(Zhang Tianzhi) [lead and Zinc Summit] Diversified Utilization of Zinc-containing wastes

SMM May 18: at the 16th lead and Zinc Summit held by Shanghai Futures Exchange and SMM in 2021, Zhang Tianxi, head of the Technology Center of Weifang Longda Zinc Industry Co., Ltd., introduced the diversified utilization of zinc-containing waste to the guests.

Picture and recording

Overview of Zinc-containing waste

Iron and steel plant zinc waste is rich in iron, carbon, zinc, lead and other substances, which has a good comprehensive utilization value. How to make efficient use of zinc-containing waste in iron and steel plants and improve its comprehensive added value to reduce environmental pollution has always been an important research topic for metallurgical enterprises and research colleges and universities.

Sources of zinc-containing waste:

1 zinc slag / zinc ash of hot-dip galvanizing industry. In the process of hot-dip galvanizing of steel, zinc consumption accounts for about 60% of the total zinc consumption, and a large amount of zinc enters into zinc slag and zinc ash.

(2) waste zinc die castings in foundry industry. Zinc die castings account for 25% of the total die castings, and the normal service life of zinc die castings is 10 to 20 years. Every year, a large number of zinc die castings are scrapped, such as scrapped auto parts, household appliances and so on.

(3) waste zinc, waste parts, scraps, scraps, chips and so on in zinc processing industry.

(4) scrap dry battery. Including zinc-manganese dry battery, alkaline zinc-manganese battery, zinc-air battery and so on.

5 steel mill ash. Zinc-containing dust mainly comes from blast furnace ash, converter ash and electric furnace ash.

(6) the scum produced in the process of smelting zinc and zinc alloy, the spatter in the casting process, zinc ash and so on.

7 zinc-containing sludge. For example: chemical industry and chemical production process of waste insurance powder, hanging white block and other production process of zinc hydroxide sludge.

8 zinc-containing smelting slag. Soot from lead and copper smelting systems.

Classification of zinc-containing wastes:

Zinc-containing waste is mainly divided into zinc ash and zinc slag, zinc ash mainly contains zinc oxide, zinc chloride, zinc ferrite or other zinc compounds, part of metal zinc; zinc slag mainly contains metal zinc, mixed with aluminum, iron, nickel and other metals.

It is worth mentioning that the National Hazardous waste list stipulates that the chemicals listed in the Hazardous Chemicals list are hazardous wastes. Item 2358 of the catalogue of Hazardous Chemicals (2016) includes zinc dust, zinc powder and zinc ash, so zinc ash should be hazardous waste.

Diversified Utilization of Zinc Slag

The essence of zinc slag is zinc alloy, including zinc-iron alloy, zinc-iron aluminum alloy and zinc-iron-aluminum-magnesium alloy. According to the difference of melting point and boiling point of metals, different metals can be separated from alloys. Corresponding to it, there are atmospheric pressure volatilization and distillation.

Atmospheric pressure volatilization

The boiling point of zinc is much lower than that of impurities in zinc slag, zinc is volatilized into zinc vapor at atmospheric pressure and high temperature, and metallic zinc is obtained after condensation. Atmospheric pressure volatilization is one of the common treatment methods of hot-dip galvanized slag, and its metal recovery rate can reach more than 98%. In addition, it also has the characteristics of simple process and so on.

Atmospheric pressure volatilization to produce zinc oxide: atmospheric pressure volatilization "metal zinc" heating / oxidation indirect zinc oxide (ZnO content ≥ 99.7%)

Distillation method

Zinc is separated from other metals by continuous fractionation processes such as evaporation, condensation and reflux in an airtight column because of the difference in boiling point between zinc and other metals in zinc slag. To achieve the purpose of purification. According to its vapor pressure or boiling point, the impurity metals that may be contained in crude zinc can be divided into two categories:

1) impurities with vapor pressure higher than (or boiling point below) zinc, such as Cd, etc.;

2) impurities whose vapor pressure is lower than (or boiling point above) zinc, such as Pb, Fe, etc.

The distillation process can be divided into two stages:

In the first stage, the hot-dip galvanizing slag is melted in the galvanizing furnace to remove the scum to obtain crude zinc.

The second stage is carried out in the cadmium tower. After repeated distillation and reflux of cadmium-containing zinc through the cadmium tower, pure zinc is produced in the lower part of the tower, and cadmium is enriched in high-cadmium zinc.

The study shows that it is feasible to recover zinc from hot-dip galvanizing slag by vacuum distillation. when the distillation temperature is 1173K, the pressure is 50m 100pa, and the distillation time is 14h, the recovery rate of metal zinc can reach 93.37%, and the product zinc reaches the international No. 1 metal zinc.

Disadvantages: vacuum distillation takes a long time and requires high equipment.

In addition to the above two separation methods, there is also the melting process. But this method can only deal with iron and zinc slag. The elements with high affinity with iron (Al, etc.) are added to the hot-dip galvanized slag to form Me-Fe alloy slag which is insoluble in molten zinc, and the effect of iron removal is achieved by using different density melting point. After removing iron, zinc is added chlorination agent (KCl, sodium chloride, etc.) to remove impurities in liquid zinc by chlorination, volatilization or slagging.

The advantages are simple process, low investment, quick effect, low process temperature, low energy consumption, and iron can be reduced to 0.0028% to 0.0078%. But the disadvantage is also obvious, a lot of scum will be produced in this process, and the direct yield of metal is only about 65%.

Diversified Utilization of Zinc Ash

The formation mechanism of zinc ash:

The properties and characteristics of zinc: easy to volatilize, easy to oxidize and easy to react.

In general, the mass fraction of zinc in all kinds of zinc ash is 5% to 70%. According to the difference of zinc content, the recovery of zinc-containing waste can be divided into leaching zinc extraction method and fire enrichment recovery method. When the zinc content is more than 15%, the leaching zinc extraction method can be used; when the zinc content is less than 15%, the fire enrichment and recovery method can be used to volatilize and enrich the zinc (which can be enriched to 45%-60% by mass), and then deal with the smoke and dust.

Wet treatment technology;

Zinc oxide is soluble in acid or alkali solution, so a suitable leaching agent can be selected by wet process to separate zinc from other impurities.

Specifically, it can be divided into acid leaching, alkaline leaching and ammonia leaching.

Fire treatment technology:

With the continuous improvement of non-ferrous metal extraction technology, the mass fraction of valuable metal elements in some zinc-containing wastes is relatively low. such as copper metallurgy waste materials (containing zinc 6% 15%, copper 0.6% 4%, lead 5% ~ 20%), high zinc slag produced in lead smelting process (5% 15% zinc), ironmaking blast furnace gas ash (6% 15% zinc), and so on.

The mass fraction of valuable metal elements in these wastes is on the low side, so it is not cost-effective to extract the metals directly by wet method. Therefore, through further enrichment, the volatile metal elements in the material must be enriched into the soot, and then various valuable metals can be extracted from the soot.

At present, domestic small and medium-sized enterprises all adopt rotary kiln volatilization method and roasting method, while large state-owned enterprises mostly adopt fuming volatilization method.