Bauxite ore is concentrated by

Explanation

To solve the question regarding how bauxite ore is concentrated, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Composition of Bauxite Ore**:
Bauxite ore primarily consists of aluminum oxide (Al2O3) along with impurities such as iron oxides and silica. The chemical formula can be represented as Al2O3·2H2O.

2. **Understand the Concept of Concentration**:
Concentration of ore involves the removal of impurities or gangue from the ore to obtain a purer form of the desired metal.

3. **Determine the Method of Concentration**:
Bauxite ore is concentrated using the process known as **leaching**. This method involves the use of a solvent to dissolve the desired mineral while leaving the impurities behind.

4. **Leaching Process**:
- First, the bauxite ore is treated with sodium hydroxide (NaOH) in an aqueous solution.
- The aluminum oxide reacts with NaOH to form a soluble complex, sodium aluminate (NaAl(OH)4).

5. **Formation of Soluble Complex**:
The reaction can be represented as:
\[ \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} + 2\text{NaOH} \rightarrow 2\text{NaAl(OH)}_4 + 2\text{H}_2\text{O} \]
Here, the aluminum is now in a soluble form, allowing for the separation from impurities.

6. **Precipitation of Aluminum Hydroxide**:
- To recover aluminum oxide from the soluble complex, carbon dioxide (CO2) gas is bubbled through the solution.
- This leads to the precipitation of aluminum hydroxide (Al(OH)3) and sodium bicarbonate (NaHCO3).

7. **Final Steps to Obtain Pure Alumina**:
- The aluminum hydroxide is then heated to remove water, yielding pure aluminum oxide (Al2O3).
- The overall reaction can be summarized as:
\[ 2\text{NaAl(OH)}_4 + \text{CO}_2 \rightarrow \text{Al}_2\text{O}_3 \cdot 2\text{H}_2\text{O} + 2\text{NaHCO}_3 \]

8. **Conclusion**:
The final product after heating is pure alumina (Al2O3), confirming that bauxite ore is concentrated by the leaching process.

### Final Answer:
Bauxite ore is concentrated by **leaching**.