Processing

Historically, in the United States, the consumption of lead in glasses and ceramics has been approximately 30,000-50,000 metric tons per year, which represents 2-3% of the total U. S. annual lead consumption [29]. If storage battery usage is not included in the annual total, as this product category represents over 86% of U. S. lead consumption annually, then glasses and ceramics represent 13-22% of the remaining demand for lead in the United States.

Litharge and the other lead oxides that are used in the production of glasses and ceramics are obtained primarily through the oxidation of refined (purified) metallic lead. Because metallic lead does not occur naturally in large quantities, it must be extracted from either primary sources (mineral ores) or secondary sources (recycled materials such as lead-acid batteries and cathode ray tubes). The processing required to refine metallic lead can be broken down into three major steps, as seen in Fig. 3:

1. Mining and concentrating

2. Extraction or smelting

3. Refining

The refining step is then followed by an oxidation step in order to produce lead oxide. Because these processes are discussed in detail in several other sources [1,30-32], the description provided below is intentionally brief.

For primary sources of lead, namely mineral ores, the process of mining and concen­trating, indeed, begins at a mine. For secondary sources, this stage of the process is replaced by separation and sorting steps to remove the components in the batteries and CRTs that do not contain lead. The remaining process steps are fundamentally the same.

Considering the primary sources of lead, although there are over forty different minerals that contain lead (see Table 2), the three most common minerals from which

Processing

pure lead is derived are galena (PbS), anglesite (PbSO4), and cerussite (PbCO3) with lead concentrations (by weight) of 87%, 68%, and 77.5%, respectively. Galena is easily recognized in the field because of its characteristic cubic shape, metallic luster, and high density. Anglesite and cerussite result from the natural weathering of galena. These three minerals exhibit the rock salt (NaCl), barite (BaSO4), and aragonite (CaCO3) crystal structures, respectively. The JCPDS cards that describe the crystallographic characteristics for these minerals are as follows: 05-0592 for galena (PbS), 36-1461 for anglesite (PbSO4), and 47-1734 for cerussite (PbCO3).

After being mined, these lead-containing minerals proceed through a concentration process that increases the lead concentration and removes waste (non-galena) rock, which is called gangue. The concentration process generally begins with crushing and grinding steps that ultimately result in particles <2 mm in size, followed by the actual concentration step, which is sometimes referred to as “beneficiation.” The most common
method used is froth flotation in which particles are separated on the basis of specific gravity. Additives, such as conditioners, are used to facilitate the separation process. Dewatering is then required, and the product remaining is primarily PbS. The concentra­tion of lead in this product is generally between 40 and 80 wt%. Other substances contained in the material at this point include iron (Fe), zinc (Zn), copper (Cu), antimony (Sb), arsenic (As), silver (Ag), gold (Au), bismuth (Bi), and lime (CaO).

The extraction, or smelting, stage requires multiple steps. First, the lead concentrate is heated to remove the sulfur (as SO2 and SO3) and to agglomerate the fine particles. This step leads to sintering and oxidation of the material, thus forming oxides of lead, zinc, iron, and silicon. Other substances such as lime, metallic lead, and residual sulfur might also be present. The next step is the actual smelting, in which the material is heated in a reducing environment so that the oxides are converted into molten metals that can be refined and separated. Various contaminants are also removed through combustion. The lead at this point is called bullion and has a concentration between 95 and 99% lead by weight. Prior to actual refining, there is one more step, the copper drossing step, which is yet another heat treatment by which copper is removed.

The lead refining stage can be divided into five steps, each of which is designed to remove (and collect) selected impurities. The impurities removed during each step are as follows:

1. Antimony (Sb), tin (Sn), and arsenic (As)

2. Silver (Ag) and gold (Au)

3. Zinc (Zn)

4. Bismuth (Bi)

5. Trace impurities

After these steps, the lead concentration is now between 99.9 and 99.99 wt%.

It should be noted that, although existing methods are well established, new methods for smelting and refining continue to be developed. For instance, “direct” smelters are available, which eliminate the need for sintering, and electrolytic refining can be used as a one-step method for simultaneously removing all impurities (except tin). Each method presents its own combination of product quality, process cost, and environ­mental management requirements.

Lead-containing glasses and ceramics do not use metallic lead as a raw material. Instead, lead oxides and lead silicates are used. Various processing techniques are used to produce the lead oxides from refined metallic lead. For instance, litharge (PbO) is the reaction product of lead and oxygen, and can thus be produced by heating lead in air or by blowing air into molten lead. Minium (lead tetroxide), which is more oxygen-rich than litharge, can be created by further oxidizing litharge in a controlled atmosphere at about 450°C. Lead silicates are made by mixing and heating litharge and sand (SiO2).

Primary and secondary sources of lead exist in the United States and throughout the world [29]. U. S. mine production of lead in concentrate is approximately 450,000-500,000 metric tons per year, which represents approximately 15% of the world production. Other countries with significant mine production of lead include Australia, Canada, China, Mexico, and Peru. Refining of secondary lead is dominated by the U. S. production, although other major sources include Canada, China, France, Germany, Italy, Peru, Spain, and the United Kingdom. In the United States, approximately 79% of the current lead refinery production is derived from secondary sources. Worldwide, secondary sources account for approximately 45% of the total 6.4 million metric tons of lead refined each year. Current prices for metallic lead and for litharge (PbO) are approximately $0.95/kg and $0.55/kg, respectively [33,34].