(Zhang Tianxi) Diversified Utilization of Zinc-containing wastes [Zinc oxide Forum]

SMM: at the ninth Green Zinc Salt and Zinc oxide Industry Summit Forum 2021 held by SMM, Zhang Tianxi, technical director of Weifang Longda New Materials Co., Ltd., introduced "diversified utilization of zinc-containing waste" for the guests present. He first talked about the source, classification and attribute identification of zinc-containing waste, and then explained the diversified utilization of zinc slag and zinc ash. Finally, the opportunities and challenges of diversified utilization of zinc-containing wastes in the future are analyzed.

I. Overview of zinc-containing waste

Overview of Zinc-containing waste sources of Zinc-containing waste

Zinc is an important basic metal, which is widely used in construction, machinery, transportation and other industries in daily life. A large amount of zinc-containing waste is inevitably produced in the process of zinc smelting. For example, the hot-dip galvanizing slag produced in the hot-dip galvanizing industry, the zinc scum produced in the process of smelting zinc and zinc alloy, and the steel plant ash produced in the process of scrap recovery.

Overview of Zinc-containing wastes Classification of Zinc-containing wastes

Different production processes produce different zinc-containing wastes. However, the phase of zinc in zinc-containing waste can be divided into two types: one is metal phase, the other is compound phase.

Hot-dip galvanizing slag, waste zinc die castings and waste zinc mainly contain metal phase zinc, among which waste zinc die castings and waste zinc are mainly returned to upstream manufacturers for reuse. therefore, the zinc waste containing metal phase zinc is mainly hot-dip galvanizing slag, hereinafter referred to as zinc slag.

Hot-dip galvanizing ash, steel plant ash, zinc-containing smelting slag and so on mainly contain compound phase zinc, this kind of zinc includes but is not limited to zinc oxide, zinc chloride and so on. Because most of them are gray or scattered slag, they are often called zinc ash.

Overview of Zinc-containing wastes attribute Identification of Zinc-containing wastes

Most of the zinc-containing wastes are hazardous wastes, and the appropriate technical route must be selected in the process of utilization.

The identification of hazardous waste in China is mainly divided into four steps: judging whether it belongs to solid waste, judging whether it is in the list of hazardous waste, judging whether it has hazardous characteristics, and expert certification.

Identification details

The National Hazardous waste list stipulates that the chemicals listed in the Hazardous Chemicals list belong to hazardous waste after being discarded. Item 2358 of the catalogue of Hazardous Chemicals (2016) includes zinc dust, zinc powder and zinc ash, so zinc ash should be hazardous waste.

Generally speaking, there are many sources of zinc-containing waste, which can be classified into zinc slag and zinc ash according to the phase of zinc; on the other hand, zinc-containing waste has the attribute of hazardous waste, and the utilization link must meet the relevant safety and environmental protection requirements. Technically: quantitative toxicity leaching analysis is difficult to avoid, management: the dangerous characteristics of zinc ash and zinc chloride are difficult to avoid, the legal level: the legal liability of waste production units and utilization units can not be avoided.

II. Diversified utilization of zinc slag

The formation process of hot-dip galvanizing slag is the process of dissolution and reaction of zinc element in zinc liquid with iron, aluminum and other elements in workpiece or alloy additive. In essence, hot-dip galvanized slag is a kind of zinc alloy.

When the temperature of liquid zinc is 450 ℃, the melting degree of iron in liquid zinc is 0.03%. Beyond this degree of melting, iron can not only continue to melt into the liquid zinc, but also precipitate solid zinc-iron metal compounds, which is what we often call zinc slag. After sampling the hot-dip galvanized slag and analyzing the XRD spectrum, it is found that the main phases of the zinc slag are metal zinc and FeZn13 compounds.

In addition, if the content of metal aluminum in zinc slag is different, the position of zinc slag is also different. Zinc scum is produced when aluminum reaches 0.135%, and bottom slag is produced when aluminum is less than 0.135%.

Generally speaking, the utilization process of hot-dip galvanizing slag is a process of metal separation.

Diversified Utilization of Zinc Slag

The commonly used methods include melting method, atmospheric pressure volatilization method and vacuum distillation method. The melting process mainly deals with zinc-iron slag and forms Al-Fe alloy with lower density which is insoluble in liquid zinc by adding aluminum element to achieve the purpose of separating iron metal. The atmospheric pressure volatilization method is mainly based on the fact that the boiling point of zinc is much lower than that of impurities in zinc slag, and zinc is volatilized into zinc vapor at atmospheric pressure and high temperature, and metallic zinc is obtained after condensation.

The vacuum distillation method mainly uses the metal separation of aluminum-zinc-silicon slag. At the same temperature, zinc can be volatilized and condensed to get metallic zinc. The remaining Al-Si slag can be used to make deoxidizer to achieve the result that all elements are effectively utilized.

Generally speaking, the utilization of hot-dip galvanized slag should be the right remedy according to different types of zinc slag, the recovery of metal zinc or zinc oxide as high as possible, while other metals should also be used.

III. Diversified utilization of zinc ash

Because of its active and volatile chemical properties, zinc is easy to react with oxygen, chlorine and other elements in the processing process to form zinc oxide and zinc chloride. In addition, the temperature is above 650 degrees Celsius. Zinc oxide reacts with ferric oxide and silica to form zinc ferrite, zinc silicate and other substances. At the same time, zinc ash may be mixed with some metal zinc, such as hot-dip galvanized ash and zinc scum.

