4

As a further example, the iron concentration in a 2000 kg zinc alloy melt is reduced from 0.025 to 0.010 w% by adding 3.4 kg of a 65 wt% Ni - 35 wt% Si binary alloy. This corresponds to a Si to Fe weight ratio of 3.5. Ni-Si is advantageously added as finely crushed material, with particles smaller than 3mm. These particles are immersed in a perforated bulb to prevent them from floating on top of the melt. After 20 minutes of mixing and 5 minutes of decanting, the resulting Fe-Si floating to the surface of the bath is skimmed off. Spreading NH₄Cl on the surface before skimming can reduce considerably the amount of zinc that is entrained with the Fe-Si.
In industrial practice, the iron-containing zinc or zinc alloy is molten, and a sample is taken to determine its iron concentration. Sampling can also be performed on the feed to the melting furnace.
The amount of molten metal in the furnace can be determined from the level of the melt in the furnace or by weighing. The total quantity of iron in the melt can thus be calculated. This amount is used to determine the needed quantity of reagents (Ni-Si, Al-Si or SiCl₄). The useful Si to Fe weight ratio can vary between 0.5 and 6, since the yield of the reaction will depend on the temperature and on the degree of mixing. Less efficient mixing and lower temperatures will result in increased reagent needs.

Claims

Process for the removal of iron from a molten zinc bath, comprising the steps of
- adding a Ni-Si alloy to the zinc bath, whereby the iron in the zinc bath reacts with said compound to produce an Fe-Si matte;

- allowing the Zn bath to settle, whereby the Fe-Si matte raises to the surface of the bath, and

- collecting the Fe-Si matte from the surface of the zinc bath.
Process according to claim 1, wherein the molten zinc is a secondary zinc product.
Process according to claim 2, comprising a preliminary process step whereby solids floating on the molten zinc bath are removed before adding the Si-bearing compound.
Process according to claim 3, wherein the secondary zinc product comprises top dross from a galvanizing bath.
Process according to either one of claims 1 to 4, wherein the Ni-Si alloy is composed of 60-70 wt% Ni and 30-40 wt% Si.
Process according to either one of claims 1 to 5, wherein the molten zinc bath is maintained at a temperature between 480 and 700 °C.