Bulk Upload Image Alt Text

TRY THIS,IT MAY HELPFUL

1. Converted to CSV file.
2. uploaded to root folder having column structure as given which contains attachment image ID and alter text

3. run this code for one time only

$row    = 1;
if( ($handle = fopen ( "Book1.csv", "r" )) !== FALSE ) {
    while ( ($data = fgetcsv ( $handle, 1000, "," )) !== FALSE ) {
        $image_id       = $data[ 0 ];
        $image_alt_text = $data[ 1 ];
        update_post_meta ( (int) $image_id, '_wp_attachment_image_alt', $image_alt_text );
    }
    fclose ( $handle );
}