Therefore, the utilization of zinc ash is generally to separate the metal zinc first and reduce the compound state zinc to metal zinc by chemical reaction.

Due to the variety of sources of zinc ash, the zinc content in all kinds of zinc ash is different, and there is a great difference, so each factory will use different methods to deal with zinc ash according to the different zinc content.

In general, the zinc content of more than 15% is directly treated by wet method, and less than 15% can be enriched by fire before wet leaching.

Diversified Utilization of Zinc Ash Fire treatment Technology

At present, the mainstream fire treatment technology is rotary kiln / rotary hearth furnace reduction volatilization. The main principle is to decompose zinc ferrite and zinc silicate in high temperature atmosphere to form zinc oxide, reduce zinc oxide to zinc vapor in reduction atmosphere, and then oxidize zinc in oxidation atmosphere to get secondary zinc oxide products.

The proportioning of rotary kiln and rotary hearth furnace, especially the ratio of carbon to zinc, is very important. Zinc ferrite can be decomposed at 1000 degrees Celsius in the presence of carbon, but in the absence of carbon reduction, 20 percent of zinc ferrite is not decomposed even at 4000 degrees Celsius.

In addition, the kiln slag produced by rotary kiln needs grinding and magnetic separation to get iron powder, but the rotary hearth furnace process can directly obtain reducing iron, so the rotary hearth furnace is widely used in iron and steel enterprises. (of course, the most important thing is the large investment and high operating cost of rotary hearth furnace equipment.)

Diversified Utilization of Zinc Ash Wet treatment Technology

The secondary zinc oxide obtained by fire treatment and the zinc ash with a content of more than 15% are generally treated by wet method. Wet treatment is generally divided into five steps: dechlorination, leaching, purification, electrodeposition, melting and casting. According to the different leaching media, zinc electrolysis is generally divided into acid method, alkali method and ammonia method.

The first step of wet treatment is generally dechlorination. In the process of acid and alkaline electrolysis, when the concentration of chloride ion exceeds a certain value, the phenomenon of "burning plate" often occurs, which reduces the life of the cathode aluminum plate by 1/3 or even more. therefore, dechlorination before electrolysis is very necessary.

In addition, there can be a part of chloride ion in ammonia electrolysis zinc electrolyte, but there is also a certain limit, and there is also selective use of chlorine-containing materials or ingredients.

The commonly used dechlorination methods are volatilization kiln fire treatment and wet alkali washing dechlorination.

Diversified Utilization of Zinc Ash by Wet process / leaching

Due to the different leaching media, the final composition of the electrolyte is different. The temperature requirements of the three kinds of electrolytic zinc are also slightly different.

Diversified Utilization of Zinc Ash Wet process / purification

In order to meet the requirements of zinc electrolysis, harmful impurities such as copper, cadmium, cobalt, nickel and lead in the leaching solution must be purified below the allowable content. The common method is zinc powder replacement.

When zinc powder is replaced, copper and lead are easy to precipitate preferentially. The removal of cadmium by replacement is more difficult than that of copper, and excessive zinc is required. In order to strengthen the replacement effect of zinc particles on cadmium, a certain ratio of copper to cadmium in the solution is required.

Sulfide precipitation method

Sulfide precipitation method takes advantage of the strong affinity between S2-in Na2S and H2S and heavy metal ions to form sulfide precipitation with positive low solubility and remove it from the solution.

The following aspects should be paid attention to in the operation of sulphide precipitation method:

The main results are as follows: (1) the vulcanized precipitation is relatively small and easy to form colloid, so macromolecular flocculant should be added to facilitate the separation.

(2) the precipitant will partially remain in the water, and the residual precipitant is a kind of pollutant, which will produce malodorous, etc., and the harmful gas H2S will be produced in the acidic environment, which will form secondary pollution.

Diversified Utilization of Zinc Ash by Wet method / Electrodeposition

Electrolysis can be divided into three types: ammonia electrolysis, acid electrolysis and alkaline electrolysis.

The energy consumption of electrowinning accounts for 70% of the total energy consumption of the whole wet treatment. The standard electrode potential of nitrogen and ammonia is obviously small, but NH4+ and NH4OH are relatively stable, and the rate of direct decomposition to nitrogen is very slow. With the increase of anodic potential, although the oxygen evolution potential exceeds the standard electrode potential of Cl- reaction, the overpotential of O2 on the plate is much larger than that of Cl2, so the anodic reaction mainly produces chlorine with strong activity, and then the decomposition reaction of NH4 + and NH4OH occurs under the catalysis of chlorine to produce nitrogen.

IV. Opportunities and challenges of diversified utilization of zinc-containing wastes

The opportunities that zinc-containing waste will face in the future are the trend of recycling of solid waste and hazardous waste, the increase of demand for non-ferrous metal products (increase in quantity), and the broad development prospect of zinc materials in the future (improvement of quality).

The challenges facing the diversified utilization of zinc-containing wastes in the future are the increasing challenges of safety and environmental protection, the challenges of energy conservation in the context of "carbon neutralization", as well as price fluctuations and financial risks.